Travel times from earthquakes near southern Kuril and their implications for the fate of subducted lithosphere

PttYSICS O F T H E EARTH ANDPLANETARY INTERIORS

ELSEVIER

Physics of the Earth and Planetary Interiors 88 (1995) 177-191

Travel times from earthquakes near southern Kuril and their implications for the fate of subducted lithosphere Mary Ann Glennon a,1, Wang-Ping Chen

a,b,,

a Department of Geology, University of Illinois, 1301 West Green Street, 245 Natural History Building, Urbana, IL 61801, USA b Department of Theoretical andApplied Mechanics, University of Illinois, 104 South Wright Street, 212 Talbot Laboratory, Urbana, IL 61801, USA

Received 15 March 1994; revision accepted 23 August 1994

Abstract

Analysis of travel time residuals from deep- and intermediate-focus earthquakes along the Kuril-Kamchatka arc has been a major impetus in advancing the hypothesis of deep slab penetration. The interpretation of travel times, however, has been controversial. The quality of arrivals reported in bulletins has also been debated. Using both digital and analog seismograms, we precisely measured approximately 400 arrival times of teleseismic P phases for seven earthquakes, most of which occurred since 1987 near southern Kuril. Travel times from many of these events have not been analyzed by other researchers, including a unique pair of shallow- and deep-focus earthquakes beneath the Island of Sakhalin. In general, differences between our precisely measured P arrivals and those reported by the International Seismological Center (ISC) are within + 0.4 s, although isolated, large differences do exist. At common stations, large differences in travel time residuals (up to 1.5 s) are observed between one of the deepest earthquakes in northern Kuril and a deep-focus event beneath the Island of Sakhalin, located some 150 km farther west of the well-defined Wadati-Benioff zone in southern Kuril. Significantly faster travel times (up to 1.2 s or more) are also observed along north-northeast and south-southwest azimuths for the deep-focus event in northern Kuril than for a shallow-focus earthquake west of the Sakhalin Island where the effect of subducted lithosphere should be minimal. In both cases, the observed differential residuals between earthquakes in northern and southern portions of the arc are too large to be caused by epicenter mislocation. Since differential travel time residuals largely reflect differences in near source structure, seismic wave speeds below depths of approximately 650 km seem to vary along this arc. The magnitude of slab-like anomalies in the lower mantle beneath Sakhalin is probably two to three times smaller than that in northern Kuril-Kamchatka. If the pattern of travel time anomalies observed for deep-focus earthquakes in northern Kuril-Kamchatka is caused by coherent subducted slab in the lower mantle, wholesale slab penetration into the lower mantle probably did not occur beneath the Sakhalin. There also appears to be a correlation between the state of strain within subducted lithosphere in the upper mantle and the extent of slab penetration into the lower mantle. Along southern Kuril, a net down-dip extensional strain is observed within the slab at intermediate-depths and persists to an unusual depth of approximately 450 km. On the other hand, in the northern Kuril-Kamchatka, down-dip compressional strain dominates at all depths in the subducted lithosphere.

* Corresponding author. 1 Present address: 2628 York Court, Woodridge, IL 60517, USA. 0031-9201/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0031-9201 (94)02991 - 1

178

M.A. Glennon, W.-P. Chen / Physics of the Earth and Planetao' Interiors 88 (1995) 177-191

itive anomalies (slow wave speed) are found in azimuths approximately normal to the arc has been postulated as evidence for wholesale penetration of subducted slab into the lower mantle (e.g. Jordan, 1977; Creager and Jordan, 1984; Okano and Suetsugu, 1992). The exact cause of travel time anomalies, however, has been a matter of ongoing debate. Many researchers contend that the travel time anomalies are caused by heterogeneities located elsewhere in the mantle, away from the source regions (e.g. Lay, 1983; Gaherty et al., 1991; Schwartz et al., 1991a,b). Images from travel time tomography seem to suggest that subducted slab penetrates into the lower mantle only in certain segments of the arc.

1. I n t r o d u c t i o n

The fate of subducted lithosphere is one of the fundamental issues in geophysics and geochemistry. In particular, the depth of penetration of subducted lithosphere constrains the extent of heat and mass transfer between the upper and the lower mantle (e.g. Silver et al., 1988; Anderson, 1989; Lay, 1994). Travel times from deep-focus earthquakes have been extensively used to investigate the extent of slab penetration. For instance, the observation that negative travel time anomalies (fast wave speed) are observed in azimuths subparallel to the trend of the Kuril-Kamchatka arc, while p o s -

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M.A. Glennon, W.-P. Chen / Physics of the Earth and Planetary Interiors 88 (1995) 177-191

For example, beneath the Sakhalin Island near s o u t h e r n Kuril, t o m o g r a p h i c i m a g e s suggest t h a t s u b d u c t e d l i t h o s p h e r e is t r a p p e d within t h e upp e r m a n t l e ( V a n d e r Hilts et al., 1991, 1993; F u k a o et al., 1992). P r o n o u n c e d r e g i o n s of fast wave s p e e d , i n t e r p r e t e d as s u b d u c t e d m a t e r i a l in t h e lower m a n t l e , a r e o b s e r v e d only in t h e n o r t h -

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e r n m o s t p o r t i o n s of t h e arc (e.g. V a n d e r Hilst et al., 1991, 1993). P a r t of t h e c o n t r o v e r s y r e g a r d i n g t h e fate o f s u b d u c t e d slab in K u r i l - K a m c h a t k a m a y also lie in the quality of o b s e r v a t i o n s . A large f r a c t i o n o f p r e v i o u s w o r k o n travel t i m e r e s i d u a l s a n d tom o g r a p h y has r e l i e d u p o n arrival times p u b l i s h e d

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180

M.A. Glennon, W.-P. Chen / Physics of the Earth and Planetary Interiors 88 (19951 177-191

by the ISC. Only T a k e i a n d S u e t s u g u (1989) calibrated ISC reports against their m e a s u r e d arrival times for events in the J a p a n slab. Recently, G r a n d (1990) r e p o r t e d that late arrivals r e p o r t e d by the ISC often seem to be associated with stations of low magnification. If this is the case, the quality of ISC data may have biased the i n t e r p r e t a t i o n of travel times. In o r d e r to provided a d d i t i o n a l c o n s t r a i n t s o n the fate of s u b d u c t e d lithosphere, we have carried out an analysis of precisely m e a s u r e d P travel times residuals from recent e a r t h q u a k e s in K u r i l - K a m c h a t k a . Several d e v e l o p m e n t s in the past few years motivated o u r study. Since a r o u n d 1987, digitally r e c o r d e d seismograms with wide dynamic r a n g e a n d b r o a d b a n d - w i d t h b e c a m e a b u n d a n t . Travel times from m u c h of this highr e s o l u t i o n data set r e m a i n unexplored. O n 12 May 1990, a large ( M 0 = 10 2o N m) e a r t h q u a k e occurred at a d e p t h of approximately 615 km b e n e a t h the Island of Sakhalin. This seemingly isolated e a r t h q u a k e is located over 150 km farther west of the well-defined W a d a t i - B e n i o f f zone in s o u t h e r n Kuril (Fig. 1) (e.g. E k s t r 6 m et al., 1990; V a n der Hilst et al., 1991; Kuge, 1994), indicating that the geometry of the W a d a t i - B e n i -

Off zone varies considerably along the arc ( G l e n n o n a n d Chen, 1993). A s e q u e n c e of large, shallow e a r t h q u a k e s in s o u t h e r n Sakhalin (e.g. C h a p m a n a n d Solomon, 1976) is also of interest, because the influence of s u b d u c t e d slab on travel times to teleseismic stations from these events should be minimal. T h e u n i q u e o c c u r r e n c e of both deep- and shallowfocus e a r t h q u a k e s b e n e a t h Sakhalin provides an o p p o r t u n i t y to test the variability in the geometry of s u b d u c t e d slab along the arc t h r o u g h residual sphere analysis. In this study, we analyzed approximately 400 precise arrival time readings. Most of the observations are from e a r t h q u a k e s n e a r s o u t h e r n Kuril that have not b e e n investigated by other researchers. For most of the events, we analyzed both digital and analog seismograms and calib r a t e d arrival times r e p o r t e d by the ISC against our precise m e a s u r e m e n t s . To facilitate the interp r e t a t i o n of observed travel time anomalies, we shall c o n c e n t r a t e on differential residuals: the difference of travel time residuals b e t w e e n pairs of events recorded at the same station. This proc e d u r e t e n d s to isolate h e t e r o g e n e i t i e s n e a r the e a r t h q u a k e sources, as ray paths b e t w e e n nearby

Table 1 Hypocenter information of earthquakes No.

Date

Origin time ~t UT

Lat. '' Long. ~ At h taxi b Az. h mh ~ Number Depth d (ON) (°E) (s) (kin) (deg) of obs. (km)

Remarks d

1746:10 1103:50 0305:51 0307:35 2346:05 /1450:ll

52.40 42.66 46.77 49.14 49.58 49.01

GCa This study GCb GCb GCa GCa

Deep and intermediate Focus

1 2 3 4 5 6

30 August, 1970 14 January, 1987 7 May, 1987 18 May, 1987 14 July, 1987 12 May, 1990

151.63 142.91 139.25 147.77 147.82 141.91

1 1 3 1 1 2

4.9 17 4.4 13 7.3 5.8

352 22 31 150 2(14 140

6.5 6.3 5.9 5.9 5.6 6.5

63 44 56 62 44 63

1337:12 46.72 141.33

2

6.9

230

6.1

61

636 _+ 19 99 435 + 15 544 + 16 583 _+ 15 615 + 15

Shallow focus

7

6 September, 1971

21

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~ Values of parameters are calculated with respect to the iasp91 travel time model (Kennett and Engdahl, 1991). b Difference between parameters reported by the ISC with respect to those determined in this study. At is the difference in origin time, I Ax I that in distance, and Az. the relative azimuth. ~ As reported by the ISC. d Depths taken from GCa (Glennon and Chen, 19931; GCb (Glennon and Chen 19951. Depth of event 6 was determined by inversion of broadband P wavetrains using the method described in Glennon and Chen (19931.

M.A. Glennon, W.-P. Chen / Physics of the Earth and Planetary Interiors 88 (1995) 177-191

earthquakes are similar in regions far away from the sources.

2. Data and analysis We collected digital seismograms for selected large- to moderate-sized earthquakes that occurred along the Kuril-Kamchatka subduction zone between 1987 and 1992. We have included deep- and intermediate-focus (>i = 100 km) earthquakes as well as shallow earthquakes unrelated to subduction. In addition, we collected analog data for events that occurred prior to 1987 in regions where more recent, large earthquakes were absent. In total, arrival times of direct P phases were read for sixteen events. In this paper, we report the results for seven events for which the azimuthal coverage of observations is favorable (Figs. 1 and 2). Short-period, broadband and mid-period digital seismograms were collected from the Data Management Center of the Incorporated Research Institutions for Seismology (IRIS DMC), the Harvard Seismic Archive Facility, the GEOSCOPE project of France (Romanowicz et al., 1991), and the National Earthquake Information Center (NEIC). We supplemented digital data with observations from analog short-period seismograms of the World-Wide Standardized Seismograph Network (WWSSN) and the Canadian Seismograph Network (CSN). For the Sakhalin Island earthquake of 12 May 1990 (event 6 in Table 1), we also obtained analog data from a regional network located in Bergen, Norway (BER). P first arrivals were read from vertical component seismograms recorded between 30 ° and 90 °. In this distance range, complications due to triplications in the upper mantle and phases interacting with the core are minimized. Arrival times were read from unprocessed seismograms for most pass-bands. Prior to picking arrival times for mid-period seismograms, we deconvolved the instrument response to obtain ground displacement because the response of these seismographs often has large phase distortions. Each observation was assigned an uncertainty, inversely proportional to

181

a visually estimated signal-to-noise ratio. For digital observations, the minimum uncertainty of arrival times were judged to be +0.1 s (2 SD). Examples of clear seismograms can be found in Glennon and Chen (1995). Analog arrival times were read with a digitizer. To maximize the precision of observations, we produced paper copies of seismograms at the largest possible scale. Since 1978, the data have been stored on microfiche which gives paper copies at a scale of approximately 14 cm per minute. Larger scales of approximately 24 cm per minute are produced for earlier data archived on 70 mm film. Considering the difference in scales, we have assumed a minimum error of +0.1 s and +0.2 s in arrival times for events that occurred before and after 1978, respectively. This is an estimate of reading error, taking into account uncertainties in picking the onset of the arrival, measuring the length of a minute, and determining the exact beginning of minute marks. Larger uncertainties were assigned for observations with low signal-to-noise ratios and for records where the minute mark is not a sharp box-car signal. For analog data, reported corrections for clock drift greater than about 0.05 s were added to measured arrival times. All arrival times, along with all applied corrections, are reported by Glennon (1994). For some P arrivals, both digital and analog observations are available at the same station. We found that the measured arrival times on both recording media generally agree to within +0.1 s. In a few cases, large differences of approximately 0.5 s do exist. In such cases, we used arrival times read from digital seismograms in our analysis. We also rejected observations with estimated uncertainty larger than 0.5 s or those with travel time residuals greater than 5 s from further analysis. Since some deep-focus earthquakes along the Kuril-Kamchatka have gradual rise times (Glennon and Chen, 1995), we paid close attention to azimuthal variations in waveforms. For events 2 and 4 (Table 1), weak onsets are observed at some azimuths. Nevertheless, for both events, first arrivals that we have picked result in consistent residuals among adjacent stations (Fig. 2). To calculate travel time residuals, we used our

182

M.A. Glennon, W.-P. Chen / Physics of the Earth and Planetary Interiors 88 (1995) 177-191

measured direct P arrival times and relocated each event listed in Table 1 using the iasp91 travel time curves (Kennett and Engdahl, 1991) and the ellipticity corrections of Dziewonski and Gilbert (1976). Except for the shallow earthquake (event 7), focal depths were independently constrained by the inversion of body waveforms (Table 1) and only epicenters and origin times were

Toy

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M.A. Glennon, W.-P. Chen / Physics of the Earth and Planetary Interiors 88 (I 995) 177-191

station coordinates between 1964 and August 1987 are obtained from ISC reports available on CD-ROM (Anonymous, 1990). For event 6, which occurred in 1990, we used station locations reported as of August 1987 for many analog instruments. However, we obtained station coordinates directly from station administrators at 15 stations and none of the locations changed between 1987 and 1990. Since there are differences between the iasp91 and other travel time curves (Kennett and Engdahl, 1991), we have tested the effect of reference travel times by relocating the events and calculating the travel times using the Jeffreys-Bullen (1940) tables, the reference used by the ISC. Except for a well-known baseline shift in travel times (Kennett and Engdahl, 1991), the overall patterns of travel time residuals did not change• For all events listed in Table 1, P arrival times have wide azimuthal coverage to ensure a stable epicenter location (Fig. 2). During relocation, we have consistently increased the weight of sparse observations in the south-southwest. The difference between epicenters determined with or without weights is generally less than 5 km. The difference between our relocated epicenters and those reported by the ISC are generally no more than 7 km (Table 1), similar to that reported by Creager and Jordan (1984) for other deep-focus events in the Kuril-Kamchatka. The difference, however, is larger for events 2 and 4 (>/12 km). We have no ready explanation for this result, because we are unable to check many readings reported by the ISC. We have investigated the effect of station corrections. Comprehensive station corrections are only available for stations that existed prior to 1985 (Toy, 1989). Consequently, only events 1 and 7 can be tested• Fig. 3 compares station corrections reported by Toy (1989) and residuals without station corrections for event 1, a deep-focus earthquake. The station corrections (Fig. 3(A)) and uncorrected residuals (Fig. 3(B)) show a similar azimuthal pattern, although the magnitude of uncorrected residuals are generally greater. Removal of Toy's station corrections does not alter the pattern of residuals much (Fig. 3(C)). Indeed, residuals and station corrections correlate with a

183

correlation coefficient (r) of approximately 0.6 (Fig. 3(D)). For stations in North America, the correlation is very pronounced with r of approximately 0.8. Not surprisingly, the application of station corrections changes the epicenter of this deep-focus earthquake by only about 6 km, confirming resuits from previous studies (e.g. Creager and Jordan, 1984; Nieman et al., 1986). For the shallowfocus event 7 (Table 1), the result is similar. The effect of event mislocation on travel time residuals will be specifically discussed when results from individual events are presented• For three deep-focus events (1, 4 and 6), we also read direct S arrival times from horizontal component seismograms in the distance range of 300-75 °. Digital seismograms were rotated to obtain the tangential component before picking arrival times. For analog observations, arrivals were read from both horizontal components, when possible. Readings from both components are generally in agreement, with a few large irreconcilable differences of more than 1 s. In all cases, we retained impulsive readings from the component closest to tangential. Uncertainty of S arrival times is approximately + 0.5-1 s. S residuals greater than 10 s were excluded from our analysis. Many digital and analog seismic stations no longer record short-period horizontal ground motion. This practice has severely limited the amount

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184

M.A. Glennon, W.-IE Chen / Physics of the Earth and Planetary In teriors 88 (1995) 177-191

of S H travel times. Consequently, we can only compare residuals between P and S phases which show a high linear correlation (r = 0.8, Fig. 4). After correcting for ellipticity, the ratio between S and P residuals is approximately 2.5. This value is slightly lower than values of 3 - 4 reported for regions away from subduction zones (e.g. Doyle and Hales, 1967; Romanowicz and Cara, 1980; Toy, 1989). The significance of this deviation, if any, cannot be addressed with the current data set and is beyond the scope of this paper.

3. Results

3.1. Quality o f I S C data

For both residual sphere analysis and travel time tomography, many researchers have relied upon arrival times reported by the ISC (e.g. Creager and Jordan, 1984, 1986; Fischer et al., 1988, 1991; Zhou and Clayton, 1990; Boyd and Creager, 1991; Van der Hilst et al., 1991, 1993; Creager and Boyd, 1992; Fukao et al., 1992). Since the reliability of arrival times reported by the ISC may depend on the magnification of seismographs (Grand, 1990), we compared our readings of P arrival times with those published by the ISC and archived on CD-ROM for six events that occurred prior to August 1987. As an example, Fig. 5 shows the result for the deep-focus event (No. 1) in northern Kuril studied by Creager and Jordan (1984) using P arrivals

A

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083070,

reported by the ISC. Our observations from the WWSSN and CSN include 18% of arrival times reported by the ISC. In this subset, we find that differences in arrival times are generally less than 0.4 s, although some large ( = 1-3 s), isolated differences are observed (Fig. 5(C)). Although this difference can be larger than the uncertainty of our arrival time readings at individual stations, the overall pattern of residuals based on our readings and those reported by the ISC are similar, with slight differences in the northeastern quadrant (Figs. 5(A) and (B)). Generally speaking, we found no major differences in the overall pattern of P residuals between our data and readings reported by the ISC for the other five events (2-5 and 7). It appears that P travel time residual analysis using ISC reports is acceptable, if smoothing is sufficient to dampen isolated noise (e.g. Takei and Suetsugu, 1989). 3.2. Southern l~ersus northern Kuril

Residuals for event 6 beneath the Sakhalin Island do not show a clear pattern of negative residuals that has often been cited as evidence of fast wave speeds below the source region (Fig. 2). In contrast, such a feature, trending north-northeasterly, is clearly visible for event 1 in the northern Kuril (Fig. 2) (Creager and Jordan, 1984). Fig. 6 shows a direct comparison of the difference between residuals for the two events at 25 seismic stations that recorded both events. Assuming Gaussian statistics, the random error of the differential residual is the square root of the sum of

ISC) Event 1

c

ISC-083070 ~vent 1

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M.A. Glennon, W.-P. Chen / Physics of the Earth and Planetary Interiors 88 (1995) 177-191

of event 6 by approximately 10 km and at increments of 45 ° in azimuth. This procedure is equivalent to superimposing a degree-one pattern on the observed residual for event 6 (Davies and McKenzie, 1969). After removing the mean, residuals calculated based on each of the perturbed epicenters are compared with those of event 1. At a focal depth of 615 km, + 10 km of mislocation in the relative position of epicenters corresponds to a perturbation of approximately + 0.5 s in relative travel times. As a result, large differential residuals observed between events 1 and 6 cannot be eliminated by changing the relative position of epicenters (Fig. 6(B)). This observation suggests that the main cause of differential residuals between deep-focus earthquakes near the Sakhalin and in northern Kuril lies near the source regions, below a depth of approximately 650 km.

the squares of the uncertainty of each individual arrival time observation. Based on the random error in measured arrival times, the random error of differential residuals is approximately + 0.3 s for all comparisons in this study. The difference between events 1 and 6 is pronounced, with faster travel times (negative differential residuals) as large as 1.5 s or more for event 1 in the southwestern quadrant (Fig. 6(A)). In principle, differential residuals isolate the cause of travel time anomalies to regions near earthquake sources because ray paths of nearby earthquakes to teleseismic stations are most distinct directly beneath the sources. Systematic error owing to the choice of reference travel times is also minimized in differential residuals. However, an important source of systematic bias that cannot be eliminated by using differential residuals is the effect of mislocation in the relative position of epicenters. For instance, Creager and Boyd (1992) and Van der Hilst and Engdahl (1992) estimated that mislocations of deep-focus epicenters are between 5 and 20 km along the K u r i l - K a m c h a t k a , if subducted slab penetrates steeply into the lower mantle. To test the effect of epicentral location on differential residuals, we displaced the epicenter

A

185

3.3. Deep versus shallow earthquakes

Event 7 is a shallow-focus earthquake that occurred near the southern tip of the Island of Sakhalin (Fig. 1). This event is distant from known subduction zones and is probably situated along the plate boundary between North America and

B 083070-051290 Events 1-6

083070- Displaced Epicenter 051290 Events 1-6

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Fig. 6. (A) Differential residuals between deep-focus events in northern Kuril-Kamchatka (event 1) and beneath the Island of Sakhalin (event 6). Differential residuals are plotted using positions calculated for event 1. (B) Differential residuals between events 1 and 6, with the epicenter of event 6 displaced by 10 km in an azimuth of approximately 90°, minimizing the differential residuals. The mean of differential residuals has been removed. Layout is the same as that of Fig. 2.

186

M.A. Glennon, 144-P Chen /Physics of the Earth and Planetary Interiors 88 (1995) 177-191

E u r a s i a (e.g. C h a p m a n a n d S o l o m o n , 1976; D e M e t s , 1992; C h e n a n d Kao, 1995). A t teleseismic distances, t h e effect o f s u b d u c t i n g lithos p h e r e on travel times f r o m this shallow event should be m i n i m a l a n d thus p r o v i d i n g an o p p o r tunity to c a l i b r a t e the p a t t e r n o f travel time a n o m a l i e s from d e e p - f o c u s events in the K u r i l Kamchatka. M o s t d i f f e r e n t i a l r e s i d u a l s b e t w e e n event 7 051290

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MM. Glennon, W.-P. Chen / Physics of the Earth and Planetary Interiors 88 (1995) 177-191

location (46.62°N, 141.51°E, depth 10 kin, origin time 13:37:10) in which some arrival times from local stations seem to have been used. The reported epicenter lies outside their local array and is about 18 km southwest of that listed in Table 1. After removing the mean, residuals at teleseismic distances calculated based on their location show a strong two-lobed pattern whose magnitude reaches + 2 s, suggestive of a location error. We estimated the effect of relative mislocation following the procedure outlined in the last section and found that differential residuals between events 7 and 6 are significantly reduced, if the epicenter of event 6 is displaced by 10 km due west from the position reported in Table 1 (Figs. 8(C) and (D)). In this case, most differential residuals are less than or equal to the minimum uncertainty of observations ( = _+0.3 s). Regardless of the true relative location of the two epicenters, we found that the largest differential

187

residuals occur in the northeastern quadrant and are positive (e.g. Figs. 8(A) and (C)), indicating faster seismic wave speed underneath the shallow event 7 than the deep event 6. On the other hand, a number of negative differential residuals greater than 1.2 s are found between events 1 and 7 (Fig. 7(A)), consistent with the notion that mantle structure beneath deep-focus earthquakes in northern and southern Kuril are distinct (Fig. 6). In fact, the pattern of differential residuals between events 1 and 7 is consistent with a north-northeast-south-southwest trending slab-like structure of high wave speed under event 1 (Creager and Jordan, 1984). Notice that if we ignore possible mislocation bias and attribute negative differential residuals between event 6 and 7 as evidence for high seismic wave speed structure trending north-northwest-south-southeast under event 6, the magnitude of most differential residuals are approxi-

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Fig. 8. (A) Differential residual between the shallow event 7 and deep-focus event 6 beneath the west coast of the Sakhalin Island. After removal of the mean, differential residuals are plotted at azimuths calculated for the deep-focus event 6. For most differential residuals, the estimated random error is approximately -t-0.3 s (see error bars). (B) Correlation between residuals for events 6 and 7. (C) Differential residuals between events 6 and 7, with the epicenter of event 6 displaced by 10 km in an azimuth of about 270 ° that minimizes the differential residuals. The layout is the same as that of (A). (D) Correlation between residuals for events 6 and 7 using epicentral parameters as those in (C).

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M.A. Glennon, W.-P. Chen / Physics of the Earth and Planetary Interiors 88 (1995) 177-191

mately a factor of 2-3 smaller than those observed between events 1 and 7 (Fig. 7(A)). Thus if large-scale structure with fast seismic wave speed exists beneath event 6, the magnitude of such an anomaly must be significantly smaller than that in the northern Kuril (Ding and Grand, 1992). Perhaps subducted slab in southern Kuril indeed piles up subhorizontally near the 660 km discontinuity, as suggested by results of travel time tomography (Van der Hilts et al., 1991, 1993; Fukao et al., 1992).

3.4. Other comparisons Differential residuals between event 4 in central Kuril and events 1 and 6 (Figs. 1 and 2) illustrate the limitations of this technique. This method is most effective, if a large number of stations common to each pair of events are available. For example, even though we have already established that residuals are significantly different between events 1 and 6 (Fig. 6), the small number of available differential residuals cannot distinguish whether the differences between events 4 and 6 are more significant than that between events 4 and 1 (Fig. 7(B)). The magnitude of available differential residuals between events 4 and the shallow event 7 is not large either (Fig. 7(A)). We cannot establish whether subducted lithosphere penetrates deep into the lower mantle in central Kuril-Kamchatka near event 4, even though such a scenario has been suggested by previous analysis of residual spheres (Creager and Jordan, 1984; Okano and Suetsugu, 1992). In map view, event 5 occurred within 50 km of event 4 (Table 1 and Fig. 1). Data coverage for event 5 in North America (northeastern quadrant) is sparse (Fig. 2). Consequently, we cannot demonstrate by differential residuals that the pattern of travel time residuals is identical for both events. Both events 4 and 5 occurred in a region where contortion of the Wadati-Benioff zone is apparent. Such a geometry further complicates the interpretation of these travel time anomalies (Glennon and Chen, 1993). The geometry of the Wadati-Benioff zone be-

tween events 3 and 6 is not clear because back ground seismicity is low in this area (Glennon and Chen, 1993). On face value, the observed residuals for event 3 are smaller than that observed for event 6 (Fig. 2). However, any difference in residuals between events 3 and 6 is too small to be resolved, if possible mislocation of + 10 km is taken into consideration (Fig. 7(C)). The difference in take-off angles owing to different focal depths not withstanding, one may argue that the overall pattern of residuals for event 2 resembles that of event 1 (Fig. 2). Event 2 occurred at a depth of approximately 100 km beneath Hokkaido where the dip of the subducted slab is shallow (e.g. Jarrard, 1986). Several researchers suggested that epicentral mislocation for such an event can be as much as 40 km (e.g. Nieman et al., 1986; Boyd and Creager, 1991; Creager and Boyd, 1992), corresponding to + 3 s of P travel time residual (Creager and Boyd, 1992). In this case, the effect of mislocation will overwhelm any pattern of travel time residuals reflecting a subducted slab of high seismic speed. In short, the evolving configuration of global seismic networks limits the number of common stations for each event pair. Consequently, given possible bias in mislocation, we cannot isolate the cause of travel time anomalies for events 2-5 by differential residuals.

3.5. Discusswn There appears to be a correlation between the state of strain within subducted lithosphere in the upper mantle, as determined from earthquake source parameters, and the extent of slab penetration into the lower mantle, as inferred from travel time analysis. Along southern Kuril where subducted lithosphere may be trapped within the transition zone (Shearer, 1991; Van der Hilts et al., 1991, 1993; Shearer and Masters, 1992; Fukao et al., 1992), a net down-dip extensional strain is observed within the slab at intermediate-depths and persists to an unusual depth of approximately 450 km (Glennon and Chen, 1993). On the other hand, in the northern Kuril-Kamchatka where travel time studies indicate slab-like features deep in the lower mantle (Jordan, 1977; Creager and

M.A. Glennon, W.-P. Chen / Physics of the Earth and Planetary Interiors 88 (1995) 177-191

Jordan, 1984; Van der Hilts et al., 1991, 1993; Fukao et al., 1992), down-dip compressional strain dominates at all depths in the subducted lithosphere (Kao and Chen, 1994). The observation that the Wadati-Benioff zone deforms at depths greater than 500 km in northern Kuril-Kamchatka (Glennon and Chen, 1993) suggests the resistance to slab penetration near the 670 km discontinuity is strong enough to deform the subducting slab and to cause down-dip compression in the entire Wadati-Benioff zone, yet not enough to prohibit at least partial recycling of subducted Pacific lithosphere into the lower mantle. 4. Conclusion The Kuril-Kamchatka arc has been proposed as a primary example where subducted lithosphere penetrates steeply into the lower mantle (e.g. Jordan; 1977; Creager and Jordan, 1984). The occurrence of several unusual earthquakes west of the Sakhalin Island near southern Kuril allowed us to investigate possible lateral variation of wave speeds in the lower mantle along this arc (Fig. 1). Using both digital and analog seismograms, we read approximately 400 precise P first arrival times (Fig. 2). We have compared our readings with those reported by the ISC. Generally speaking, the difference is about +0.4 s or less (Fig. 5). Confirming the report of Creager and Jordan (1984) who used ISC arrivals, the deep-focus event of 30 August 1970 in northern Kuril (event 1, Table 1) showed large negative P travel time residuals (fast wave speeds) in azimuths along north-northeast-south-southwest, subparallel to the local strike of the Wadati-Benioff zone (Figs. 1 and 3). However, these negative residuals are absent for both shallow- and deep-focus earthquakes beneath the Sakhalin Island. Differences in travel time residuals between earthquakes in northern Kuril and near Sakhalin are also evident in differential residuals. Given that observed differential residuals are too large to be absorbed by mislocation bias and that differential residuals tend to isolate effects of near source structures, our observation suggests that the lower mantle

189

wave speed structure is significantly different between northern Kuril and the Sakhalin region. On the other hand, it is not possible to distinguish the variations, if any, between central and northern Kuril, or that between central Kuril and Sakhalin from differential residuals. Our results are consistent with the notion that subducted slab penetrates deep into the lower mantle in northern Kuril (e.g. Jordan, 1977; Creager and Jordan, 1984). Such a configuration, however, probably does not occur beneath the Sakhalin Island where subducted lithosphere may lie subhorizontally near the boundary between the upper and lower mantle (Shearer, 1991; Van der Hilts et al., 1991, 1993; Shearer and Masters, 1992; Fukao et al., 1992).

Acknowledgments We benefited from discussion with K. Creager, X. Ding, K. Eckhardt, S. Grand and T. Lay. Chen's interest in travel times was largely reactivated by S. Grand's encouragement. We also thank two anonymous reviewers for helpful comments on the manuscript. We are grateful for a large number of analog seismograms sent to us from seismograph stations around the world. Other analog seismograms were collected at the Lamont-Doherty Earth Observatory of Columbia University. We thank T. Ahern (IRIS DMC), C.R. Hutt and R. Woodward (USGS) for answering many questions on digital data. The following colleagues either granted us permission to access their data, or helped us along the way: T. Ahern, M. Palmer, and R. Titus (DMC of IRIS); J.-P. Montanger, B. Romanowicz, G. Roult, F. Basset, H. Lyon-Caen, E. Okal, and A. Pyrolley (GEOSCOPE); G. Ekstr6m and R. Woodward (Harvard Seismic Archive Facility); R. Buland, R. Engdahl, and M. Zirbes (NEIC); D. Deloatch (LamontDoherty). This research was supported by NSF grants EAR90-18321 and EAR93-16012.

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