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Faulting parameters of earthquakes in the New Madrid, Missouri, region
ENG[NEEPdNG GEOLOGY ELSEVIER
Engineering Geology 46 (1997) 299-311
Faulting parameters of earthquakes in the New Madrid, Missouri, region Robert B. Herrmann *, Charles J. Amrnon 1 Department of Earth and Atmospheric Sciences, Saint Louis University, 3507 Laclede Avenue, St. Louis, MO 63103, USA Received 6 July 1996; accepted 10 November 1996
Abstract
The advent of high-resolution digital seismic recording and advances in computer technology enable the combination of traditional regional seismic network observations with direct seismogram modeling to improve estimates of small earthquake faulting geometry, depth, and size. We illustrate a combined modeling approach using observations from three earthquakes that occurred within the environs of the New Madrid Seismic Zone: two Missouri earthquakes from Septmnber 26, 1990 and May 4, 1991; and the southern Illinois earthquake of February 5, 1994. We also re-examine the faulting geometry for two events from the 1960s that are inconsistent with the current estimate of the regional stress field. Based on direct modeling of the long-period seismograms associated with these events, we revise earlier estimates of the earthquake parameters for the March 3, 1963 and July 21, 1967 Missouri earthquakes. Comparing the new and revised results with existing earthquake mechanisms in the region, we find that tension-axes are generally aligned in a N-S to NW-SE direction, while the compression-axes trend in a NE to E direction. An interesting exception to this pattern are the March 3, 1963 and two nearby earthquakes that lie within a well-defined 30-km long left step in seismicity near New Madrid. Keyworda: Earthquake; Surface wave; Focal mechanism; Stress orientation; New Madrid; Seismic movement; Waveform modeling
1. Imrodn~ion Modeling b r o a d b a n d seismograms recorded at distances up to a few hundred kilometers from an earthquake can constrain the earthquake faulting geometry, depth, size, and slip duration (e.g., Dreger and Helmberger, 1991, 1993). However, * Corresponding author. Tel: + 1 314 977 3120; fax: + 1 314 977 3117; e-mail:rbht~.as.slu.edu 1Tel: 314 977 3197; Fax: 314 977 3117; e-mail: [email protected]. 0013-7952/97/$17.00 © 1997 ElsevierScienceB.V. All rights reserved. PI1 S0013-7952(97)00008-2
since ground motions depend on the earthquake parameters and earth structure, to extract accurate source information (the strike and dip of the fault, and the rake, or direction of slip on the fault), we must account for the effects of earth structure on the seismograms. In general, the influence of structure on the shape of a seismogram increases with increasing wave frequency and epicentral distance. For large earthquakes, we can minimize these on earthquake modeling by relying on low-frequency seismic waves. Studying smaller earthquakes is more challenging because small events do not
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R.B. Herrmann, C.J. Ammon / Engineering Geology 46 (1997) 299-311
excite large low-frequency waves. Thus, we must utilize higher-frequency observations and reduce the influence of earth structure by other means. For example, Zhao and Helmberger (1994) proposed separately analyzing segments of the waveform to reduce the effects of the unknown earth structure on the estimates of the earthquake parameters. An alternative is to improve our ability to model high-frequency observations by constructing a more detailed regional-structure model using independent observations (e.g., Herrmann, 1995). A third approach is to exploit seismograms recorded close to the earthquake, which are less distorted by the effects of propagation. In the following, we illustrate the utility of waveform modeling to constrain the depth, size, and fault parameters of small-to-moderate-size central United States earthquakes. In each analysis, we exploit a sparse set of observations recorded close to the epicenter. We proceed by example, beginning with a simultaneous analysis of waveforms and regional P-wave polarities. Next, we present an example relying solely on waveforms, but check the consistency of these results with observed P-wave polarities. Then we focus our attention on several earthquakes from the 1960s that were studied prior to the general accessibility
of the computational power needed to model seismograms. The locations of the seismic stations used and the earthquakes studied are shown in Fig. 1.
2. Faulting parameters of three recent central US earthquakes
2.1. The September 26, 1990 earthquake The September 26, 1990 earthquake occurred on the periphery of the regional seismic networks operated by Saint Louis and Memphis State (now University of Memphis) Universities, and the hypocenter depth estimated using arrival times is poorly constrained. In addition, the faulting geometry estimated using first-motion polarities
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Fig. 2. Comparison of observed and predicted (upper and lower traces, respectively, for each component) seismograms for the September 26, 1990 earthquake recorded at station CCM. Each waveform was filtered with a 15-100 WWSSN LP instrument response with a unit peak gain. A total of 51.1 s of time history is displayed, starting 20 s after the event origin time. The lower hemisphere equal area projections to the fight show the observed P-wave first motion polarities from the main event (upper), and the main and aftershock first motion data (lower). The mechanisms correspond to the predicted waveforms. A circle or plus sign indicates a compression, and a triangle or minus sign indicates a dilatation. Positive Z, R and T values represent motion up, away from the source, and in a direction clockwise about the source.
R.B. Herrmann, CJ. Ammon / Engineering Geology46 (1997) 299-311
is ambiguous due to limited azimuthal coverage (Fig. 2). In Fig. 2, we plot the seismograms generated by this earthquake and recorded at seismic station CCM (Cathedral Caves, Missouri). CCM is located 175kin from the epicenter along a source-station azimuth of 305 °. We also show the predicted seismograms computed by solving the equations of motion in a cylindrical symmetric medium (plane-layered earth model) using the wavenumber integration technique (Wang and Hemuann, 1980). Each waveform includes a 15-100 World Wide Standard Seismograph Network (WWSSN) long-period (LP) instrument response with unit peak magnification. The WWSSN LP response is a low-pass filter which eases the comparison of recent observations with those of older events. We exploit the full bandwidth of the seismograms later when we examine the high-frequency signals. The vertical (Z), radial (R) (horizontal component of motion in the direction connecting the epicenter and seismometer) and transverse (T) (horizontal direction perpendicular to the radial) components of ground motion are displayed and the peak trace amplitude in cm is shown below each signal. We modeled these observations by direct comparison with predicted seismograms computed by systematically changing the values of strike, dip, rake, and source depth. Since we are searching over a small number of parameters, each with a limited range, the grid search can be implemented interactively using a flexible misfit measure, such as simultaneously matching the main features in the seismograms and the least-ambiguous P-wave polarities. During the search, the waveform fit is judged on absolute amplitude of the signals and relative amplitudes o f the P, S, Love, and Rayleigh waves. One acceptable match is shown in Fig. 2 (the predicted seismograms were computed using the fault parameters listed in Table 1 and the CUS earth model is listed in Table 2). The fit to the observed seismograms is very good at the lower frequencies (below 0.5 Hz) and the first-motion match is also good, with only four inconsistencies that could be related to the dependence of the takeoff angle upon the earth model. The composite of polarities plotted in Fig. 2 are mainshock and
301
aftershocks polarities obtained from the regional networks and a portable aftershock deployment (Taylor and Wuenscher, 1990). The 15-kin depth inferred from the waveforms is consistent with 10-16-km aftershock depth range constrained by readings from portable seismographs deployed directly above the aftershocks (Taylor and Wuenseher, 1990). In Fig. 2, one glaring inconsistency between the observed and predicted seismograms is the different arrival times of the high frequency S-wave on the vertical component. Simple earth models inadequately describe the complexity of the Earth at small scale lengths and direct seismogram modeling to estimate faulting parameters is most accurate for waveforms recorded a short distance (measured in wavelengths) from the event. In all cases we must assess the sensitivity of earthquake parameters on the assumed earth models. One way to assess structure sensitivity is to examine the faulting parameters estimated using another earth model. Herrmann (1995) modeled these CCM waveforms to refine the CUS earth model. The result of his analysis is the HAMBURG earth model, which is listed in Table2. Using the HAMBURG model the optimal estimates for hypocentral depth is 15 kin, fault strike is 150°, dip is 70 °, rake is 60 °, and the seismic moment is 2.5x 1022dyne-cm. These strike, dip, and rake values differ from those estimated using the CUS model by 10° and the moment differs by about 15% directly indicating the uncertainty in our estimates of these parameters. In Fig. 3, we compare predicted and observed ground velocities using the HAMBURG structure for a sequence of low-pass filters with comer frequencies of 0.10, 0.20, 0.50 and 1.0 Hz. Note that the S-wave is relatively well modeled in terms of arrival time and shape at higher frequency, even though the peak amplitudes and complexity are under-predicted at 1 Hz. For an accurate prediction of these high-frequency ground motions, substantial effort was required to calibrate the structure between the source and receiver. In this case, the feature of the earth model that improves the high-frequency seismogram fit is the gradational crust-mantle transition. The main points of this exercise are that we can
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K B. Herrmann, C.J. Ammon / Engineermg Geology 46 (1997) 299-311
Table 1
Event information Date
Previous results 020262 030363 081465 102165 072167 110968 111770 040374 061375 032576 061087
New additions in this work 092690 050491 020594 Revisions in this work 030363 072167
Origin time (UT)
Lat. (°N)
Long. (°W)
Depth (kin)
Strike (0)
Dip (°)
0643 1730 1313 0204 0914 1701 0213 2305 2240 0041 2348
36.37 36.64 37.22 37.48 37.44 37.91 35.86 38.55 36.54 35.59 38.71
89.51 90.05 89.31 90.94 90.44 88.37 89.95 88.07 89.68 90.48 87.95
7.5 15 1.5 5 15 22 16 15 9 12 10
350 124 280 260 350 0 220 310 85 220 135
84 78 70 40 60 46 75 70 60 65 70
1318 0118 1455
37.16 36.56 37.36
89.58 89.83 89.19
15 8 16
140 90 30
75 67.5 70
1730 0914
36.64 37.44
90.05 90.44
15 15
304 350
78 60
Rake (°)
Moment (dyne-on)
Ref.
145 152 -20 -70 -45 79 150 0 -20 150 15
2.5E+22 1.1E+23 2.9E+21 9.0E+22 1.3E+22 9.7E + 23 1.6E+22 3.7E+22 4.6E+21 9.8E+22 3.1E+23
(1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (3)
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1.1E+23 1.3E+22
(4) (4)
(1) Origin time and location from Gordon (1988); (2) depth, seismicmoment and mechanism from Herrmann (1979); (3) Taylor et al. (1989); (4) This paper.
match the higher frequency signals with modest changes to the earth structure, and that the difference in mechanisms achieved after this greater effort is insignificant for first-order tectonic investigations. This illustration justifies the use of the simple CUS model (or slight modifications of it) to estimate the faulting parameters of earthquakes in the region.
2.2. The May 4, 1991 earthquake The second earthquake we examine occurred on May 4, 1991, near New Madrid, Missouri, and near the center of the regional seismic network and the temporary, dense Memphis State PANDA deployment. Due to its location well within the networks, the earthquake origin time and hypocenter depth (8 km) are well constrained. The available first-motion polarities (Fig. 4) suggest a strike-slip solution, but permit considerable variation in the nodal plane orientations. We compare the 15-100 WWSSN LP filtered observed and predicted seismograms for station CCM in Fig. 4.
CCM is located 209 km from the epicenter, along an azimuth of 323 °. Waveform modeling with the CUS model match the first-order features in the waveforms, but the predicted surface-waves arrived earlier than observed. To match the observations better, a modified CUS earth model, denoted by M A L D E N ( H e n m a n n , 1995) in Table 2, was used in the grid search and to compute the predicted seismograms shown in Fig. 4. The best-fitting fault parameters (listed in Table 1) produce an excellent fit to the main features of the observed waveforms and match the observed P-wave polarities. The waveforms also favor an 8-km depth, consistent with the regional-network location.
2.3. The February 5, 1994 earthquake The final example is the small southern Illinois earthquake on February 5, 1994, an event that is significantly smaller than those discussed above (Table 1). The waveform inversion technique described by Ammon et al. (1996) was applied to
R.B. Herrmann, CJ. Ammon / Engineering Geology 46 (1997) 299-311 Table 2 Earth models
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Qfl- 1
2.5 2.7 2.9 3.0 3.4
0.0050 0.0005 0.0005 0.0005 0.0005
0.010 0.001 0.001 0.001 0.001
2.83 3.44 3.59 3.72 3.74 4.60 4.65
2.5 2.7 2.8 2.8 2.8 3.3 3.3
0.0050 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005
0.010 0.001 0.001 0.001 0.001 0.001 0.001
2.85 3.32 3.60 3.54 3.60 3.70 3.67 3.80 3.77 3.89 3.83 4.23 4.25 4.48 4.52 4.54 4.65 4.94
2.48 2.65 2.77 2.74 2.77 2.82 2.80 2.87 2.85 2.91 2.88 3.08 3.09 3.23 3.25 3.26 3.33 3.51
0.0050 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005
0.010 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001
Thickness
P, velocity
S-velocity
Density
(Van)
(kin/s)
(kin/s)
(g/cm3)
5.00 6.10 6.40 6.70 8.15
2.89 3.52 3.70 3.87 4.70
4.89 5.98 6.21 6.44 6.47 7.97 8.05 HAMBURG 1.00 4.93 2.00 5.76 2.00 6.25 3.00 6.13 3.00 6.23 4.00 6.41 4.00 6.36 5.50 6.58 5.50 6.53 5.50 6.74 5.50 6.63 1.25 7.33 1.25 7.36 2.50 7.76 2.50 7.84 5.00 7.87 2.50 8.06 oo 8.56
CUS 1.0 9.0 10.0 20.0 MALDEN 1.0 9.0 10.0 10.0 10.0 20.0
the data of two broadband stations: CCM and MIAR (Mountain Ida, Arkansas, 34.546N, 93.573W). The approach is a least-squares inversion of the observed seismograms for a deviatoric seismic moment tensor. Regional network P-wave polarities are not included in the inversion. Prior to inversion the observed seismograms were filtered to include only frequencies in the band between 0.02 and 0.1 Hz. In this case, the primary waves modeled were Love and Rayleigh waves, which are the largest waves in this frequency range recorded a few hundred km from this size earthquake. The comparison of the observed and predicted seismograms is shown in Fig. 5. In this
303
instance, the waves sensitive to hypocentral depth (the Rayleigh and body waves) are below the background noise level, so the depth was fixed near that estimated using regional P and S arrival times (16km). The observed amplitudes are matched very well, although some phase inconsistencies are apparent. The Rayleigh waves are near nodal for both stations but Love waves have substantial amplitude and are nicely matched. Our confidence in these results is increased by the consistency with the independent P-wave polarities (Fig. 5). To summarize, direct seismogram modeling provides valuable constraints on the earthquake parameters of regionally-recorded earthquakes. The value of waveform modeling increases when the P-wave first motions are sparse or have a limited azimuthal distribution around the source. In these cases, a match to the main features in the observed seismograms can improve constraints on the depth, strike, dip, and rake of the earthquake and provide valuable constraints on the seismic moment and depth of the earthquake.
3. A re-evaluation of two central US focal
mechanisms Our best estimates of the central US compressional stress orientation is an ENE direction (Zoback and Zoback, 1991; Zoback, 1992). Much of the stress information available in this region is based on earthquake faulting parameters determined by matching surface-wave radiation patterns (Heumann, 1974; Heiimann, 1979; Taylor et al., 1989) and several exceptions to this stress pattern exist. Zoback (1992) notes that of the fault parameters in Henmann (1979), two in the New Madrid region are associated with normal faulting (October 21, 1965; July 21, 1967) and one has a compression axis trending NS rather than EW (March 3, 1963). As mentioned above, the faulting parameters for these three events were estimated by He~¥r,~aun ( 1974, 1979) using surface-wave radiation patterns. A systematic search over the strike, dip, rake, and hypoeenter depth was performed to find the solutions providing the best fit to the observed radia-
304
R.B. Herrmann, CJ. Ammon / Engineering Geology 46 (1997) 299-311
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tion pattern for surface waves with periods between 5 and 50 s. However, since only fundamental-mode surface-wave spectral amplitudes were used, there exist inherent ambiguities of 180° in strike and rake due to the symmetry of theoretical radiation patterns. Thus, four solutions are possible for each event (two have the same focal mechanism and hence associated stress directions). To select one, HeJ-fmann (1974, 1979) checked the consistency of the surface-wave solutions with observed
P-wave polarities. Unfortunately, for many of the earthquakes, the quality of the P-wave observations was marginal. After processing a large number of earthquakes, more faith was placed in the surface-wave constraints on nodal plane orientation and only impulsive P-wave first motions were used. Since the study of He~mann (1979), computational techniques for modeling seismic waveforms have improved significantly. It is timely to revisit
R.R Herrmann, C.J. Ammon / Engineering Geology 46 (1997) 299-311
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earlier earthquake solutions, and if possible, resolve the apparent incompatibility with the present model of the mid-continent stress field. Of the three events that are inconsistent with the regional stress pattern, the faulting parameters of the October 21, 1965 event have been estimated by Mitchell (1973), Patton (1976), and HeHmann (1979). Each of these investigators discovered that the observations were fit best with a normalfaulting earthquake, and the result is robust. In contrast, Heitma~,ln's solutions (HeHmann, 1979) for the March 3, 1963, and July 21, 1967, earthquakes fit the spectral ampfitude observations well but did not match the phase spectra. In light of the poor match to the phase observations, we re-examine these earthquakes using direct seismogram comparison with recordings from seismometers located closest to the epicenters. To check the earlier earthquake parameter estimates, we model analog seismograms recorded close to the events. We assume that the earlier
Fig. 5. Modeling results for the February 2, 1994 southern Illinois earthquake. (Upper fight) Station distribution used in the inversion. (Left) Seismogram fits computed using a moment-tensor inversion (observations are shown with a solid line). (Lower right) Major double couple estimated in the inversion with the first-motion polarities from the Saint Louis University regional seismic network.
surface-wave spectral amplitude study resolved the earthquake nodal planes to within a 180° ambiguity in the strike and rake, and that the published parameters suffered only from insufficient P-wave first motion constraints. We answer what today is a simple question: do the published faulting parameters of H e m , a n n (1979) yield predicted seismograms that agree with nearby long-period observations? If they do not, then we determine which of the three focal mechanisms is consistent with the spectral amplitudes and best fits the waveforms? We fix the seismic moment, source depth, strike, dip, and rake to those estimated earlier by Herrmann (1974, 1979). We show the seismograph and earthquake locations in Fig. 1 and the published earthquake parameters are listed in Table 1. 3.1. March 3, 1963
Of the ten earthquakes studied by Herrmann (1974), this event had the greatest number of observations because of its size. The 1963 event was well recorded by 39 seismograph stations in North America, and provided the best fit to
R.B. Herrmann, CJ. Ammon / Engineering Geology 46 (1997) 299-311
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observed radiation patterns. Unfortunately, the P-wave first motions were of poor quality. In Fig. 6 we compare the observed and predicted long-period seismograms recorded at stations FLO and SLM, located at Florissant and St. Louis, Missouri respectively. At this time FLO had a
30-100 WWSSN long-period response, and SLM had a response similar to the 15-100 WWSSN response. Both the observed and synthetic traces have been adjusted to a unit peak instrument gain. Given the quality of these long-period seismograms, we chose not to rotate the horizontal
R.B. Herrrnann, C.J. Amman / Engineering Geology46 (1997) 299-311
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components to form radial and transverse components of motion (although in this instance north is approximately radial and east is almost aligned with transverse). Fig. 6(a) r e p r i n t s the prediction from the previously published mechanism (Table 1). Note that the polarity of the P-wave motion on the Z and N components is incorrect.
The traces annotated by Fig. 6(b) have the same nodal planes as those in Fig. 6(a) but correspond to an interchange of the compression (P) and tension (T) axes. This corrects the P-wave motion, but contradicts initial motion on the East component at FLO. Finally the traces indicated by Fig. 6(c) correspond to an interchange of the P-
R.B. Herrmann, C.J. Ammon / EngineeringGeology46 (1997) 299-311
308
3.2. July 21, 1967
and T-axes as well as a rotation of the nodal planes by 180 + which improves the fit to the FLO East component. Based on the improved match to the seismograms, we favor solution in Fig. 6(c) for this earthquake. The revised fault parameters are listed in Table 1. To check this revision, we re-examined the impulsive P-wave first motions of this earthquake. These impulsive observations have fewer inconsistencies with Fig. 6(c) than with Fig. 6(b) and many fewer than with the original mechanism. In terms of the stress field in the central US, the important implication is that the P- and T-axes deduced earlier from the 1963 earthquake must be interchanged. -92.00
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R.B. Herrmann, C.J. Arnmon / Engineering Geology46 (1997) 299-311
strike by 180° (Fig. 7(c)) yields pulse shapes that do not match the Rayleigh wave as well. Our conclusion is that the original fault parameters should be revised as indicated in the last entry in Table 1.
4. Patterns in New Madrid faulting In Fig. 8 we overlay the revised, new, and unmodified focal mechanisms listed in Table 1 onto the map of regional micro-earthquake seismicity for the time period 1975-1992. The observed seismicity terminates at the lower left corner of the plot. Interestingly, both the March 25, 1976 and the March 3, 1963 earthquakes occurred at the end of linear seismicity trends and these were the largest earthquakes, Mo "~ 10 23, in this mapped region during the last 30 years. Fig. 9 is a larger view of the New Madrid region and is interesting in that the March 3, 1963 earthquake lies at the end of a WNW trending seismicity pattern emanating from the NNW seismicity trend at New Madrid. One of the nodal planes for each of the three focal mechanisms can be associated with the seismicity trend, and thus may be identified as the fault plane. In Fig. 10 we present the orientations of the Tand P-axes corresponding to the focal mechanisms. The length of each symbol is a function of the cosine of the plunge of the axis (the projection of the axis onto the surface). Within the resolution of the data, the November 9, 1968 earthquake was a pure reverse-faulting event, while the October 21, 1965 earthquake is a pure normal-faulting event and hence have no projections onto the surface. All other earthquakes had significant components of strike-slip motion. In general, the Paxes trend in an E-ENE direction, in agreement with the broad central US pattern observed by Zoback (1992). The T-axes are more variable, but often trend N-NW. The orientation of the stress axes corresponding to earthquakes associated with the left stepping cross trend in seismicity near 36.5°N are different from those of the other events. This may represent a rotation of the local stress field as a result of complex intersections of structural features. Available observations cannot
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resolve either issue but indicate the cross-step feature is worthy of further study.
5. Discussion We applied seismic waveform modeling to constrain recent faulting parameters and test previously published results for two events from the 1960s. For each event, only a few traces were recorded at distances less than 200 kin, a range where inaccuracies in earth models do not strongly influence the predicted seismograms. We showed that the published fault parameters for the March 3, 1963 and the July 21, 1967 earthquakes are
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R.B. Herrmann, CJ. Ammon / Engineering Geology 46 (1997) 299-311
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retrieval of faulting parameters from small earthquakes, and expand our understanding of the potential regional high-frequency ground motions.
Acknowledgment
We thank Arthur Snoke and an anonymous reviewer for constructive reviews that greatly helped us improve this manuscript. Seismic station C C M is maintained by Saint Louis University and the I R I S Consortium and M I A R is part of the US National Seismic Network. The photographic seismograms were diligently digitized by Wei Liu. This research was sponsored in part by the US Geological Survey under Grants 14-08-0001-G2138, 14-08-0001-G2142, and 1434-95-G2607.
References
Ammon, C.J., Herrmann, R.B., Langston, C.A. and Benz., H, 1996. Source parameters of the January 16, 1994Wyomissing
R.B. Herrmann, C J. Ammon / Engineering Geology 46 (1997) 299-311 Hills, Pennsylvania earthquakes. Seismol. Res. Lett., in press. Dmger, D.S. and Helmberger, D.V., 1991. Complex faulting deduced from broadband modeling of the 28 February 1990 Upland earthquake, (ME=5.2). Bull. Seismol. Soc. Am., 81: 1129-1144. Dreger, D.S. and Helmberger, D.V., 1993. Determination of source parameters at regional distances with three-component sparse network data. J. Geophys. Res., 98: 8107-8125. Gordon, D.W., 1988. Revised instrumental hypocenters and correlation of earthquake locations and tectonics in the central United States, US Geological Survey Prof. Paper 1364, US Government Printing Office, Washington, 69 pp. Herrmann, R.B., 1974. Surface wave generation by central United States earthquakes, Ph.D. Dissertation, Saint Louis University, 263 pp. Herrmann, R.B., 1979. Surface wave focal mechanisms for eastern North American earthquakes with tectonic implications. J. Geophys. Res., 84: 3543-3552. Herrmann, R.B., 1995. Broadband seismology and small regional seismic networks, in, K.M. Shediock and A.C. Johnston (Editors), Investigations of the New Madrid Seismic Zone, US Geological Survey Prof. Paper 1538 (in draft). Mitchell, B., 1973. Radiation pattern of surface waves from the southeastern Missouri earthquake of October 21, 1965. J. Geophys. Res., 886.
311
Patton, H., 1976. A note on the source mechanism of the southeastern Missouri earthquake of October 21, 1965. J. Geophys. Res., 81: 1483-1486. Taylor, K.B., Herrmann, R.B., Hamburger, M.W., Pavlis, G.L., Johnston, A., Langer, C. and Lam, C., 1989. The southeastern Illinois earthquake of 10 June 1987. Seismol. Res. Lett., 60: 101-110. Taylor, K.B. and Wuenscher, M.E., 1990. Special investigations of seismic activity: The Ripley, Tenn. earthquake of 29 Aug. 1990 and the New Hamburg, Missouri earthquake of 26 Sept. 1990, in Central Mississippi Valley Earthquake Bulletin: Quarterly Bulletin 66, Third Quarter 1990, Saint Louis University. Wang, C.Y. and Herrmann, R.H., 1980. A numerical study of P-, SV-, and SH-wave generation in a plane layered medum. Bull. Seismol. Soc. Am., 70: 1015-1036. Zhoa, L.-S. and Helmberger, D.V., 1994. Source estimation from broadband regional seismograms. Bull. Seismol. Soc. Am., 84: 91-104. Zoback, M.D. and Zoback, M.L, 1991. Tectonics stress field of North America and relative plate motions, in Neotectonics of North America, Geol. Soc. Am., pp. 339-366. Zoback, M.L., 1992. Stress field constraints on intraplate seismicity in eastern North America. J. Geophys. Res., 97: 11761-11782.
Engineering Geology 46 (1997) 299-311
Faulting parameters of earthquakes in the New Madrid, Missouri, region Robert B. Herrmann *, Charles J. Amrnon 1 Department of Earth and Atmospheric Sciences, Saint Louis University, 3507 Laclede Avenue, St. Louis, MO 63103, USA Received 6 July 1996; accepted 10 November 1996
Abstract
The advent of high-resolution digital seismic recording and advances in computer technology enable the combination of traditional regional seismic network observations with direct seismogram modeling to improve estimates of small earthquake faulting geometry, depth, and size. We illustrate a combined modeling approach using observations from three earthquakes that occurred within the environs of the New Madrid Seismic Zone: two Missouri earthquakes from Septmnber 26, 1990 and May 4, 1991; and the southern Illinois earthquake of February 5, 1994. We also re-examine the faulting geometry for two events from the 1960s that are inconsistent with the current estimate of the regional stress field. Based on direct modeling of the long-period seismograms associated with these events, we revise earlier estimates of the earthquake parameters for the March 3, 1963 and July 21, 1967 Missouri earthquakes. Comparing the new and revised results with existing earthquake mechanisms in the region, we find that tension-axes are generally aligned in a N-S to NW-SE direction, while the compression-axes trend in a NE to E direction. An interesting exception to this pattern are the March 3, 1963 and two nearby earthquakes that lie within a well-defined 30-km long left step in seismicity near New Madrid. Keyworda: Earthquake; Surface wave; Focal mechanism; Stress orientation; New Madrid; Seismic movement; Waveform modeling
1. Imrodn~ion Modeling b r o a d b a n d seismograms recorded at distances up to a few hundred kilometers from an earthquake can constrain the earthquake faulting geometry, depth, size, and slip duration (e.g., Dreger and Helmberger, 1991, 1993). However, * Corresponding author. Tel: + 1 314 977 3120; fax: + 1 314 977 3117; e-mail:rbht~.as.slu.edu 1Tel: 314 977 3197; Fax: 314 977 3117; e-mail: [email protected]. 0013-7952/97/$17.00 © 1997 ElsevierScienceB.V. All rights reserved. PI1 S0013-7952(97)00008-2
since ground motions depend on the earthquake parameters and earth structure, to extract accurate source information (the strike and dip of the fault, and the rake, or direction of slip on the fault), we must account for the effects of earth structure on the seismograms. In general, the influence of structure on the shape of a seismogram increases with increasing wave frequency and epicentral distance. For large earthquakes, we can minimize these on earthquake modeling by relying on low-frequency seismic waves. Studying smaller earthquakes is more challenging because small events do not
300
R.B. Herrmann, C.J. Ammon / Engineering Geology 46 (1997) 299-311
excite large low-frequency waves. Thus, we must utilize higher-frequency observations and reduce the influence of earth structure by other means. For example, Zhao and Helmberger (1994) proposed separately analyzing segments of the waveform to reduce the effects of the unknown earth structure on the estimates of the earthquake parameters. An alternative is to improve our ability to model high-frequency observations by constructing a more detailed regional-structure model using independent observations (e.g., Herrmann, 1995). A third approach is to exploit seismograms recorded close to the earthquake, which are less distorted by the effects of propagation. In the following, we illustrate the utility of waveform modeling to constrain the depth, size, and fault parameters of small-to-moderate-size central United States earthquakes. In each analysis, we exploit a sparse set of observations recorded close to the epicenter. We proceed by example, beginning with a simultaneous analysis of waveforms and regional P-wave polarities. Next, we present an example relying solely on waveforms, but check the consistency of these results with observed P-wave polarities. Then we focus our attention on several earthquakes from the 1960s that were studied prior to the general accessibility
of the computational power needed to model seismograms. The locations of the seismic stations used and the earthquakes studied are shown in Fig. 1.
2. Faulting parameters of three recent central US earthquakes
2.1. The September 26, 1990 earthquake The September 26, 1990 earthquake occurred on the periphery of the regional seismic networks operated by Saint Louis and Memphis State (now University of Memphis) Universities, and the hypocenter depth estimated using arrival times is poorly constrained. In addition, the faulting geometry estimated using first-motion polarities
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Fig. 2. Comparison of observed and predicted (upper and lower traces, respectively, for each component) seismograms for the September 26, 1990 earthquake recorded at station CCM. Each waveform was filtered with a 15-100 WWSSN LP instrument response with a unit peak gain. A total of 51.1 s of time history is displayed, starting 20 s after the event origin time. The lower hemisphere equal area projections to the fight show the observed P-wave first motion polarities from the main event (upper), and the main and aftershock first motion data (lower). The mechanisms correspond to the predicted waveforms. A circle or plus sign indicates a compression, and a triangle or minus sign indicates a dilatation. Positive Z, R and T values represent motion up, away from the source, and in a direction clockwise about the source.
R.B. Herrmann, CJ. Ammon / Engineering Geology46 (1997) 299-311
is ambiguous due to limited azimuthal coverage (Fig. 2). In Fig. 2, we plot the seismograms generated by this earthquake and recorded at seismic station CCM (Cathedral Caves, Missouri). CCM is located 175kin from the epicenter along a source-station azimuth of 305 °. We also show the predicted seismograms computed by solving the equations of motion in a cylindrical symmetric medium (plane-layered earth model) using the wavenumber integration technique (Wang and Hemuann, 1980). Each waveform includes a 15-100 World Wide Standard Seismograph Network (WWSSN) long-period (LP) instrument response with unit peak magnification. The WWSSN LP response is a low-pass filter which eases the comparison of recent observations with those of older events. We exploit the full bandwidth of the seismograms later when we examine the high-frequency signals. The vertical (Z), radial (R) (horizontal component of motion in the direction connecting the epicenter and seismometer) and transverse (T) (horizontal direction perpendicular to the radial) components of ground motion are displayed and the peak trace amplitude in cm is shown below each signal. We modeled these observations by direct comparison with predicted seismograms computed by systematically changing the values of strike, dip, rake, and source depth. Since we are searching over a small number of parameters, each with a limited range, the grid search can be implemented interactively using a flexible misfit measure, such as simultaneously matching the main features in the seismograms and the least-ambiguous P-wave polarities. During the search, the waveform fit is judged on absolute amplitude of the signals and relative amplitudes o f the P, S, Love, and Rayleigh waves. One acceptable match is shown in Fig. 2 (the predicted seismograms were computed using the fault parameters listed in Table 1 and the CUS earth model is listed in Table 2). The fit to the observed seismograms is very good at the lower frequencies (below 0.5 Hz) and the first-motion match is also good, with only four inconsistencies that could be related to the dependence of the takeoff angle upon the earth model. The composite of polarities plotted in Fig. 2 are mainshock and
301
aftershocks polarities obtained from the regional networks and a portable aftershock deployment (Taylor and Wuenscher, 1990). The 15-kin depth inferred from the waveforms is consistent with 10-16-km aftershock depth range constrained by readings from portable seismographs deployed directly above the aftershocks (Taylor and Wuenseher, 1990). In Fig. 2, one glaring inconsistency between the observed and predicted seismograms is the different arrival times of the high frequency S-wave on the vertical component. Simple earth models inadequately describe the complexity of the Earth at small scale lengths and direct seismogram modeling to estimate faulting parameters is most accurate for waveforms recorded a short distance (measured in wavelengths) from the event. In all cases we must assess the sensitivity of earthquake parameters on the assumed earth models. One way to assess structure sensitivity is to examine the faulting parameters estimated using another earth model. Herrmann (1995) modeled these CCM waveforms to refine the CUS earth model. The result of his analysis is the HAMBURG earth model, which is listed in Table2. Using the HAMBURG model the optimal estimates for hypocentral depth is 15 kin, fault strike is 150°, dip is 70 °, rake is 60 °, and the seismic moment is 2.5x 1022dyne-cm. These strike, dip, and rake values differ from those estimated using the CUS model by 10° and the moment differs by about 15% directly indicating the uncertainty in our estimates of these parameters. In Fig. 3, we compare predicted and observed ground velocities using the HAMBURG structure for a sequence of low-pass filters with comer frequencies of 0.10, 0.20, 0.50 and 1.0 Hz. Note that the S-wave is relatively well modeled in terms of arrival time and shape at higher frequency, even though the peak amplitudes and complexity are under-predicted at 1 Hz. For an accurate prediction of these high-frequency ground motions, substantial effort was required to calibrate the structure between the source and receiver. In this case, the feature of the earth model that improves the high-frequency seismogram fit is the gradational crust-mantle transition. The main points of this exercise are that we can
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K B. Herrmann, C.J. Ammon / Engineermg Geology 46 (1997) 299-311
Table 1
Event information Date
Previous results 020262 030363 081465 102165 072167 110968 111770 040374 061375 032576 061087
New additions in this work 092690 050491 020594 Revisions in this work 030363 072167
Origin time (UT)
Lat. (°N)
Long. (°W)
Depth (kin)
Strike (0)
Dip (°)
0643 1730 1313 0204 0914 1701 0213 2305 2240 0041 2348
36.37 36.64 37.22 37.48 37.44 37.91 35.86 38.55 36.54 35.59 38.71
89.51 90.05 89.31 90.94 90.44 88.37 89.95 88.07 89.68 90.48 87.95
7.5 15 1.5 5 15 22 16 15 9 12 10
350 124 280 260 350 0 220 310 85 220 135
84 78 70 40 60 46 75 70 60 65 70
1318 0118 1455
37.16 36.56 37.36
89.58 89.83 89.19
15 8 16
140 90 30
75 67.5 70
1730 0914
36.64 37.44
90.05 90.44
15 15
304 350
78 60
Rake (°)
Moment (dyne-on)
Ref.
145 152 -20 -70 -45 79 150 0 -20 150 15
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(1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (1,2) (3)
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(4) (4)
(1) Origin time and location from Gordon (1988); (2) depth, seismicmoment and mechanism from Herrmann (1979); (3) Taylor et al. (1989); (4) This paper.
match the higher frequency signals with modest changes to the earth structure, and that the difference in mechanisms achieved after this greater effort is insignificant for first-order tectonic investigations. This illustration justifies the use of the simple CUS model (or slight modifications of it) to estimate the faulting parameters of earthquakes in the region.
2.2. The May 4, 1991 earthquake The second earthquake we examine occurred on May 4, 1991, near New Madrid, Missouri, and near the center of the regional seismic network and the temporary, dense Memphis State PANDA deployment. Due to its location well within the networks, the earthquake origin time and hypocenter depth (8 km) are well constrained. The available first-motion polarities (Fig. 4) suggest a strike-slip solution, but permit considerable variation in the nodal plane orientations. We compare the 15-100 WWSSN LP filtered observed and predicted seismograms for station CCM in Fig. 4.
CCM is located 209 km from the epicenter, along an azimuth of 323 °. Waveform modeling with the CUS model match the first-order features in the waveforms, but the predicted surface-waves arrived earlier than observed. To match the observations better, a modified CUS earth model, denoted by M A L D E N ( H e n m a n n , 1995) in Table 2, was used in the grid search and to compute the predicted seismograms shown in Fig. 4. The best-fitting fault parameters (listed in Table 1) produce an excellent fit to the main features of the observed waveforms and match the observed P-wave polarities. The waveforms also favor an 8-km depth, consistent with the regional-network location.
2.3. The February 5, 1994 earthquake The final example is the small southern Illinois earthquake on February 5, 1994, an event that is significantly smaller than those discussed above (Table 1). The waveform inversion technique described by Ammon et al. (1996) was applied to
R.B. Herrmann, CJ. Ammon / Engineering Geology 46 (1997) 299-311 Table 2 Earth models
Q~-I
Qfl- 1
2.5 2.7 2.9 3.0 3.4
0.0050 0.0005 0.0005 0.0005 0.0005
0.010 0.001 0.001 0.001 0.001
2.83 3.44 3.59 3.72 3.74 4.60 4.65
2.5 2.7 2.8 2.8 2.8 3.3 3.3
0.0050 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005
0.010 0.001 0.001 0.001 0.001 0.001 0.001
2.85 3.32 3.60 3.54 3.60 3.70 3.67 3.80 3.77 3.89 3.83 4.23 4.25 4.48 4.52 4.54 4.65 4.94
2.48 2.65 2.77 2.74 2.77 2.82 2.80 2.87 2.85 2.91 2.88 3.08 3.09 3.23 3.25 3.26 3.33 3.51
0.0050 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005
0.010 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001
Thickness
P, velocity
S-velocity
Density
(Van)
(kin/s)
(kin/s)
(g/cm3)
5.00 6.10 6.40 6.70 8.15
2.89 3.52 3.70 3.87 4.70
4.89 5.98 6.21 6.44 6.47 7.97 8.05 HAMBURG 1.00 4.93 2.00 5.76 2.00 6.25 3.00 6.13 3.00 6.23 4.00 6.41 4.00 6.36 5.50 6.58 5.50 6.53 5.50 6.74 5.50 6.63 1.25 7.33 1.25 7.36 2.50 7.76 2.50 7.84 5.00 7.87 2.50 8.06 oo 8.56
CUS 1.0 9.0 10.0 20.0 MALDEN 1.0 9.0 10.0 10.0 10.0 20.0
the data of two broadband stations: CCM and MIAR (Mountain Ida, Arkansas, 34.546N, 93.573W). The approach is a least-squares inversion of the observed seismograms for a deviatoric seismic moment tensor. Regional network P-wave polarities are not included in the inversion. Prior to inversion the observed seismograms were filtered to include only frequencies in the band between 0.02 and 0.1 Hz. In this case, the primary waves modeled were Love and Rayleigh waves, which are the largest waves in this frequency range recorded a few hundred km from this size earthquake. The comparison of the observed and predicted seismograms is shown in Fig. 5. In this
303
instance, the waves sensitive to hypocentral depth (the Rayleigh and body waves) are below the background noise level, so the depth was fixed near that estimated using regional P and S arrival times (16km). The observed amplitudes are matched very well, although some phase inconsistencies are apparent. The Rayleigh waves are near nodal for both stations but Love waves have substantial amplitude and are nicely matched. Our confidence in these results is increased by the consistency with the independent P-wave polarities (Fig. 5). To summarize, direct seismogram modeling provides valuable constraints on the earthquake parameters of regionally-recorded earthquakes. The value of waveform modeling increases when the P-wave first motions are sparse or have a limited azimuthal distribution around the source. In these cases, a match to the main features in the observed seismograms can improve constraints on the depth, strike, dip, and rake of the earthquake and provide valuable constraints on the seismic moment and depth of the earthquake.
3. A re-evaluation of two central US focal
mechanisms Our best estimates of the central US compressional stress orientation is an ENE direction (Zoback and Zoback, 1991; Zoback, 1992). Much of the stress information available in this region is based on earthquake faulting parameters determined by matching surface-wave radiation patterns (Heumann, 1974; Heiimann, 1979; Taylor et al., 1989) and several exceptions to this stress pattern exist. Zoback (1992) notes that of the fault parameters in Henmann (1979), two in the New Madrid region are associated with normal faulting (October 21, 1965; July 21, 1967) and one has a compression axis trending NS rather than EW (March 3, 1963). As mentioned above, the faulting parameters for these three events were estimated by He~¥r,~aun ( 1974, 1979) using surface-wave radiation patterns. A systematic search over the strike, dip, rake, and hypoeenter depth was performed to find the solutions providing the best fit to the observed radia-
304
R.B. Herrmann, CJ. Ammon / Engineering Geology 46 (1997) 299-311
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tion pattern for surface waves with periods between 5 and 50 s. However, since only fundamental-mode surface-wave spectral amplitudes were used, there exist inherent ambiguities of 180° in strike and rake due to the symmetry of theoretical radiation patterns. Thus, four solutions are possible for each event (two have the same focal mechanism and hence associated stress directions). To select one, HeJ-fmann (1974, 1979) checked the consistency of the surface-wave solutions with observed
P-wave polarities. Unfortunately, for many of the earthquakes, the quality of the P-wave observations was marginal. After processing a large number of earthquakes, more faith was placed in the surface-wave constraints on nodal plane orientation and only impulsive P-wave first motions were used. Since the study of He~mann (1979), computational techniques for modeling seismic waveforms have improved significantly. It is timely to revisit
R.R Herrmann, C.J. Ammon / Engineering Geology 46 (1997) 299-311
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Fig. 4. Comparison of observed and predicted (upper and lower traces, respectively, for each component) seismograms for the May 4, 1991 earthquake recorded at CCM. Both sets are passed through a 15-100 WWSSN LP instrument response with a peak gain of 1. A total of 80 s of time history is displayed, starting 20 s after the event origin time. The lower hemisphere equal area projections to the right show the observed P-wave first motion data from the event with (upper) and without (lower) the nodal planes. Circles and triangles indicate compressional or dilatational P-wave first-motion, respectively.
earlier earthquake solutions, and if possible, resolve the apparent incompatibility with the present model of the mid-continent stress field. Of the three events that are inconsistent with the regional stress pattern, the faulting parameters of the October 21, 1965 event have been estimated by Mitchell (1973), Patton (1976), and HeHmann (1979). Each of these investigators discovered that the observations were fit best with a normalfaulting earthquake, and the result is robust. In contrast, Heitma~,ln's solutions (HeHmann, 1979) for the March 3, 1963, and July 21, 1967, earthquakes fit the spectral ampfitude observations well but did not match the phase spectra. In light of the poor match to the phase observations, we re-examine these earthquakes using direct seismogram comparison with recordings from seismometers located closest to the epicenters. To check the earlier earthquake parameter estimates, we model analog seismograms recorded close to the events. We assume that the earlier
Fig. 5. Modeling results for the February 2, 1994 southern Illinois earthquake. (Upper fight) Station distribution used in the inversion. (Left) Seismogram fits computed using a moment-tensor inversion (observations are shown with a solid line). (Lower right) Major double couple estimated in the inversion with the first-motion polarities from the Saint Louis University regional seismic network.
surface-wave spectral amplitude study resolved the earthquake nodal planes to within a 180° ambiguity in the strike and rake, and that the published parameters suffered only from insufficient P-wave first motion constraints. We answer what today is a simple question: do the published faulting parameters of H e m , a n n (1979) yield predicted seismograms that agree with nearby long-period observations? If they do not, then we determine which of the three focal mechanisms is consistent with the spectral amplitudes and best fits the waveforms? We fix the seismic moment, source depth, strike, dip, and rake to those estimated earlier by Herrmann (1974, 1979). We show the seismograph and earthquake locations in Fig. 1 and the published earthquake parameters are listed in Table 1. 3.1. March 3, 1963
Of the ten earthquakes studied by Herrmann (1974), this event had the greatest number of observations because of its size. The 1963 event was well recorded by 39 seismograph stations in North America, and provided the best fit to
R.B. Herrmann, CJ. Ammon / Engineering Geology 46 (1997) 299-311
306
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(SEC) Fig. 6. Comparison of seismograms at FLO (top) and SLM (bottom) for the March 3, 1963 earthquake. In each display, the digitized trace is indicated, the trace predicted on the basis of the published focal mechanism by (a), the trace for the same nodal planes but with P- and T-axes interchanged by (b), and the trace with the nodal planes rotated by 180 ° and the P- and T-axes interchanged by (c). The time histories start 20 s after the origin time and a total of 102.2 s is displayed. FLO is 235 km at an azimuth of 354 ° from the source, and SLM is 215.48 km along an azimuth of 357 °. Peak trace amplitudes are in cm for a peak instrument magnification of 1.0. Positive Z, N and E values, indicate motion up, north and east.
observed radiation patterns. Unfortunately, the P-wave first motions were of poor quality. In Fig. 6 we compare the observed and predicted long-period seismograms recorded at stations FLO and SLM, located at Florissant and St. Louis, Missouri respectively. At this time FLO had a
30-100 WWSSN long-period response, and SLM had a response similar to the 15-100 WWSSN response. Both the observed and synthetic traces have been adjusted to a unit peak instrument gain. Given the quality of these long-period seismograms, we chose not to rotate the horizontal
R.B. Herrrnann, C.J. Amman / Engineering Geology46 (1997) 299-311
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Fig. 7. Comparison of seismograms for the July 21, 1967 tmrthquake recorded at FLO and SLM: observed, (a) prediction based on published mechanism, (b) prediction with published nodal pbums but with P- and T-ax~ intcrchaaged, and (c) prediction with nodal plane strike increments! by 180° and the P- and T-axes intm'chaagml. FLO is 145 km from the event along an azimuth of 1°, and SLM is 127 km away along an azimuth of 6 °. The traces start at 20 s a t ~ the event origin and continue for 51.0 s. Both instruments have WWSSN 15-100 LP responses and a l~ak gain of 1.0.
components to form radial and transverse components of motion (although in this instance north is approximately radial and east is almost aligned with transverse). Fig. 6(a) r e p r i n t s the prediction from the previously published mechanism (Table 1). Note that the polarity of the P-wave motion on the Z and N components is incorrect.
The traces annotated by Fig. 6(b) have the same nodal planes as those in Fig. 6(a) but correspond to an interchange of the compression (P) and tension (T) axes. This corrects the P-wave motion, but contradicts initial motion on the East component at FLO. Finally the traces indicated by Fig. 6(c) correspond to an interchange of the P-
R.B. Herrmann, C.J. Ammon / EngineeringGeology46 (1997) 299-311
308
3.2. July 21, 1967
and T-axes as well as a rotation of the nodal planes by 180 + which improves the fit to the FLO East component. Based on the improved match to the seismograms, we favor solution in Fig. 6(c) for this earthquake. The revised fault parameters are listed in Table 1. To check this revision, we re-examined the impulsive P-wave first motions of this earthquake. These impulsive observations have fewer inconsistencies with Fig. 6(c) than with Fig. 6(b) and many fewer than with the original mechanism. In terms of the stress field in the central US, the important implication is that the P- and T-axes deduced earlier from the 1963 earthquake must be interchanged. -92.00
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In Fig. 7 we compare the digitized traces at FLO and SLM with the predictions based on the published mechanism (Fig. 7(a)). The SH pulse on the East component disagrees with the observations, as does some of the P-wave characteristics on the Vertical and North components. This is a small event, and the maximum trace amplitude on the original data was at most several millimeters. If the original nodal planes are kept, but the P- and T-axes are interchanged, the corresponding predictions (Fig. 7(b)) match the observations much better. The result further changing the nodal plane
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R.B. Herrmann, C.J. Arnmon / Engineering Geology46 (1997) 299-311
strike by 180° (Fig. 7(c)) yields pulse shapes that do not match the Rayleigh wave as well. Our conclusion is that the original fault parameters should be revised as indicated in the last entry in Table 1.
4. Patterns in New Madrid faulting In Fig. 8 we overlay the revised, new, and unmodified focal mechanisms listed in Table 1 onto the map of regional micro-earthquake seismicity for the time period 1975-1992. The observed seismicity terminates at the lower left corner of the plot. Interestingly, both the March 25, 1976 and the March 3, 1963 earthquakes occurred at the end of linear seismicity trends and these were the largest earthquakes, Mo "~ 10 23, in this mapped region during the last 30 years. Fig. 9 is a larger view of the New Madrid region and is interesting in that the March 3, 1963 earthquake lies at the end of a WNW trending seismicity pattern emanating from the NNW seismicity trend at New Madrid. One of the nodal planes for each of the three focal mechanisms can be associated with the seismicity trend, and thus may be identified as the fault plane. In Fig. 10 we present the orientations of the Tand P-axes corresponding to the focal mechanisms. The length of each symbol is a function of the cosine of the plunge of the axis (the projection of the axis onto the surface). Within the resolution of the data, the November 9, 1968 earthquake was a pure reverse-faulting event, while the October 21, 1965 earthquake is a pure normal-faulting event and hence have no projections onto the surface. All other earthquakes had significant components of strike-slip motion. In general, the Paxes trend in an E-ENE direction, in agreement with the broad central US pattern observed by Zoback (1992). The T-axes are more variable, but often trend N-NW. The orientation of the stress axes corresponding to earthquakes associated with the left stepping cross trend in seismicity near 36.5°N are different from those of the other events. This may represent a rotation of the local stress field as a result of complex intersections of structural features. Available observations cannot
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resolve either issue but indicate the cross-step feature is worthy of further study.
5. Discussion We applied seismic waveform modeling to constrain recent faulting parameters and test previously published results for two events from the 1960s. For each event, only a few traces were recorded at distances less than 200 kin, a range where inaccuracies in earth models do not strongly influence the predicted seismograms. We showed that the published fault parameters for the March 3, 1963 and the July 21, 1967 earthquakes are
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R.B. Herrmann, CJ. Ammon / Engineering Geology 46 (1997) 299-311
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retrieval of faulting parameters from small earthquakes, and expand our understanding of the potential regional high-frequency ground motions.
Acknowledgment
We thank Arthur Snoke and an anonymous reviewer for constructive reviews that greatly helped us improve this manuscript. Seismic station C C M is maintained by Saint Louis University and the I R I S Consortium and M I A R is part of the US National Seismic Network. The photographic seismograms were diligently digitized by Wei Liu. This research was sponsored in part by the US Geological Survey under Grants 14-08-0001-G2138, 14-08-0001-G2142, and 1434-95-G2607.
References
Ammon, C.J., Herrmann, R.B., Langston, C.A. and Benz., H, 1996. Source parameters of the January 16, 1994Wyomissing
R.B. Herrmann, C J. Ammon / Engineering Geology 46 (1997) 299-311 Hills, Pennsylvania earthquakes. Seismol. Res. Lett., in press. Dmger, D.S. and Helmberger, D.V., 1991. Complex faulting deduced from broadband modeling of the 28 February 1990 Upland earthquake, (ME=5.2). Bull. Seismol. Soc. Am., 81: 1129-1144. Dreger, D.S. and Helmberger, D.V., 1993. Determination of source parameters at regional distances with three-component sparse network data. J. Geophys. Res., 98: 8107-8125. Gordon, D.W., 1988. Revised instrumental hypocenters and correlation of earthquake locations and tectonics in the central United States, US Geological Survey Prof. Paper 1364, US Government Printing Office, Washington, 69 pp. Herrmann, R.B., 1974. Surface wave generation by central United States earthquakes, Ph.D. Dissertation, Saint Louis University, 263 pp. Herrmann, R.B., 1979. Surface wave focal mechanisms for eastern North American earthquakes with tectonic implications. J. Geophys. Res., 84: 3543-3552. Herrmann, R.B., 1995. Broadband seismology and small regional seismic networks, in, K.M. Shediock and A.C. Johnston (Editors), Investigations of the New Madrid Seismic Zone, US Geological Survey Prof. Paper 1538 (in draft). Mitchell, B., 1973. Radiation pattern of surface waves from the southeastern Missouri earthquake of October 21, 1965. J. Geophys. Res., 886.
311
Patton, H., 1976. A note on the source mechanism of the southeastern Missouri earthquake of October 21, 1965. J. Geophys. Res., 81: 1483-1486. Taylor, K.B., Herrmann, R.B., Hamburger, M.W., Pavlis, G.L., Johnston, A., Langer, C. and Lam, C., 1989. The southeastern Illinois earthquake of 10 June 1987. Seismol. Res. Lett., 60: 101-110. Taylor, K.B. and Wuenscher, M.E., 1990. Special investigations of seismic activity: The Ripley, Tenn. earthquake of 29 Aug. 1990 and the New Hamburg, Missouri earthquake of 26 Sept. 1990, in Central Mississippi Valley Earthquake Bulletin: Quarterly Bulletin 66, Third Quarter 1990, Saint Louis University. Wang, C.Y. and Herrmann, R.H., 1980. A numerical study of P-, SV-, and SH-wave generation in a plane layered medum. Bull. Seismol. Soc. Am., 70: 1015-1036. Zhoa, L.-S. and Helmberger, D.V., 1994. Source estimation from broadband regional seismograms. Bull. Seismol. Soc. Am., 84: 91-104. Zoback, M.D. and Zoback, M.L, 1991. Tectonics stress field of North America and relative plate motions, in Neotectonics of North America, Geol. Soc. Am., pp. 339-366. Zoback, M.L., 1992. Stress field constraints on intraplate seismicity in eastern North America. J. Geophys. Res., 97: 11761-11782.