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Simultaneous inversion of local and teleseismic data for the crust and mantle structure of southern California
PHYSICS O F T H E EARTH AND PLANETARY INTERIORS ELSEVIER
Physics of the Earth and Planetary Interiors 93 (1996) 191-214
Simultaneous inversion of local and teleseismic data for the crust and mantle structure of southern California Dapeng Zhao a,,, Hiroo Kanamori a, Eugene Humphreys b a Seismological Laboratory, California Institute of Technology, Pasadena, CA 91125, USA b Department of Geological Sciences, Universityof Oregon, Eugene, OR 97403, USA Received 3 October 1994; revision accepted 25 May 1995
Abstract We determined a detailed three-dimensional seismic velocity structure of the crust and mantle to a depth of 800 km beneath southern California using local and teleseismic data simultaneously. We used 131372 P-wave arrival times from 6437 local earthquakes and 12 134 travel time residuals from 193 teleseismic events recorded by 251 seismic stations of the Caltech-US Geological Survey Southern California Seismic Network. Compared with previous local and teleseismic studies, the station coverage is considerably improved, and the number of data used is greatly increased in the present study. This, together with the local and teleseismic joint inversion approach, results in a unified three-dimensional (3-D) P-wave velocity model of the crust and mantle in southern California with a higher resolution than the previous models. The result obtained shows very heterogeneous structures in the crust and upper mantle in southern California. Shallow crustal structures correlate well with surface geological features. Sedimentary basins such as the Los Angeles Basin, Ventura Basin and Santa Maria Basin are imaged well as low velocities, and batholiths such as the Peninsular Ranges and San Gabriel Mountains are imaged as high velocities. The velocity in the crust is generally low in the Mojave Desert. The crustal velocities change abruptly across major faults such as the San Andreas, San Jacinto, Elsinore and Garlock faults. In the upper mantle, three major anomalies are found, beneath the Transverse Ranges, Salton Trough and Southern Sierra Nevada. The Transverse Ranges feature appears as a curtain-like, east-trending and north-dipping high-velocity anomaly with a thickness of approximately 50 kin, and extends most deeply at its eastern end to about 250 km. The Salton Trough feature is composed of low velocities which trend southeast and extend to approximately 200 km depth. The Southern Sierra Nevada feature is a slab-like high-velocity anomaly which extends to about 250 kin. The mantle velocity is generally low in the upper 100-200 km beneath the volcanic areas in southern California. The seismic velocity anomalies in the mantle beneath southern California are considered to result from small-scale mantle convection.
1. Introduction
" Corresponding author present address: Department of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130, USA.
S o u t h e r n C a l i f o r n i a consists o f several significantly d i f f e r e n t t e c t o n i c provinces, i n c l u d i n g t h e S a l t o n T r o u g h w h i c h is a m o d e r n crustal pulla p a r t a s s o c i a t e d with t h e o p e n i n g o f t h e G u l f o f
0031-9201/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSD! 0031-9201(95)03076-X
D. Zhao et al. / Physics of the Earth and Planetary Interiors 93 (1996) 191-214
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California, and a major transform plate boundary separating the Pacific plate and the North American plate; its major surface expression is the San Andreas Fault (see Fig. 1). It is reasonable to expect that these surface features are accompanied by structural heterogeneities at depth in the crust and upper mantle. Many researchers have investigated the heterogeneous seismic velocity structure of the crust and upper mantle beneath the entire southern California region by using arrival time data from either local earthquakes (e.g. Ergas and Jackson, 1981; Hearn, 1984; Hearn and Clayton, 1986a,b; Sung and Jackson, 1992; Zhao and Kanamori, 1992; Magistrale et al., 1992), or teleseismic events (Hadley and Kanamori, 1977; Raikes, 1980; Walck and Minster, 1982; Humphreys et al., 1984; Humphreys and Clayton, 1990). The data used in these studies are recorded by the Southern California Seismic Network (SCSN) jointly run by the
California Institute of Technology (Caltech) and the US Geological Survey (USGS). The data set provided by the SCSN is very extensive in the number and distribution of both stations and sources, and hence the structure there can be determined very well (Humphreys and Clayton, 1990). These previous studies revealed significant structural complexities in the crust and upper mantle in southern California, which seem to correlate with the surface geological features. As mentioned above, so far local earthquake tomography and teleseismic tomography methods have been used separately to study the seismic velocity structure of southern California. The two approaches have inherent advantages and limitations. Local earthquake tomography uses arrival times from shallow earthquakes and can determine a detailed structure of the crust and uppermost mantle, but cannot determine the deeper structure. This is simply because no deep earth-
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Fig. 1. Simplified geological map of southern California. The locations of major faults (continuous lines), volcanic areas and other geographic areas referred to in the text are shown. LAB, Los Angeles Basin; PRB, Peninsular Ranges Batholith; SBM, San Bcrnardino Mountains; SGM, San Gabriel Mountains; SMB, Santa Maria Basin; SMM, Santa Monica Mountains; SSNB, Southern Sierra Nevada Batholith; ST, Salton Trough; VB, Ventura Basin.
D. Zhao et al. ~Physics of the Earth and Planetary Interiors 93 (1996) 191-214
quake occurs in southern California, and seismic rays of direct and refracted waves from the local crustal earthquakes can only sample the areas from the surface to the uppermost mantle. In contrast, teleseismic tomography uses relative travel time residuals from distant events with epicentral distances longer than approximately 30°, and can determine the deep structure of the mantle. Teleseismic tomography, however, usually cannot determine well the crustal structure because teleseismic rays basically travel in a subvertical direction and do not crisscross well near the surface. Therefore it is usual in teleseismic tomography that a crustal model (either a threedimensional (3-D) velocity model or simply station corrections) obtained by a previous study is used to correct for the complex shallow structure, and only the mantle structure is determined through the teleseismic inversions (e.g. Humphreys and Clayton, 1990). If an inadequate crustal model is used, the accuracy of the mantle structure obtained, particularly that of the uppermost mantle, will be affected. Recently, a new approach has been proposed to determine tomographic images of an area by inverting data from local and teleseismic events simultaneously (Roecker et al., 1993; Zhao et al., 1993, 1994). This local and teleseismic joint inversion approach preserves the advantages of the two separate approaches, and overcomes their limitations (Zhao et al., 1994). Moreover, when data from both local and teleseismic events are used, the horizontally propagating local rays and vertically travelling teleseismic rays will crisscross well in the lower crust and uppermost mantle, which improves the resolution there. The successful application by Roecker et al. (1993) and Zhao et al. (1993, 1994) demonstrates the effectiveness of the joint inversion approach. In the present study, we have applied this new approach to a large number of arrival times from local and teleseismic events to determine a detailed 3-D P-wave velocity structure of the crust and mantle down to 800 km depth beneath southern California. We have taken advantage of the high density of seismic stations, and the high quality and great quantity of the data base of the Caltech-USGS Southern California Seismic Net-
193
work. Compared with the previous studies for this area, the station coverage is considerably improved, and the number of data used is greatly increased. This, together with the use of the local and teleseismic joint inversion approach and the modern tomographic method, results in a unified 3-D P-wave velocity model of the crust and mantle in southern California with a higher resolution than the previous models. The obtained result casts new light on the complex structure and tectonics of southern California.
2. Data and method
In this study, we combined two sets of seismic data to conduct tomographic inversions. One consists of arrival times from local crustal earthquakes which occurred in and around southern California, and the other of relative travel time residuals from teleseismic events.
2.1. Local earthquake data We have used arrival times recorded by the SCSN in the period from January 1981 to May 1992. Fig. 2 shows the present study area and geographic locations of 251 seismic stations we used. The stations are densely and uniformly distributed in the study area except for the Great Valley region. The station spacing ranges from 10 to 40 km. 121W
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More than 100 000 local earthquakes have been recorded by the seismic network during the period of 12 years. To select the best set of events for the inversion, we divide the crust of the study area into blocks with spatial sizes of 8 km x 8 km x 2 km. Among the earthquakes within each block, we selected the event which has the greatest number of first P-wave arrivals and the smallest uncertainty for the hypocentral location. As a result, we selected 6437 earthquakes; their epicentral locations are shown in Fig. 3. All the events have at least ten P arrivals, many of them have more than 100 P arrivals. The events with fewer arrivals occurred in the Great Valley, and the eastern and western edges of the study area, where there are fewer stations and seismicity is
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D. Zhao et aL /Physics of the Earth and Planetary Interiors 93 (1996) 191-214
lower. The uncertainty for the hypocentral locations is less than 2 km for most of the events. We can see that the events selected are well distributed in the entire study area. Many events are concentrated on the major fault zones in southern California, such as the San Andreas, San Jacinto, Elsinore, and Garlock faults. T h e r e are, however, many earthquakes which occurred in places where no fault appears on the Earth's surface. T h e total n u m b e r of first P-wave arrivals from the 6437 local earthquakes is 131 372. The accuracy of arrival time is estimated for each picking by the SCSN analyst. Most of the local data we used have accuracy of approximately 0.1 s; some data with large epicentral distances may have an
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accuracy of approximately 0.2 s. Arrival times with larger picking errors are not used. 2.1 Teleseismic data We have used a total of 193 teleseismic events which are located at epicentral distances between 30 ° and 95 ° from southern California. O f the 193 events, 180 occurred during the period from April 1974 to October 1986. Some of these data have been used by Raikes (1980) and Humphreys and Clayton (1990) to study the upper-mantle structure of southern California. Arrival times of 13 events that occurred from October 1989 to August 1990 were collected by Mori and Frankel (1992) to investigate the correlation of P-wave
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196
D. Zhao et al. / Physics of the Earth and Planetary Interiors 93 (1996) 191-214
amplitudes and travel time residuals for teleseisms in southern California. Fig. 4 shows the geographic locations of the 193 teleseismic events we used in the present study. We can see that these events have a fairly complete azimuthal coverage except for a gap at 150-225 ° from north. Most of the events are earthquakes that occurred in the subduction zones of the North and West Pacific, Southeast Asia, and Middle and South America. Some earthquakes occurred in the oceanic ridges and the Eurasian continent. In total, 12134 P-wave arrival times were collected from the 193 teleseismic events. The number of arrivals for each event ranges from 18 to 153. The average number of arrivals is 63. There are 19 events having more than 100 arrivals. The accuracy of the arrival times is estimated to be approximately 0.1-0.2 s for the teleseismic data. We use relative travel time residuals of the teleseismic events to minimize source effects such as errors introduced by hypocentral mislocations and origin times, and the path effect outside the modeling space. (For the calculation of relative residuals, see Zhao et al. (1994).) Hypocenters were obtained from the Bulletin of the International Seismological Centre. Theoretical travel times are calculated by using the iasp91 Earth model (Kennett and Engdahl, 1991). For the crust and mantle right under the study area, the model as shown below in Fig. 8 is used (for details of the model, see Section 3). We display the teleseismic data in two ways. First, the relative residuals for each station are averaged over all the 193 events, resulting in a mean relative travel time residual. These are plotted in Fig. 5(e). Second, we separate the events into source quadrants according to the azimuth of the source from the network, also referred to as the source azimuth or source direction. We calculate the mean relative residuals for four quadrants: NE 0°, 90°; SE 90~, 180°; SW 180°, 270°; NW 270°, 360°. The mean residuals for each source quadrant are plotted in Figs. 5(a)5(d). The mean relative residuals, referred to simply as residuals, averaged for all the 193 events (Fig. 5(e)) produce a spatial pattern of early arrivals at
stations in the Transverse Ranges, eastern Mojave Desert, Peninsular Ranges, and some stations in Southern Sierra Nevada, and late arrivals at stations in Salton Trough, Santa Maria Basin, Ventura Basin, Los Angeles Basin, and along the San Andreas Fault zone in the northwestern part of the study area. This pattern is generally consistent with that found by the previous investigators (e.g. Raikes, 1980; Humphreys and Clayton, 1990). The pattern and magnitude of the residuals for the events from the four separate quadrants (Figs. 5(a)-5(d)) are, as a whole, all similar to those in Fig. 5(e). The most significant variation is the spatial shift of the Transverse Ranges feature of early arrivals with the change of the incident direction of the teleseismic rays. For the teleseismic events in the northeastern quadrant (Fig. 5(a)), all the early arrivals are confined to regions southwest of the San Andreas Fault. For the events in the southwestern quadrant (Fig. 5(c)), however, the early arrivals spread into most of the Mojave Desert. For events in the northwestern and southeastern quadrants (Figs. 5(d) and 5(b), respectively), the Transverse Ranges early feature shifts to south and north, respectively. This systematic spatial pattern provides convincing evidence that a high-velocity anomaly extends deep in the mantle beneath the Transverse Ranges. The magnitude and pattern for the early arrivals in Southern Sierra Nevada also changes for events in different quadrants. The Salton Trough feature of late arrivals which trends NWSE is relatively stable for events in all the quadrants. 2.3. Method
In this study we have used the method of Zhao et al. (1994) to combine teleseismic residuals with local earthquake arrival times in tomographic inversions. This method can deal with a general velocity model in which complex velocity discontinuities exist and the velocity changes in three dimensions (see also Zhao et al. (1992)). Local earthquakes are relocated in the inversion process. An efficient 3-D ray tracing technique (Zhao et al., 1992) is used to calculate travel times and ray paths accurately and rapidly. The LSQR algo-
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denser net of grid nodes in the crust and uppermost mantle. Figs. 6 and 7 show the configurations of the grid net in the horizontal and vertical planes, respectively. The separation between grid nodes is 0.25 ° (approximately 25 kin) in the horizontal plane. In the crust and uppermost mantle, four grid mesh layers are set up at 2, 10, 22, and 35 km. Between 35 and 695 km, 22 layers of grid mesh are set up with an interval of 30 km. From 695 to 800 km, three grid mesh layers are set up with an interval of 35 km. The total number of grid nodes is 17 x 27 × 29 = 13311, Velocity perturbations at the grid nodes and hypocentral parameters of local earthquakes are taken as unknown parameters. The total number of unknowns is (4 × 6437) + 13311 = 39059. It should be noted that the total number of local and teleseismic data is 143 506.
3. Analysis and result 3.1. Inversion Fig. 8 shows the one-dimensional (l-D) P-wave velocity model which is used as the starting model
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Fig. 8. One-dimensional P-wave velocity model which is taken as the starting velocity model for the tomographic inversions in this study. The crustal model is modified from that of Kanamori and Hadley (1975); the mantle model is the GCA model determined by Walck (1984).
D. Zhao et al. / Physics of the Earth and Planetary Interiors 93 (1996) 191-214
198
short-period travel-time and waveform data recorded by the SCSN. The GCA model is found to be an appropriate average 1-D mantle velocity model for southern California (Walck, 1984; Humphreys and Clayton, 1990). The two firstorder discontinuities in the GCA model are at 390 and 660 km depth, where the velocity jump is 4.7% and 2.8%, respectively. Applying the technique of Zhao et al. (1994) to all the local and teleseismic data and adopting the starting velocity model (Fig. 8) and the grid net (Figs. 6 and 7) as described above, we performed a number of inversions by changing the depth of the modeling space from 200 to 800 km with an interval of 100 km. We found that the final results and final travel time residuals are essentially the same for the inversions with the
for the tomographic inversions in the present study. The crustal model is modified slightly from the Kanamori and Hadley (1975) model, which was constructed by using accurate arrival times from large quarry blasts in the central part of southern California. The standard velocity model for the routine earthquake location by the SCSN was derived from this structure. The model consists of four constant-velocity layers with discontinuities at 4, 27.4, and 32.4 km, respectively. The lower part of the second layer (from 18 to 27.4 km) was modified to have a velocity gradient by Zhao and Kanamori (1992). The modified crustal model is found to be more appropriate for the present study area (Zhao and Kanamori, 1992). The mantle velocity model in Fig. 8 is the GCA model determined by Walck (1984) using
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D. Zhao et al. ~Physics of the Earth and Planetary Interiors 93 (1996) 191-214
model depth from 600 to 800 km. For the inversions with shallower model depths, there are some variations in the results and final travel time residuals become larger. This means that velocity anomalies in the upper 500 km depth beneath southern California are significant for the data set used in the present study. Below 500 km, velocity anomalies are either less significant or they cannot be resolved by the present data set. In the following, we show the result of the inversion with the modeling depth of 800 km. We solved the inverse problem simultaneously for 6437 x 4 hypocentral parameters and 11212 velocity perturbations for the grid nodes with hit counts (number of rays passing through each node) larger than ten. More than 99% of the nodes have hit counts greater than 50. A variance reduction of 42% is achieved after the inversion.
199
Fig. 9 shows fractional P-wave velocity perturbations (in percent) on horizontal planes determined by the inversion. Only the images down to 400 km depth are shown because the velocity changes in the deeper areas are insignificant. Fig. 10 shows locations of six vertical cross-sections of the tomographic images as shown in Fig. 11. The cross-sections are shown to 600 km depth for clarity, because no substantial structural anomaly is found in the deeper areas. Fig. 12 shows a 3-D image of the P-wave velocity perturbations viewed from the northwest. 3.2. Resolution
Before describing the main features of the obtained result, we first show the resolution of the tomographic image. We first used the
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D. Zhao et aL / Physics of the Earth and Planetary Interiors 93 (1996) 191-214
structed for most of the study area for the layers in the crust and uppermost mantle. The resolution is poor around the Great Valley region for the layers in the crust because of lack of stations there. For the layers shallower than 300 km in the upper mantle, the resolution is good and the checkerboard pattern is correctly reconstructed mainly in the central part of the study area. For the layers in the depth range of 300-600 km, the checkerboard pattern is restored only in some limited areas. Below 600 km depth, the resolution is very poor. Fig. 14 shows the result of another checkerboard resolution test with a grid separation of 0.4 ° (approximately 40 km). Only eight representative layers are shown. We can see in Fig. 14 that the resolution is high and the checkerboard pattern is correctly reconstructed for most of the study area.
checkerboard resolution test (Zhao et al., 1992) to assess the adequacy of the ray coverage and to evaluate the resolution. To make a checkerboard, we assign positive and negative velocity perturbations of 6% to the 3-D grid nodes, the image of which is straightforward and easy to remember. Therefore, seeing the image of the inverted synthetic checkerboard pattern, one can easily understand where the resolution is good and where it is poor. It should be noted that we added random errors having a normal distribution with a standard deviation of 0.1 s to the synthetic data calculated for the synthetic models in all the resolution tests we conducted. Fig. 13 shows the result of one test with a grid separation of 0.25 ° (approximately 25 km) for the study area. The resolution is generally high and the checkerboard pattern is correctly recon-
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Recently, Leveque et al. (1993) showed that in some cases small structures as in the checkerboard test can be well retrieved, whereas larger
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structures are poorly retrieved. To find whether there is such a problem in our present tomographic images, we made a synthetic test as shown in Fig. 15. The synthetic model has a simple and large-scale structure of more than 200 kin, and the grid separation is 40 km. The inverted images show that the large-scale structure is generally well reconstructed for all the depth levels. We also made a number of other synthetic tests by using synthetic models with different structural geometries and grid separations; the inverted results always give reasonally well retrieved images of the synthetic model. From these resolution tests, we can say that both the small and the large structures can be retrieved well by our present technique and data set. The tomographic image obtained in the present study has a spatial resolution of about 25 km for the upper 250 km depths and about 40 km for the deeper areas.
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D. Zhao et al. /Physics of the Earth and Planetary Interiors 93 (1996) 191-214
however, still shows high velocity. In addition, high velocities exist in the Salton Buttes and Great Valley region, and low velocities exist in the eastern Mojave Desert, Colorado Desert, Coso volcanic area and Coastal Ranges. The abrupt velocity change across the northern part of the San Andreas Fault is noteworthy. In the lower crust (22 km depth), the velocity is low in the Mojave Desert, Salton Trough and the volcanic areas, and is high north of the Garlock Fault, Coastal Ranges, and in a region including the southern Peninsular Ranges and the Elsinore and the San Jacinto fault zones. The velocity is also high beneath the Continental Borderland. The velocity changes abruptly across the southern part of the San Andreas Fault. In the uppermost mantle (35 km depth), high velocities exist in the eastern Mojave Desert, Salton Trough, and in Pacific coastal regions, whereas low velocities exist in the southern Sierra
3.3. 3-D crust and mantle structure
In the following, we describe the main features of the obtained tomographic images that have reliable resolution. The crustal structure determined in this study is similar to that found by Zhao and Kanamori (1992, 1993, 1995). At shallow depths (2 km), the P-wave image correlates well with the major surface geological features. The Ventura, Santa Maria and Los Angeles basins, and the Salton Trough are imaged as low velocities, and the batholiths of the Peninsular Ranges, San Gabriel Mountains and Southern Sierra Nevada are imaged as high velocities. All of the six volcanic areas in southern California are underlain by low-velocity anomalies. In the middle crust (10 km depth), the velocity anomalies for the sedimentary basins and batholiths seen at 2 km depth disappear or become less prominent. The Peninsular Ranges batholith,
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D. Zhao et at/Physics o/the Earth and Planetary Interiors 93 (1996) 191-214
Nevada, Peninsular Ranges, and around the Cima and Coso volcanic areas. Across the Garlock Fault, the velocity changes significantly. In the upper mantle, the maximum amplitude of velocity anomalies decreases significantly with depth. It is approximately 5-6% at depths shallower than 150 km, approximately 3-4% from 150 to 400 km, approximately 2-3% from 400 to 600 km, and less than 2% below 600 km. The pattern of velocity variations is approximately the same for all the layers in the upper mantle. The three most striking velocity anomalies in the upper mantle are beneath the Transverse Ranges, Salton Trough and western foothills of the Southern Sierra Nevada (named the 'Isabella anomaly' by Raikes (1980) and Jones et al. (1994)). The Transverse Ranges feature appears as a slab-like, east-trending and north-dipping highvelocity anomaly with a thickness of 50-60 km (see Fig. 9, layers from 65 to 245 km, and Fig. 11,
cross-sections AAg, BB', and DD'). Beneath the eastern part of the Transverse Ranges, this feature extends most deeply to about 250 km (see Fig. 11, cross-section AA¢); in the western part, it extends to about 150 km (see Fig. 11, cross-section DD'). This high-velocity feature seems continuous to the surface and is sandwiched between low-velocity anomalies on its upper and lower sides (see Fig. 11, cross-sections AA¢ and DD'). The high-velocity zone is thin (approximately 2030 km) at shallower depth (0-50 km), and become thicker (50-60 km) in the upper mantle. The Isabella feature is a slab-like high-velocity anomaly which extends to about 250 km, with its main part to 200 km depth (see Fig. 9, layers from 65 to 335 km, and Fig. 11, cross-sections CC' and DD'). This feature continues to the northern boundary of the present study area (36.5°N). The Salton Trough feature is composed of low velocities which trend southeast and ex-
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204
D. Zhao et al. / Physics of the Earth and Planetary Interiors 93 (1996) 191-214
reduced to 0.23 s for the 3-D velocity model (Figs. 9 and 11). The variance reduction is 45%. This means that the teleseismic data are well explained by the present 3-D velocity model.
tend to about 200 km (see Fig. 9, layers from 65 to 185 km, and Fig. 11, cross-sections EE' and FF'). Low velocities are visible in the Pacific coastal regions in the depth ranges of 65-155 km (see Fig. 9). Below the volcanic areas, the velocity is generally low, and the low-velocity anomalies extend to 100-200 km depth (see Fig. 9, layers from 65 to 185 km, and Fig. 11, cross-sections AP~, CC', EE' and FF'). It seems that these volcanic areas have a deep extension in the upper mantle. Fig. 16 shows synthetic teleseismic residuals per source quadrant calculated for the 3-D P-wave velocity structural model determined in this study (Figs. 9 and 11; Fig. 12). Comparing the synthetic residual distributions with those of the observed data (Fig. 5), we can see that they are consistent in both the distribution pattern and residual magnitude. The r.m.s, residual of the tel•seismic data is 0.31 s for the 1-D velocity model (Fig. 8); it is
S a n
4. Discussion Previous studies found a Pn velocity anisotropy in southern California by using Pn waves with epicentral distances as great as 600 km (Vetter and Minster, 1981; Hearn, 1984; Sung and Jackson, 1992). In this study, we did not take into account the Pn velocity anisotropy, which may have some effect on the obtained P-wave image. However, the effect is believed to be very small because the Pn rays we have used traverse the structure in many directions, tending to cancel out any anisotropic contribution. In addition, all the Pn arrivals we used have epicentral distances
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D. Zhao et al. / Physics of the Earth and Planetary Interiors 93 (1996) 191-214
shorter than 200 km. These Pn waves do not propagate within the uppermost mantle further than 60 km. Although early investigators reported large changes of crustal thickness in southern California (e.g. Press, 1956; Shor and Raitt, 1956; Roller and Healy, 1963), later detailed studies using high-quality data from local quarries and explosions at the Nevada Test Site have revealed that
the crustal thickness throughout much of southern California is relatively uniform and in the range of 30-35 km (Kanamori and Hadley, 1975; Hadley and Kanamori, 1977). The exceptions may be the thinning crust offshore and in the Salton Trough region. The crust is found to be approximately 22-26 km in the Imperial Valley (Fuis et al., 1984). We did not include the Moho depth variation in this study. The obtained P-wave im-
Fig. 12. Three-dimensionalimage of the P-wavetomographyof southern California viewed from the northwest. The depth range is 0-600 km. Red and blue colors show isosurfaces of 2% low- and 2% high-velocityanomalies, respectively.
D. Zhao et al. / Physics of the Earth and Planetary Interiors 93 (1996) 191-214
age for the lower crust and uppermost mantle may be influenced, to some degree, by the Moho depth variation for the areas with an anomalous crust. In this study we obtained a high-velocity anomaly in the uppermost mantle (35 km) beneath the Salton Trough, which is probably caused by the unrealistic Moho depth (32.4 km) in the 1-D starting model for that area. It is desirable to determine the lateral velocity variations in the crust and uppermost mantle and the lateral depth variations of the Moho discontinuity simultaneously. However, the data we used in this study are only the first P-wave arrival times, which are not effective in distinguishing the two structural features. Reflected a n d / o r converted waves at the Moho discontinuity, which are very effective in determining the structure around the discontinuity, are needed to constrain well the Moho geometry. A joint use of the first and later arrivals in future studies may allow more accurate determination of the structure around the Moho. As described in the preceding section, we have imaged more clearly the three major velocity anomalies in the upper mantle beneath southern California in Isabella, the Transverse Ranges and the Salton Trough. The Isabella anomaly, which is located below the western foothill of the Southern Sierra Nevada, was first noted by Raikes (1980) using teleseismic residuals. She suggested that this anomaly may be a southward continuation of a 'dead slab' with high seismic velocity detected by Solomon and Butler (1974) beneath the Northern Sierra Nevada and Cascade Ranges. Her suggestion was confirmed by Biasi and Humphreys (1992), who determined P-wave velocity structure of the upper mantle beneath central California and southern Nevada. Recently, Jones et al. (1994) investigated in detail the Pwave velocity structure of the crust and upper mantle in Southern Sierra Nevada by using teleseismic residuals and Pn arrival times. They found little dip on the Moho and estimated a crustal thickness of about 32 km beneath the Sierra Nevada. They also found a considerably low velocity anomaly in the uppermost mantle beneath the high eastern Sierra Nevada. These features
207
are in good agreement with the present result (see Fig. 9, 35 km depth layer). Jones et al. (1994) suggested that the Isabella anomaly is unrelated to the regional tectonics of the Sierra Nevada, but is simply the downgoing part of a small-scale local convection system, similar to that in the Transverse Ranges. They listed two observations to support their interpretation. First, they found that the Isabella anomaly they imaged exists in the depth range from about 100 to 200 km, and the anomaly does not exist at shallower depth. Thus they inferred that the anomaly may represent cooler, denser material from the upper mantle that is now descending from near the base of the crust to depths near 200 km. Second, they found in their teleseismic tomographic image a low-velocity body in the upper 100 km of the mantle under the Tehachapi Mountains, which is located 50-60 km south of Lake Isabella. They suggested that the Tehachapi low represents low-velocity material replacing the high-velocity material now descending to form the Isabella anomaly to the north. From the tomographic image we obtained in this study (Fig. 11, cross-section DD'), we can see that the Isabella anomaly exists in the depth range from 50 to about 250 km, which is much larger than that determined by Jones et al. (1994). The Tehachapi low is also visible from 50 to 150 km in the present image; however, it is much less significant than the Isabella anomaly. Moreover, the Tehachapi anomaly is closely adjoining the Isabella anomaly. It is difficult to imagine that the two anomalies with a length of 100-200 km in depth may form a convection system in such a narrow range of about 60 km in the horizontal direction. We prefer the interpretation by Raikes (1980) that the Isabella anomaly is a southward continuation of a 'dead slab' beneath the Northern Sierra Nevada and Cascade Ranges, and so it is related to the regional tectonics of the Sierra Nevada. Many previous studies (Wilson, 1965; Atwater, 1970; Moore, 1973) have suggested that the Gulf of California is the locus of a spreading ridge along which the Pacific plate is being rifted away from the North American plate at a rate of as
D. Zhao et al / Physics of the Earth and Planetary Interiors 93 (1996) 191-214
208
and scattered Quaternary volcanism (Lomnitz et al., 1970; Elders et al., 1972; Robinson et al., 1976). Magnetic anomaly patterns so characteristic of most oceanic spreading ridges are well expressed only near the mouth of the Gulf of California. The Imperial Valley of southern California, the northern landward extension of the gulf, is a broad structural depression also termed
much as 6 cm year-1 (Larson, 1972). This spreading ridge appears to be broken into a number of short segments by a series of en echelon transform faults that show seismic activity compatible with the plate tectonics model. The Gulf of California is also characterized by locally high heat flow (Von Herzen, 1963), a crust intermediate between oceanic and continental (Moore, 1973),
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D. Zhao et aL ~Physicso f the Earth and Planetary Interiors 93 (1996) 191-214
the Salton Trough. Although tectonically similar to the rest of the Gulf of California, the trough is filled with as much as 6 km of sediments derived primarily from the Colorado River (Muffler and Doc, 1968). Elders et al. (1972) showed that spreading extends landward under the Salton Trough and that some of the areas of high heat flow in the trough may be located over active spreading centers as part of a leaky transform fault system. Gravity data (Biehler, 1964) indicate that the crust beneath the trough is isostatically compensated even though the trough is underlain by this great thickness of relatively low-density sediments. Biehler (1971) interpreted this relationship as indicating that the continental crust beneath the Salton Trough is undergoing thin-
209
ning and basification, probably by intrusion of basaltic rocks at depth. The low seismic velocity anomalies under the Salton Trough as imaged by this study are consistent with these previous geophysical observations, and can be understood in the concept of a northward continuation of the spreading center from the Gulf of California into the Imperial Valley. The existence of the Transverse Ranges anomaly was first noted by Hadley and Kanamori (1977), and was subsequently confirmed by Raikes (1980), Humphreys et al. (1984), and Humphreys and Clayton (1990). Our present study provides a d e a r e r image of this anomaly. It is the most striking feature in the upper mantle beneath southern California. These significant seismic ve-
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D. Zhao et aL /Physics o f the Earth and Planetary Interiors 93 (1990) 191-214
ring beneath southern California (Humphreys and Hager, 1990). Bird and Rosenstoek (1984) first invoked convergence and downwelling of the subcrustal lithosphere beneath the Transverse Ranges. Recently, Humphreys and Hager (1990) proposed a model for the young and continuing
locity anomalies in the upper mantle imply large variations in temperature. Because such laterally variable thermal structure cannot be maintained by any reasonable static conditions, their creation must be a recent or still continuing event; thus some form of mantle convection must be occur-
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D. Zhao et al. /Physics of the Earth and Planetary Interiors 93 (1996) 191-214
sidered to be created through the convergence and sinking of the entire thickness of subcrustal lithosphere. This anomaly is estimated to be about 375°C colder and 1% denser than average southern California mantle at the same depth by using the values of temperature derivatives of elastic
kinematic deformation of the crust and upper mantle beneath southern California. They have taken into account both the seismic velocity structure of the mantle and the known late Cenozoic geologic record. The high-velocity upper-mantle anomaly beneath the Transverse Ranges is con-
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D. Zhao et al. / Physics of the Earth and Planetary Interiors 93 (1996) 191-214
212
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wave velocities for mantle materials estimated by Karato (1993). Subcrustal lithosphere from both sides of the convergent zone is considered to participate nearly equally in the construction of the anomaly, and the low-velocity upper-mantle anomaly beneath the Salton Trough region is attributed to high temperatures and 1-4% partial melt related to adiabatic decompression during mantle upwelling. Therefore these upper-mantle anomalies can be interpreted as results of smallscale convection beneath southern California, to which much of the recent tectonic activity in the region can be attributed.
5. Conclusion
We have used a great number of data from local earthquakes and teleseismic events simulta-
neously to determine a unified and detailed 3-D P-velocity model of the crust and mantle down to 800 km depth beneath southern California. The spatial resolution is approximately 25 km for the crust and uppermost mantle, and 25-40 km for the deeper areas. The main results are summarized as follows. (1) Significant heterogeneities are imaged in the crust and upper mantle in southern California. Shallow crustal structures correlate well with surface geological features. Sedimentary basins such as the Los Angeles, Ventura and Santa Maria basins are imaged well as low velocities, and batholiths such as the Peninsular Ranges and San Gabriel Mountains are imaged as high velocities. The velocity in the crust is generally low in the Mojave Desert. (2) The crustal velocities change abruptly across the major faults in southern California,
D. Zhao et al. / Physics of the Earth and Planetary Interiors 93 (1996) 191-214
such as the San Andreas, San Jacinto and Garlock fault zones, suggesting the deep extent of these fault zones in the crust. (3) Three major velocity anomalies are found in the upper mantle in southern California. The Transverse Ranges feature appears as a curtainlike, east-trending and north-dipping high-velocity anomaly with a thickness of about 50 km, and extends most deeply at its eastern end to approximately 250 km. The Salton Trough feature is composed of low velocities which trend southeast and extend to about 150 kin. The Isabella feature is a slab-like high-velocity anomaly which extends to 250-300 km. (4) We prefer the interpretation of upper-mantle anomalies as results of small-scale mantle convection: the Transverse Ranges anomaly represents downwelling material, whereas the Salton Trough anomaly is an upwelling flow. The Isabella anomaly is considered to be a southward continuation of a 'dead slab' as revealed beneath the Northern Sierra Nevada and Cascade Range. (5) The mantle velocity is generally low in the upper 100-200 km depth beneath the volcanic areas in southern California, suggesting that these volcanic areas may have a deep extension in the upper mantle.
Acknowledgments This work was funded by grants from the National Science Foundation (EAR-92-04748), the US Geological Survey (USGS 1434-93-G-2287), and the Southern California Earthquake Center (SCEC). We thank J. Mori for providing the P-wave data from 13 teleseismic events. G. Biasi and K. Dueker kindly prepared some of the teleseismic data to be used for this study. We appreciate helpful discussions with D.L. Anderson. Two anonymous referees critically read the manuscript and provided helpful comments. W. Su helped us to produce the color image in Fig. 12. This paper is Contribution 5454, Division of Geological and Planetary Sciences, California Institute of Technology, and SCEC Publication 124.
213
References Atwater, T., 1970. Implications of plate tectonics for the Cenozoic tectonic evolution of western North America. Geol. Soc. Am. Bull., 81: 3513-3536. Biasi, G.P. and Humphreys, E., 1992. P-Wave image of the upper mantle structure of central California and southern Nevada. Geophys. Res. Lett., 19: 1161-1164. Biehler, S., 1964. Geophysical study of the Salton Trough of southern California. Ph.D. Thesis, California Institute of Technology, Pasadena, 139 pp. Biehler, S., 1971. Gravity models of the crustal structure of the Salton Trough. Geol. Soc. Am. Abstr. Prog., 3(7): 506. Bird, P. and Rosenstock, R.W., 1984. Kinematics of present crust and mantle flow in southern California. Geol. Soc. Am. Bull., 95: 946-957. Elders, W.A., Rex, R.W., Meidav, T., Robinson, P.T. and Biehler, S., 1972. Crustal spreading in southern California. Science, 178: 15-24. Ergas, R.A. and Jackson, D.D., 1981. Spatial variations of seismic velocities in southern California. Bull. Seismol. Soc. Am., 71: 671-689. Fuis, G.S., Mooney, W.D., Healy, J.H., McMechan, J.A. and Lutter, W.J., 1984. A seismic refraction study of the Imperial Valley region, California. J. Geophys. Res., 89: 11651189. Hadley, D.M. and Kanamori, H., 1977. Seismic structure of the Transverse Ranges, California. Geol. Soc. Am. Bull., 88: 1461-1478. Hearn, T.M., 1984. Pn travel times in southern California. J. Geophys. Res., 89: 1843-1855. Hearn, T.M. and Clayton, R.W., 1986a. Lateral velocity variations in southern California I. Results for the upper crust from Pg waves. Bull. Seismol. Soc. Am., 76: 495-509. Hearn, T.M. and Clayton, R.W., 1986b. Lateral velocity variations in southern California II. Results for the lower ernst from Pn waves. Bull. Seismol. Soc. Am., 76: 511-520. Humphreys, E. and Clayton, R.W., 1990. Tomographic image of southern California mantle. J. Geophys. Res., 95: 19725-19746. Humphreys, E. and Hager, B.H., 1990. A kinematic model for the late Cenozoic development of southern California crust and upper mantle. J. Geophys. Res., 95: 19747-19762. Humphreys, E., Clayton, R.W. and Hager, B.H., 1984. A tomographic image of mantle structure beneath southern California. Geophys. Res. Lett., 11: 625-627. Jones, C.H., Kanamori, H. and Roecker, S.W., 1994. Missing roots and mantle 'drips': regional Pn and teleseismic arrival times in the southern Sierra Nevada and vicinity, California, J. Geophys. Res., 99: 4567-4601. Kanamori, H. and Hadley, D., 1975. Crustal structure and temporal velocity change in Southern California. Pure Appi. Geophys., 113: 257-280. Karato, S., 1993. Importance of anelasticity in the interpretation of seismic tomography. Geophys. Res. Lett., 20: 1623-1626.
214
D. Zhao et al. /Physics of the Earth and Planetary Interiors 93 (1996) 191-214
Kennett, B.L.N. and Engdahl, E.R., 1991. Traveltimes for global earthquake location and phase identification. Geophys. J. Int., 105: 429-465. Larson, R., 1972. Bathymetry, magnetic anomalies, and plate tectonics history of the mouth of the Gulf of California. Geol. Soc. Am. Bull., 83: 3345-3360. Leveque, J., Rivera, L. and Wittlinger, G., 1993. On the use of the checker-board test to assess the resolution of tomographic inversions. Geophys. J. Int., 115: 313-318. Lomnitz, C., Mooser, F., Allen, C.R., Brune, J.N. and Thatcher, W., 1970. Seismicity and tectonics of the northern Gulf of California region, Mexico - - preliminary resuits. Inst. Geofis. Int. An., 10(2): 37-48. Magistrale, H., Kanamori, H. and Jones, C., 1992. Forward and inverse three-dimensional P wave velocity models of the southern California crust. J. Geophys. Res., 97: 1411514135. Moore, D.G., 1973. Plate edge deformation and crustal growth of Gulf of California structural province. Geol. Soc. Am. Bull., 84: 1883-1906. Mori, J. and Frankel, A., 1992. Correlation of P wave amplitudes and travel time residuals for teleseisms recorded on the Southern California Seismic Network. J. Geophys. Res., 97: 6661-6674. Muffler, L.J.P. and Doe, B.R., 1968. Composition and mean age of detritus of the Colorado River delta in the Salton Trough, southeastern California. J. Sediment. Petrol., 38: 384-399. Paige, C.C. and Saunders, M.A., 1982. LSQR: an algorithm for sparse linear equations and sparse least squares. Assoc. Comput. Mach. Trans. Math. Software, 8: 43-71. Press, F., 1956. Determination of crustal structure from phase velocity of Rayleigh waves, Part 1: Southern California. Bull. Geol. Soc. Am., 67: 1647-1658. Raikes, S.A., 1980. Regional variations in upper mantle structure beneath southern California. Geophys. J.R. Astron. Soc., 63: 187-216. Robinson, P.T., Elders, W.A. and Muffler, L.J.P., 1976. Quaternary volcanism in the Salton Sea geothermal field, Imperial Valley, California. Geol. Soc. Am. Bull., 87: 347-360. Roecker, S.W., Sabitova, T.M., Vinnik, L.P., Burmakov, Y.A., Golvanov, M.I., Mamatkanova, R. and Munirova, L., 1993. Three-dimensional elastic wave velocity structure of the western and central Tien Shah. J~ Geophys. Res., 98: 15779-15795. Roller, J.C. and Healy, J.H., 1963. Seismic refraction mea-
surement between Santa Monica Bay and Lake Mead. J. Geophys. Res., 68: 5837-5849. Shor, G.G. and Raitt, R.W., 1956. Seismic studies in the Southern California borderland. 20th International Geological Congress Mexico, Geofis. Aplicada (Secc. 9), Vol. 2, pp. 243-259. Solomon, S.C. and Butler, R.G., 1974. Prospecting for dead slabs. Earth Planet. Sci. Lett., 21: 421-430. Sung, L.Y. and Jackson, D.D., 1992. Crustal and uppermost mantle structure under southern California. Bull. Seismol. Soc. Am., 82: 934-961. Vetter, V. and Minster, J.B., 1981. Pn velocity anistropy in Southern California. Bull. Seismol. Soc. Am., 71: 15111530. Von Herzen, R.P., 1963. Geothermal heat' flow in the gulfs of California and Aden. Science, 140: 1207-1208. Walck, M.C., 1984. The P-wave upper mantle structure beneath an active spreading centre: the Gulf of California. Geophys. J.R. Astron. Soc., 76: 697-723. Walck, M.C. and Minster, J.B., 1982. Relative array analysis of upper mantle lateral velocity variations in southern California. J. Geophys. Res., 87: 1754-1772. Wilson, J.T., 1965. A new class of faults and their bearing on continental drift. Nature, 207: 343-347. Zhao, D. and Hasegawa, A., 1993. P wave tomographic imaging of the crust and upper mantle beneath the Japan Islands. J. Geophys. Res., 98: 4333-4353. Zhao, D. and Kanamori, H., 1992. P wave image of the crust and uppermost mantle in southern California. Geophys. Res. Lett., 19: 2329-2332. Zhao, D. and Kanamori, H., 1993. The 1992 Landers earthquake sequences: earthquake occurrence and structural heterogeneities. Geophys. Res. Lett., 20: 1083-1086. Zhao, D. and Kanamori, H., 1995. The 1994 Northridge earthquake: 3-D crustal structure in the rupture zone and its relation to the aftershock locations and mechanisms. Geophys. Res. Lett., 22: 763-766. Zhao, D., Hasegawa, A. and Horiuchi, S., 1992. Tomographic imaging of P and S wave velocity structure beneath northeastern Japan. J. Geophys. Res., 97: 19909-19928. Zhao, D., Hasegawa, A. and Kanamori, H., 1993. Slab structure beneath Japan as derived from local, regional and teleseismic events. Seismol. Res. Lett., 64(1): 16. Zhao, D., Hasegawa, A. and Kanamori, H., 1994. Deep structure of Japan subduction zone as derived from local, regional and teleseismic events. J. Geophys. Res., 99: 22313-22329.
Physics of the Earth and Planetary Interiors 93 (1996) 191-214
Simultaneous inversion of local and teleseismic data for the crust and mantle structure of southern California Dapeng Zhao a,,, Hiroo Kanamori a, Eugene Humphreys b a Seismological Laboratory, California Institute of Technology, Pasadena, CA 91125, USA b Department of Geological Sciences, Universityof Oregon, Eugene, OR 97403, USA Received 3 October 1994; revision accepted 25 May 1995
Abstract We determined a detailed three-dimensional seismic velocity structure of the crust and mantle to a depth of 800 km beneath southern California using local and teleseismic data simultaneously. We used 131372 P-wave arrival times from 6437 local earthquakes and 12 134 travel time residuals from 193 teleseismic events recorded by 251 seismic stations of the Caltech-US Geological Survey Southern California Seismic Network. Compared with previous local and teleseismic studies, the station coverage is considerably improved, and the number of data used is greatly increased in the present study. This, together with the local and teleseismic joint inversion approach, results in a unified three-dimensional (3-D) P-wave velocity model of the crust and mantle in southern California with a higher resolution than the previous models. The result obtained shows very heterogeneous structures in the crust and upper mantle in southern California. Shallow crustal structures correlate well with surface geological features. Sedimentary basins such as the Los Angeles Basin, Ventura Basin and Santa Maria Basin are imaged well as low velocities, and batholiths such as the Peninsular Ranges and San Gabriel Mountains are imaged as high velocities. The velocity in the crust is generally low in the Mojave Desert. The crustal velocities change abruptly across major faults such as the San Andreas, San Jacinto, Elsinore and Garlock faults. In the upper mantle, three major anomalies are found, beneath the Transverse Ranges, Salton Trough and Southern Sierra Nevada. The Transverse Ranges feature appears as a curtain-like, east-trending and north-dipping high-velocity anomaly with a thickness of approximately 50 kin, and extends most deeply at its eastern end to about 250 km. The Salton Trough feature is composed of low velocities which trend southeast and extend to approximately 200 km depth. The Southern Sierra Nevada feature is a slab-like high-velocity anomaly which extends to about 250 kin. The mantle velocity is generally low in the upper 100-200 km beneath the volcanic areas in southern California. The seismic velocity anomalies in the mantle beneath southern California are considered to result from small-scale mantle convection.
1. Introduction
" Corresponding author present address: Department of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130, USA.
S o u t h e r n C a l i f o r n i a consists o f several significantly d i f f e r e n t t e c t o n i c provinces, i n c l u d i n g t h e S a l t o n T r o u g h w h i c h is a m o d e r n crustal pulla p a r t a s s o c i a t e d with t h e o p e n i n g o f t h e G u l f o f
0031-9201/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSD! 0031-9201(95)03076-X
D. Zhao et al. / Physics of the Earth and Planetary Interiors 93 (1996) 191-214
192
California, and a major transform plate boundary separating the Pacific plate and the North American plate; its major surface expression is the San Andreas Fault (see Fig. 1). It is reasonable to expect that these surface features are accompanied by structural heterogeneities at depth in the crust and upper mantle. Many researchers have investigated the heterogeneous seismic velocity structure of the crust and upper mantle beneath the entire southern California region by using arrival time data from either local earthquakes (e.g. Ergas and Jackson, 1981; Hearn, 1984; Hearn and Clayton, 1986a,b; Sung and Jackson, 1992; Zhao and Kanamori, 1992; Magistrale et al., 1992), or teleseismic events (Hadley and Kanamori, 1977; Raikes, 1980; Walck and Minster, 1982; Humphreys et al., 1984; Humphreys and Clayton, 1990). The data used in these studies are recorded by the Southern California Seismic Network (SCSN) jointly run by the
California Institute of Technology (Caltech) and the US Geological Survey (USGS). The data set provided by the SCSN is very extensive in the number and distribution of both stations and sources, and hence the structure there can be determined very well (Humphreys and Clayton, 1990). These previous studies revealed significant structural complexities in the crust and upper mantle in southern California, which seem to correlate with the surface geological features. As mentioned above, so far local earthquake tomography and teleseismic tomography methods have been used separately to study the seismic velocity structure of southern California. The two approaches have inherent advantages and limitations. Local earthquake tomography uses arrival times from shallow earthquakes and can determine a detailed structure of the crust and uppermost mantle, but cannot determine the deeper structure. This is simply because no deep earth-
Cenozoic Bslin Deposits
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Fig. 1. Simplified geological map of southern California. The locations of major faults (continuous lines), volcanic areas and other geographic areas referred to in the text are shown. LAB, Los Angeles Basin; PRB, Peninsular Ranges Batholith; SBM, San Bcrnardino Mountains; SGM, San Gabriel Mountains; SMB, Santa Maria Basin; SMM, Santa Monica Mountains; SSNB, Southern Sierra Nevada Batholith; ST, Salton Trough; VB, Ventura Basin.
D. Zhao et al. ~Physics of the Earth and Planetary Interiors 93 (1996) 191-214
quake occurs in southern California, and seismic rays of direct and refracted waves from the local crustal earthquakes can only sample the areas from the surface to the uppermost mantle. In contrast, teleseismic tomography uses relative travel time residuals from distant events with epicentral distances longer than approximately 30°, and can determine the deep structure of the mantle. Teleseismic tomography, however, usually cannot determine well the crustal structure because teleseismic rays basically travel in a subvertical direction and do not crisscross well near the surface. Therefore it is usual in teleseismic tomography that a crustal model (either a threedimensional (3-D) velocity model or simply station corrections) obtained by a previous study is used to correct for the complex shallow structure, and only the mantle structure is determined through the teleseismic inversions (e.g. Humphreys and Clayton, 1990). If an inadequate crustal model is used, the accuracy of the mantle structure obtained, particularly that of the uppermost mantle, will be affected. Recently, a new approach has been proposed to determine tomographic images of an area by inverting data from local and teleseismic events simultaneously (Roecker et al., 1993; Zhao et al., 1993, 1994). This local and teleseismic joint inversion approach preserves the advantages of the two separate approaches, and overcomes their limitations (Zhao et al., 1994). Moreover, when data from both local and teleseismic events are used, the horizontally propagating local rays and vertically travelling teleseismic rays will crisscross well in the lower crust and uppermost mantle, which improves the resolution there. The successful application by Roecker et al. (1993) and Zhao et al. (1993, 1994) demonstrates the effectiveness of the joint inversion approach. In the present study, we have applied this new approach to a large number of arrival times from local and teleseismic events to determine a detailed 3-D P-wave velocity structure of the crust and mantle down to 800 km depth beneath southern California. We have taken advantage of the high density of seismic stations, and the high quality and great quantity of the data base of the Caltech-USGS Southern California Seismic Net-
193
work. Compared with the previous studies for this area, the station coverage is considerably improved, and the number of data used is greatly increased. This, together with the use of the local and teleseismic joint inversion approach and the modern tomographic method, results in a unified 3-D P-wave velocity model of the crust and mantle in southern California with a higher resolution than the previous models. The obtained result casts new light on the complex structure and tectonics of southern California.
2. Data and method
In this study, we combined two sets of seismic data to conduct tomographic inversions. One consists of arrival times from local crustal earthquakes which occurred in and around southern California, and the other of relative travel time residuals from teleseismic events.
2.1. Local earthquake data We have used arrival times recorded by the SCSN in the period from January 1981 to May 1992. Fig. 2 shows the present study area and geographic locations of 251 seismic stations we used. The stations are densely and uniformly distributed in the study area except for the Great Valley region. The station spacing ranges from 10 to 40 km. 121W
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D. Zhao et al. / Physics of the Earth and Planetary Interiors 93 (1996) 191-214
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More than 100 000 local earthquakes have been recorded by the seismic network during the period of 12 years. To select the best set of events for the inversion, we divide the crust of the study area into blocks with spatial sizes of 8 km x 8 km x 2 km. Among the earthquakes within each block, we selected the event which has the greatest number of first P-wave arrivals and the smallest uncertainty for the hypocentral location. As a result, we selected 6437 earthquakes; their epicentral locations are shown in Fig. 3. All the events have at least ten P arrivals, many of them have more than 100 P arrivals. The events with fewer arrivals occurred in the Great Valley, and the eastern and western edges of the study area, where there are fewer stations and seismicity is
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and
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D. Zhao et aL /Physics of the Earth and Planetary Interiors 93 (1996) 191-214
lower. The uncertainty for the hypocentral locations is less than 2 km for most of the events. We can see that the events selected are well distributed in the entire study area. Many events are concentrated on the major fault zones in southern California, such as the San Andreas, San Jacinto, Elsinore, and Garlock faults. T h e r e are, however, many earthquakes which occurred in places where no fault appears on the Earth's surface. T h e total n u m b e r of first P-wave arrivals from the 6437 local earthquakes is 131 372. The accuracy of arrival time is estimated for each picking by the SCSN analyst. Most of the local data we used have accuracy of approximately 0.1 s; some data with large epicentral distances may have an
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accuracy of approximately 0.2 s. Arrival times with larger picking errors are not used. 2.1 Teleseismic data We have used a total of 193 teleseismic events which are located at epicentral distances between 30 ° and 95 ° from southern California. O f the 193 events, 180 occurred during the period from April 1974 to October 1986. Some of these data have been used by Raikes (1980) and Humphreys and Clayton (1990) to study the upper-mantle structure of southern California. Arrival times of 13 events that occurred from October 1989 to August 1990 were collected by Mori and Frankel (1992) to investigate the correlation of P-wave
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196
D. Zhao et al. / Physics of the Earth and Planetary Interiors 93 (1996) 191-214
amplitudes and travel time residuals for teleseisms in southern California. Fig. 4 shows the geographic locations of the 193 teleseismic events we used in the present study. We can see that these events have a fairly complete azimuthal coverage except for a gap at 150-225 ° from north. Most of the events are earthquakes that occurred in the subduction zones of the North and West Pacific, Southeast Asia, and Middle and South America. Some earthquakes occurred in the oceanic ridges and the Eurasian continent. In total, 12134 P-wave arrival times were collected from the 193 teleseismic events. The number of arrivals for each event ranges from 18 to 153. The average number of arrivals is 63. There are 19 events having more than 100 arrivals. The accuracy of the arrival times is estimated to be approximately 0.1-0.2 s for the teleseismic data. We use relative travel time residuals of the teleseismic events to minimize source effects such as errors introduced by hypocentral mislocations and origin times, and the path effect outside the modeling space. (For the calculation of relative residuals, see Zhao et al. (1994).) Hypocenters were obtained from the Bulletin of the International Seismological Centre. Theoretical travel times are calculated by using the iasp91 Earth model (Kennett and Engdahl, 1991). For the crust and mantle right under the study area, the model as shown below in Fig. 8 is used (for details of the model, see Section 3). We display the teleseismic data in two ways. First, the relative residuals for each station are averaged over all the 193 events, resulting in a mean relative travel time residual. These are plotted in Fig. 5(e). Second, we separate the events into source quadrants according to the azimuth of the source from the network, also referred to as the source azimuth or source direction. We calculate the mean relative residuals for four quadrants: NE 0°, 90°; SE 90~, 180°; SW 180°, 270°; NW 270°, 360°. The mean residuals for each source quadrant are plotted in Figs. 5(a)5(d). The mean relative residuals, referred to simply as residuals, averaged for all the 193 events (Fig. 5(e)) produce a spatial pattern of early arrivals at
stations in the Transverse Ranges, eastern Mojave Desert, Peninsular Ranges, and some stations in Southern Sierra Nevada, and late arrivals at stations in Salton Trough, Santa Maria Basin, Ventura Basin, Los Angeles Basin, and along the San Andreas Fault zone in the northwestern part of the study area. This pattern is generally consistent with that found by the previous investigators (e.g. Raikes, 1980; Humphreys and Clayton, 1990). The pattern and magnitude of the residuals for the events from the four separate quadrants (Figs. 5(a)-5(d)) are, as a whole, all similar to those in Fig. 5(e). The most significant variation is the spatial shift of the Transverse Ranges feature of early arrivals with the change of the incident direction of the teleseismic rays. For the teleseismic events in the northeastern quadrant (Fig. 5(a)), all the early arrivals are confined to regions southwest of the San Andreas Fault. For the events in the southwestern quadrant (Fig. 5(c)), however, the early arrivals spread into most of the Mojave Desert. For events in the northwestern and southeastern quadrants (Figs. 5(d) and 5(b), respectively), the Transverse Ranges early feature shifts to south and north, respectively. This systematic spatial pattern provides convincing evidence that a high-velocity anomaly extends deep in the mantle beneath the Transverse Ranges. The magnitude and pattern for the early arrivals in Southern Sierra Nevada also changes for events in different quadrants. The Salton Trough feature of late arrivals which trends NWSE is relatively stable for events in all the quadrants. 2.3. Method
In this study we have used the method of Zhao et al. (1994) to combine teleseismic residuals with local earthquake arrival times in tomographic inversions. This method can deal with a general velocity model in which complex velocity discontinuities exist and the velocity changes in three dimensions (see also Zhao et al. (1992)). Local earthquakes are relocated in the inversion process. An efficient 3-D ray tracing technique (Zhao et al., 1992) is used to calculate travel times and ray paths accurately and rapidly. The LSQR algo-
197
D. Zhao et al. / Physics of the Earth and Planetary Interiors 93 (1996) 191-214 121W
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denser net of grid nodes in the crust and uppermost mantle. Figs. 6 and 7 show the configurations of the grid net in the horizontal and vertical planes, respectively. The separation between grid nodes is 0.25 ° (approximately 25 kin) in the horizontal plane. In the crust and uppermost mantle, four grid mesh layers are set up at 2, 10, 22, and 35 km. Between 35 and 695 km, 22 layers of grid mesh are set up with an interval of 30 km. From 695 to 800 km, three grid mesh layers are set up with an interval of 35 km. The total number of grid nodes is 17 x 27 × 29 = 13311, Velocity perturbations at the grid nodes and hypocentral parameters of local earthquakes are taken as unknown parameters. The total number of unknowns is (4 × 6437) + 13311 = 39059. It should be noted that the total number of local and teleseismic data is 143 506.
3. Analysis and result 3.1. Inversion Fig. 8 shows the one-dimensional (l-D) P-wave velocity model which is used as the starting model
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D. Zhao et al. / Physics of the Earth and Planetary Interiors 93 (1996) 191-214
198
short-period travel-time and waveform data recorded by the SCSN. The GCA model is found to be an appropriate average 1-D mantle velocity model for southern California (Walck, 1984; Humphreys and Clayton, 1990). The two firstorder discontinuities in the GCA model are at 390 and 660 km depth, where the velocity jump is 4.7% and 2.8%, respectively. Applying the technique of Zhao et al. (1994) to all the local and teleseismic data and adopting the starting velocity model (Fig. 8) and the grid net (Figs. 6 and 7) as described above, we performed a number of inversions by changing the depth of the modeling space from 200 to 800 km with an interval of 100 km. We found that the final results and final travel time residuals are essentially the same for the inversions with the
for the tomographic inversions in the present study. The crustal model is modified slightly from the Kanamori and Hadley (1975) model, which was constructed by using accurate arrival times from large quarry blasts in the central part of southern California. The standard velocity model for the routine earthquake location by the SCSN was derived from this structure. The model consists of four constant-velocity layers with discontinuities at 4, 27.4, and 32.4 km, respectively. The lower part of the second layer (from 18 to 27.4 km) was modified to have a velocity gradient by Zhao and Kanamori (1992). The modified crustal model is found to be more appropriate for the present study area (Zhao and Kanamori, 1992). The mantle velocity model in Fig. 8 is the GCA model determined by Walck (1984) using
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D. Zhao et al. ~Physics of the Earth and Planetary Interiors 93 (1996) 191-214
model depth from 600 to 800 km. For the inversions with shallower model depths, there are some variations in the results and final travel time residuals become larger. This means that velocity anomalies in the upper 500 km depth beneath southern California are significant for the data set used in the present study. Below 500 km, velocity anomalies are either less significant or they cannot be resolved by the present data set. In the following, we show the result of the inversion with the modeling depth of 800 km. We solved the inverse problem simultaneously for 6437 x 4 hypocentral parameters and 11212 velocity perturbations for the grid nodes with hit counts (number of rays passing through each node) larger than ten. More than 99% of the nodes have hit counts greater than 50. A variance reduction of 42% is achieved after the inversion.
199
Fig. 9 shows fractional P-wave velocity perturbations (in percent) on horizontal planes determined by the inversion. Only the images down to 400 km depth are shown because the velocity changes in the deeper areas are insignificant. Fig. 10 shows locations of six vertical cross-sections of the tomographic images as shown in Fig. 11. The cross-sections are shown to 600 km depth for clarity, because no substantial structural anomaly is found in the deeper areas. Fig. 12 shows a 3-D image of the P-wave velocity perturbations viewed from the northwest. 3.2. Resolution
Before describing the main features of the obtained result, we first show the resolution of the tomographic image. We first used the
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200
D. Zhao et aL / Physics of the Earth and Planetary Interiors 93 (1996) 191-214
structed for most of the study area for the layers in the crust and uppermost mantle. The resolution is poor around the Great Valley region for the layers in the crust because of lack of stations there. For the layers shallower than 300 km in the upper mantle, the resolution is good and the checkerboard pattern is correctly reconstructed mainly in the central part of the study area. For the layers in the depth range of 300-600 km, the checkerboard pattern is restored only in some limited areas. Below 600 km depth, the resolution is very poor. Fig. 14 shows the result of another checkerboard resolution test with a grid separation of 0.4 ° (approximately 40 km). Only eight representative layers are shown. We can see in Fig. 14 that the resolution is high and the checkerboard pattern is correctly reconstructed for most of the study area.
checkerboard resolution test (Zhao et al., 1992) to assess the adequacy of the ray coverage and to evaluate the resolution. To make a checkerboard, we assign positive and negative velocity perturbations of 6% to the 3-D grid nodes, the image of which is straightforward and easy to remember. Therefore, seeing the image of the inverted synthetic checkerboard pattern, one can easily understand where the resolution is good and where it is poor. It should be noted that we added random errors having a normal distribution with a standard deviation of 0.1 s to the synthetic data calculated for the synthetic models in all the resolution tests we conducted. Fig. 13 shows the result of one test with a grid separation of 0.25 ° (approximately 25 km) for the study area. The resolution is generally high and the checkerboard pattern is correctly recon-
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Recently, Leveque et al. (1993) showed that in some cases small structures as in the checkerboard test can be well retrieved, whereas larger
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structures are poorly retrieved. To find whether there is such a problem in our present tomographic images, we made a synthetic test as shown in Fig. 15. The synthetic model has a simple and large-scale structure of more than 200 kin, and the grid separation is 40 km. The inverted images show that the large-scale structure is generally well reconstructed for all the depth levels. We also made a number of other synthetic tests by using synthetic models with different structural geometries and grid separations; the inverted results always give reasonally well retrieved images of the synthetic model. From these resolution tests, we can say that both the small and the large structures can be retrieved well by our present technique and data set. The tomographic image obtained in the present study has a spatial resolution of about 25 km for the upper 250 km depths and about 40 km for the deeper areas.
202
D. Zhao et al. /Physics of the Earth and Planetary Interiors 93 (1996) 191-214
however, still shows high velocity. In addition, high velocities exist in the Salton Buttes and Great Valley region, and low velocities exist in the eastern Mojave Desert, Colorado Desert, Coso volcanic area and Coastal Ranges. The abrupt velocity change across the northern part of the San Andreas Fault is noteworthy. In the lower crust (22 km depth), the velocity is low in the Mojave Desert, Salton Trough and the volcanic areas, and is high north of the Garlock Fault, Coastal Ranges, and in a region including the southern Peninsular Ranges and the Elsinore and the San Jacinto fault zones. The velocity is also high beneath the Continental Borderland. The velocity changes abruptly across the southern part of the San Andreas Fault. In the uppermost mantle (35 km depth), high velocities exist in the eastern Mojave Desert, Salton Trough, and in Pacific coastal regions, whereas low velocities exist in the southern Sierra
3.3. 3-D crust and mantle structure
In the following, we describe the main features of the obtained tomographic images that have reliable resolution. The crustal structure determined in this study is similar to that found by Zhao and Kanamori (1992, 1993, 1995). At shallow depths (2 km), the P-wave image correlates well with the major surface geological features. The Ventura, Santa Maria and Los Angeles basins, and the Salton Trough are imaged as low velocities, and the batholiths of the Peninsular Ranges, San Gabriel Mountains and Southern Sierra Nevada are imaged as high velocities. All of the six volcanic areas in southern California are underlain by low-velocity anomalies. In the middle crust (10 km depth), the velocity anomalies for the sedimentary basins and batholiths seen at 2 km depth disappear or become less prominent. The Peninsular Ranges batholith,
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D. Zhao et at/Physics o/the Earth and Planetary Interiors 93 (1996) 191-214
Nevada, Peninsular Ranges, and around the Cima and Coso volcanic areas. Across the Garlock Fault, the velocity changes significantly. In the upper mantle, the maximum amplitude of velocity anomalies decreases significantly with depth. It is approximately 5-6% at depths shallower than 150 km, approximately 3-4% from 150 to 400 km, approximately 2-3% from 400 to 600 km, and less than 2% below 600 km. The pattern of velocity variations is approximately the same for all the layers in the upper mantle. The three most striking velocity anomalies in the upper mantle are beneath the Transverse Ranges, Salton Trough and western foothills of the Southern Sierra Nevada (named the 'Isabella anomaly' by Raikes (1980) and Jones et al. (1994)). The Transverse Ranges feature appears as a slab-like, east-trending and north-dipping highvelocity anomaly with a thickness of 50-60 km (see Fig. 9, layers from 65 to 245 km, and Fig. 11,
cross-sections AAg, BB', and DD'). Beneath the eastern part of the Transverse Ranges, this feature extends most deeply to about 250 km (see Fig. 11, cross-section AA¢); in the western part, it extends to about 150 km (see Fig. 11, cross-section DD'). This high-velocity feature seems continuous to the surface and is sandwiched between low-velocity anomalies on its upper and lower sides (see Fig. 11, cross-sections AA¢ and DD'). The high-velocity zone is thin (approximately 2030 km) at shallower depth (0-50 km), and become thicker (50-60 km) in the upper mantle. The Isabella feature is a slab-like high-velocity anomaly which extends to about 250 km, with its main part to 200 km depth (see Fig. 9, layers from 65 to 335 km, and Fig. 11, cross-sections CC' and DD'). This feature continues to the northern boundary of the present study area (36.5°N). The Salton Trough feature is composed of low velocities which trend southeast and ex-
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204
D. Zhao et al. / Physics of the Earth and Planetary Interiors 93 (1996) 191-214
reduced to 0.23 s for the 3-D velocity model (Figs. 9 and 11). The variance reduction is 45%. This means that the teleseismic data are well explained by the present 3-D velocity model.
tend to about 200 km (see Fig. 9, layers from 65 to 185 km, and Fig. 11, cross-sections EE' and FF'). Low velocities are visible in the Pacific coastal regions in the depth ranges of 65-155 km (see Fig. 9). Below the volcanic areas, the velocity is generally low, and the low-velocity anomalies extend to 100-200 km depth (see Fig. 9, layers from 65 to 185 km, and Fig. 11, cross-sections AP~, CC', EE' and FF'). It seems that these volcanic areas have a deep extension in the upper mantle. Fig. 16 shows synthetic teleseismic residuals per source quadrant calculated for the 3-D P-wave velocity structural model determined in this study (Figs. 9 and 11; Fig. 12). Comparing the synthetic residual distributions with those of the observed data (Fig. 5), we can see that they are consistent in both the distribution pattern and residual magnitude. The r.m.s, residual of the tel•seismic data is 0.31 s for the 1-D velocity model (Fig. 8); it is
S a n
4. Discussion Previous studies found a Pn velocity anisotropy in southern California by using Pn waves with epicentral distances as great as 600 km (Vetter and Minster, 1981; Hearn, 1984; Sung and Jackson, 1992). In this study, we did not take into account the Pn velocity anisotropy, which may have some effect on the obtained P-wave image. However, the effect is believed to be very small because the Pn rays we have used traverse the structure in many directions, tending to cancel out any anisotropic contribution. In addition, all the Pn arrivals we used have epicentral distances
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D. Zhao et al. / Physics of the Earth and Planetary Interiors 93 (1996) 191-214
shorter than 200 km. These Pn waves do not propagate within the uppermost mantle further than 60 km. Although early investigators reported large changes of crustal thickness in southern California (e.g. Press, 1956; Shor and Raitt, 1956; Roller and Healy, 1963), later detailed studies using high-quality data from local quarries and explosions at the Nevada Test Site have revealed that
the crustal thickness throughout much of southern California is relatively uniform and in the range of 30-35 km (Kanamori and Hadley, 1975; Hadley and Kanamori, 1977). The exceptions may be the thinning crust offshore and in the Salton Trough region. The crust is found to be approximately 22-26 km in the Imperial Valley (Fuis et al., 1984). We did not include the Moho depth variation in this study. The obtained P-wave im-
Fig. 12. Three-dimensionalimage of the P-wavetomographyof southern California viewed from the northwest. The depth range is 0-600 km. Red and blue colors show isosurfaces of 2% low- and 2% high-velocityanomalies, respectively.
D. Zhao et al. / Physics of the Earth and Planetary Interiors 93 (1996) 191-214
age for the lower crust and uppermost mantle may be influenced, to some degree, by the Moho depth variation for the areas with an anomalous crust. In this study we obtained a high-velocity anomaly in the uppermost mantle (35 km) beneath the Salton Trough, which is probably caused by the unrealistic Moho depth (32.4 km) in the 1-D starting model for that area. It is desirable to determine the lateral velocity variations in the crust and uppermost mantle and the lateral depth variations of the Moho discontinuity simultaneously. However, the data we used in this study are only the first P-wave arrival times, which are not effective in distinguishing the two structural features. Reflected a n d / o r converted waves at the Moho discontinuity, which are very effective in determining the structure around the discontinuity, are needed to constrain well the Moho geometry. A joint use of the first and later arrivals in future studies may allow more accurate determination of the structure around the Moho. As described in the preceding section, we have imaged more clearly the three major velocity anomalies in the upper mantle beneath southern California in Isabella, the Transverse Ranges and the Salton Trough. The Isabella anomaly, which is located below the western foothill of the Southern Sierra Nevada, was first noted by Raikes (1980) using teleseismic residuals. She suggested that this anomaly may be a southward continuation of a 'dead slab' with high seismic velocity detected by Solomon and Butler (1974) beneath the Northern Sierra Nevada and Cascade Ranges. Her suggestion was confirmed by Biasi and Humphreys (1992), who determined P-wave velocity structure of the upper mantle beneath central California and southern Nevada. Recently, Jones et al. (1994) investigated in detail the Pwave velocity structure of the crust and upper mantle in Southern Sierra Nevada by using teleseismic residuals and Pn arrival times. They found little dip on the Moho and estimated a crustal thickness of about 32 km beneath the Sierra Nevada. They also found a considerably low velocity anomaly in the uppermost mantle beneath the high eastern Sierra Nevada. These features
207
are in good agreement with the present result (see Fig. 9, 35 km depth layer). Jones et al. (1994) suggested that the Isabella anomaly is unrelated to the regional tectonics of the Sierra Nevada, but is simply the downgoing part of a small-scale local convection system, similar to that in the Transverse Ranges. They listed two observations to support their interpretation. First, they found that the Isabella anomaly they imaged exists in the depth range from about 100 to 200 km, and the anomaly does not exist at shallower depth. Thus they inferred that the anomaly may represent cooler, denser material from the upper mantle that is now descending from near the base of the crust to depths near 200 km. Second, they found in their teleseismic tomographic image a low-velocity body in the upper 100 km of the mantle under the Tehachapi Mountains, which is located 50-60 km south of Lake Isabella. They suggested that the Tehachapi low represents low-velocity material replacing the high-velocity material now descending to form the Isabella anomaly to the north. From the tomographic image we obtained in this study (Fig. 11, cross-section DD'), we can see that the Isabella anomaly exists in the depth range from 50 to about 250 km, which is much larger than that determined by Jones et al. (1994). The Tehachapi low is also visible from 50 to 150 km in the present image; however, it is much less significant than the Isabella anomaly. Moreover, the Tehachapi anomaly is closely adjoining the Isabella anomaly. It is difficult to imagine that the two anomalies with a length of 100-200 km in depth may form a convection system in such a narrow range of about 60 km in the horizontal direction. We prefer the interpretation by Raikes (1980) that the Isabella anomaly is a southward continuation of a 'dead slab' beneath the Northern Sierra Nevada and Cascade Ranges, and so it is related to the regional tectonics of the Sierra Nevada. Many previous studies (Wilson, 1965; Atwater, 1970; Moore, 1973) have suggested that the Gulf of California is the locus of a spreading ridge along which the Pacific plate is being rifted away from the North American plate at a rate of as
D. Zhao et al / Physics of the Earth and Planetary Interiors 93 (1996) 191-214
208
and scattered Quaternary volcanism (Lomnitz et al., 1970; Elders et al., 1972; Robinson et al., 1976). Magnetic anomaly patterns so characteristic of most oceanic spreading ridges are well expressed only near the mouth of the Gulf of California. The Imperial Valley of southern California, the northern landward extension of the gulf, is a broad structural depression also termed
much as 6 cm year-1 (Larson, 1972). This spreading ridge appears to be broken into a number of short segments by a series of en echelon transform faults that show seismic activity compatible with the plate tectonics model. The Gulf of California is also characterized by locally high heat flow (Von Herzen, 1963), a crust intermediate between oceanic and continental (Moore, 1973),
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D. Zhao et aL ~Physicso f the Earth and Planetary Interiors 93 (1996) 191-214
the Salton Trough. Although tectonically similar to the rest of the Gulf of California, the trough is filled with as much as 6 km of sediments derived primarily from the Colorado River (Muffler and Doc, 1968). Elders et al. (1972) showed that spreading extends landward under the Salton Trough and that some of the areas of high heat flow in the trough may be located over active spreading centers as part of a leaky transform fault system. Gravity data (Biehler, 1964) indicate that the crust beneath the trough is isostatically compensated even though the trough is underlain by this great thickness of relatively low-density sediments. Biehler (1971) interpreted this relationship as indicating that the continental crust beneath the Salton Trough is undergoing thin-
209
ning and basification, probably by intrusion of basaltic rocks at depth. The low seismic velocity anomalies under the Salton Trough as imaged by this study are consistent with these previous geophysical observations, and can be understood in the concept of a northward continuation of the spreading center from the Gulf of California into the Imperial Valley. The existence of the Transverse Ranges anomaly was first noted by Hadley and Kanamori (1977), and was subsequently confirmed by Raikes (1980), Humphreys et al. (1984), and Humphreys and Clayton (1990). Our present study provides a d e a r e r image of this anomaly. It is the most striking feature in the upper mantle beneath southern California. These significant seismic ve-
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D. Zhao et aL /Physics o f the Earth and Planetary Interiors 93 (1990) 191-214
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D. Zhao et al. /Physics of the Earth and Planetary Interiors 93 (1996) 191-214
sidered to be created through the convergence and sinking of the entire thickness of subcrustal lithosphere. This anomaly is estimated to be about 375°C colder and 1% denser than average southern California mantle at the same depth by using the values of temperature derivatives of elastic
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D. Zhao et al. / Physics of the Earth and Planetary Interiors 93 (1996) 191-214
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wave velocities for mantle materials estimated by Karato (1993). Subcrustal lithosphere from both sides of the convergent zone is considered to participate nearly equally in the construction of the anomaly, and the low-velocity upper-mantle anomaly beneath the Salton Trough region is attributed to high temperatures and 1-4% partial melt related to adiabatic decompression during mantle upwelling. Therefore these upper-mantle anomalies can be interpreted as results of smallscale convection beneath southern California, to which much of the recent tectonic activity in the region can be attributed.
5. Conclusion
We have used a great number of data from local earthquakes and teleseismic events simulta-
neously to determine a unified and detailed 3-D P-velocity model of the crust and mantle down to 800 km depth beneath southern California. The spatial resolution is approximately 25 km for the crust and uppermost mantle, and 25-40 km for the deeper areas. The main results are summarized as follows. (1) Significant heterogeneities are imaged in the crust and upper mantle in southern California. Shallow crustal structures correlate well with surface geological features. Sedimentary basins such as the Los Angeles, Ventura and Santa Maria basins are imaged well as low velocities, and batholiths such as the Peninsular Ranges and San Gabriel Mountains are imaged as high velocities. The velocity in the crust is generally low in the Mojave Desert. (2) The crustal velocities change abruptly across the major faults in southern California,
D. Zhao et al. / Physics of the Earth and Planetary Interiors 93 (1996) 191-214
such as the San Andreas, San Jacinto and Garlock fault zones, suggesting the deep extent of these fault zones in the crust. (3) Three major velocity anomalies are found in the upper mantle in southern California. The Transverse Ranges feature appears as a curtainlike, east-trending and north-dipping high-velocity anomaly with a thickness of about 50 km, and extends most deeply at its eastern end to approximately 250 km. The Salton Trough feature is composed of low velocities which trend southeast and extend to about 150 kin. The Isabella feature is a slab-like high-velocity anomaly which extends to 250-300 km. (4) We prefer the interpretation of upper-mantle anomalies as results of small-scale mantle convection: the Transverse Ranges anomaly represents downwelling material, whereas the Salton Trough anomaly is an upwelling flow. The Isabella anomaly is considered to be a southward continuation of a 'dead slab' as revealed beneath the Northern Sierra Nevada and Cascade Range. (5) The mantle velocity is generally low in the upper 100-200 km depth beneath the volcanic areas in southern California, suggesting that these volcanic areas may have a deep extension in the upper mantle.
Acknowledgments This work was funded by grants from the National Science Foundation (EAR-92-04748), the US Geological Survey (USGS 1434-93-G-2287), and the Southern California Earthquake Center (SCEC). We thank J. Mori for providing the P-wave data from 13 teleseismic events. G. Biasi and K. Dueker kindly prepared some of the teleseismic data to be used for this study. We appreciate helpful discussions with D.L. Anderson. Two anonymous referees critically read the manuscript and provided helpful comments. W. Su helped us to produce the color image in Fig. 12. This paper is Contribution 5454, Division of Geological and Planetary Sciences, California Institute of Technology, and SCEC Publication 124.
213
References Atwater, T., 1970. Implications of plate tectonics for the Cenozoic tectonic evolution of western North America. Geol. Soc. Am. Bull., 81: 3513-3536. Biasi, G.P. and Humphreys, E., 1992. P-Wave image of the upper mantle structure of central California and southern Nevada. Geophys. Res. Lett., 19: 1161-1164. Biehler, S., 1964. Geophysical study of the Salton Trough of southern California. Ph.D. Thesis, California Institute of Technology, Pasadena, 139 pp. Biehler, S., 1971. Gravity models of the crustal structure of the Salton Trough. Geol. Soc. Am. Abstr. Prog., 3(7): 506. Bird, P. and Rosenstock, R.W., 1984. Kinematics of present crust and mantle flow in southern California. Geol. Soc. Am. Bull., 95: 946-957. Elders, W.A., Rex, R.W., Meidav, T., Robinson, P.T. and Biehler, S., 1972. Crustal spreading in southern California. Science, 178: 15-24. Ergas, R.A. and Jackson, D.D., 1981. Spatial variations of seismic velocities in southern California. Bull. Seismol. Soc. Am., 71: 671-689. Fuis, G.S., Mooney, W.D., Healy, J.H., McMechan, J.A. and Lutter, W.J., 1984. A seismic refraction study of the Imperial Valley region, California. J. Geophys. Res., 89: 11651189. Hadley, D.M. and Kanamori, H., 1977. Seismic structure of the Transverse Ranges, California. Geol. Soc. Am. Bull., 88: 1461-1478. Hearn, T.M., 1984. Pn travel times in southern California. J. Geophys. Res., 89: 1843-1855. Hearn, T.M. and Clayton, R.W., 1986a. Lateral velocity variations in southern California I. Results for the upper crust from Pg waves. Bull. Seismol. Soc. Am., 76: 495-509. Hearn, T.M. and Clayton, R.W., 1986b. Lateral velocity variations in southern California II. Results for the lower ernst from Pn waves. Bull. Seismol. Soc. Am., 76: 511-520. Humphreys, E. and Clayton, R.W., 1990. Tomographic image of southern California mantle. J. Geophys. Res., 95: 19725-19746. Humphreys, E. and Hager, B.H., 1990. A kinematic model for the late Cenozoic development of southern California crust and upper mantle. J. Geophys. Res., 95: 19747-19762. Humphreys, E., Clayton, R.W. and Hager, B.H., 1984. A tomographic image of mantle structure beneath southern California. Geophys. Res. Lett., 11: 625-627. Jones, C.H., Kanamori, H. and Roecker, S.W., 1994. Missing roots and mantle 'drips': regional Pn and teleseismic arrival times in the southern Sierra Nevada and vicinity, California, J. Geophys. Res., 99: 4567-4601. Kanamori, H. and Hadley, D., 1975. Crustal structure and temporal velocity change in Southern California. Pure Appi. Geophys., 113: 257-280. Karato, S., 1993. Importance of anelasticity in the interpretation of seismic tomography. Geophys. Res. Lett., 20: 1623-1626.
214
D. Zhao et al. /Physics of the Earth and Planetary Interiors 93 (1996) 191-214
Kennett, B.L.N. and Engdahl, E.R., 1991. Traveltimes for global earthquake location and phase identification. Geophys. J. Int., 105: 429-465. Larson, R., 1972. Bathymetry, magnetic anomalies, and plate tectonics history of the mouth of the Gulf of California. Geol. Soc. Am. Bull., 83: 3345-3360. Leveque, J., Rivera, L. and Wittlinger, G., 1993. On the use of the checker-board test to assess the resolution of tomographic inversions. Geophys. J. Int., 115: 313-318. Lomnitz, C., Mooser, F., Allen, C.R., Brune, J.N. and Thatcher, W., 1970. Seismicity and tectonics of the northern Gulf of California region, Mexico - - preliminary resuits. Inst. Geofis. Int. An., 10(2): 37-48. Magistrale, H., Kanamori, H. and Jones, C., 1992. Forward and inverse three-dimensional P wave velocity models of the southern California crust. J. Geophys. Res., 97: 1411514135. Moore, D.G., 1973. Plate edge deformation and crustal growth of Gulf of California structural province. Geol. Soc. Am. Bull., 84: 1883-1906. Mori, J. and Frankel, A., 1992. Correlation of P wave amplitudes and travel time residuals for teleseisms recorded on the Southern California Seismic Network. J. Geophys. Res., 97: 6661-6674. Muffler, L.J.P. and Doe, B.R., 1968. Composition and mean age of detritus of the Colorado River delta in the Salton Trough, southeastern California. J. Sediment. Petrol., 38: 384-399. Paige, C.C. and Saunders, M.A., 1982. LSQR: an algorithm for sparse linear equations and sparse least squares. Assoc. Comput. Mach. Trans. Math. Software, 8: 43-71. Press, F., 1956. Determination of crustal structure from phase velocity of Rayleigh waves, Part 1: Southern California. Bull. Geol. Soc. Am., 67: 1647-1658. Raikes, S.A., 1980. Regional variations in upper mantle structure beneath southern California. Geophys. J.R. Astron. Soc., 63: 187-216. Robinson, P.T., Elders, W.A. and Muffler, L.J.P., 1976. Quaternary volcanism in the Salton Sea geothermal field, Imperial Valley, California. Geol. Soc. Am. Bull., 87: 347-360. Roecker, S.W., Sabitova, T.M., Vinnik, L.P., Burmakov, Y.A., Golvanov, M.I., Mamatkanova, R. and Munirova, L., 1993. Three-dimensional elastic wave velocity structure of the western and central Tien Shah. J~ Geophys. Res., 98: 15779-15795. Roller, J.C. and Healy, J.H., 1963. Seismic refraction mea-
surement between Santa Monica Bay and Lake Mead. J. Geophys. Res., 68: 5837-5849. Shor, G.G. and Raitt, R.W., 1956. Seismic studies in the Southern California borderland. 20th International Geological Congress Mexico, Geofis. Aplicada (Secc. 9), Vol. 2, pp. 243-259. Solomon, S.C. and Butler, R.G., 1974. Prospecting for dead slabs. Earth Planet. Sci. Lett., 21: 421-430. Sung, L.Y. and Jackson, D.D., 1992. Crustal and uppermost mantle structure under southern California. Bull. Seismol. Soc. Am., 82: 934-961. Vetter, V. and Minster, J.B., 1981. Pn velocity anistropy in Southern California. Bull. Seismol. Soc. Am., 71: 15111530. Von Herzen, R.P., 1963. Geothermal heat' flow in the gulfs of California and Aden. Science, 140: 1207-1208. Walck, M.C., 1984. The P-wave upper mantle structure beneath an active spreading centre: the Gulf of California. Geophys. J.R. Astron. Soc., 76: 697-723. Walck, M.C. and Minster, J.B., 1982. Relative array analysis of upper mantle lateral velocity variations in southern California. J. Geophys. Res., 87: 1754-1772. Wilson, J.T., 1965. A new class of faults and their bearing on continental drift. Nature, 207: 343-347. Zhao, D. and Hasegawa, A., 1993. P wave tomographic imaging of the crust and upper mantle beneath the Japan Islands. J. Geophys. Res., 98: 4333-4353. Zhao, D. and Kanamori, H., 1992. P wave image of the crust and uppermost mantle in southern California. Geophys. Res. Lett., 19: 2329-2332. Zhao, D. and Kanamori, H., 1993. The 1992 Landers earthquake sequences: earthquake occurrence and structural heterogeneities. Geophys. Res. Lett., 20: 1083-1086. Zhao, D. and Kanamori, H., 1995. The 1994 Northridge earthquake: 3-D crustal structure in the rupture zone and its relation to the aftershock locations and mechanisms. Geophys. Res. Lett., 22: 763-766. Zhao, D., Hasegawa, A. and Horiuchi, S., 1992. Tomographic imaging of P and S wave velocity structure beneath northeastern Japan. J. Geophys. Res., 97: 19909-19928. Zhao, D., Hasegawa, A. and Kanamori, H., 1993. Slab structure beneath Japan as derived from local, regional and teleseismic events. Seismol. Res. Lett., 64(1): 16. Zhao, D., Hasegawa, A. and Kanamori, H., 1994. Deep structure of Japan subduction zone as derived from local, regional and teleseismic events. J. Geophys. Res., 99: 22313-22329.