Rupture Model of the 1989 Loma Prieta Earthquake from the Inversion of Strong-motion and Broadband Teleseismic Data

We have used 24 broadband teleseismic and 48 components of local strong-motion velocity records of the 1989 Loma Prieta earthquake in a formal inversion to determine the temporal and spatial distribution of slip. Separate inversions of the teleseismic data (periods of 3 to 30 sec) or strong-motion data (periods of 1 to 5 sec) result in similar models. The data require bilateral rupture with relatively little slip in the region directly updip from the hypocenter. Slip is concentrated in two patches: one centered 6 km northwest of the hypocenter at a depth of 12 km and with a maximum slip of 350 cm, and the other centered about 5 km southeast of the hypocenter at a depth of 16 km and with a maximum slip of 460 cm. The bilateral nature of the rupture results in large amplitude ground motions at sites located along the fault strike, both to the northwest and the southeast. However, the northwestern patch has a larger moment and overall stress drop and is, consequently, the source of the largest ground motion velocities, consistent with the observed recordings. This bilateral rupture also produces relatively modest ground motign amplitudes directly updip from the hypocenter, which is in agreement with the velocity ground motions observed at Corralitos. There is clear evidence of a foreshock (magnitude between 3.5 and 5.0) or a slow rupture nucleation about 2 sec before the main part of the rupture; the origin time implied by strong-motion trigger times is systematically 2 sec later than the time predicted from the high-gain regional network data. The seismic moment obtained from either of the separate data sets or both sets combined is about 3.0 × 1026 dyne-cm and the potency is 0.95 km 3. INTRODUCTION In this study, we use a least-squares linear inversion of strong motion and teleseismic waveform data to solve for the temporal and spatial distribution of slip vectors during the 1989 Loma Prieta earthquake (M s = 7.1). Although the geometry of the fault plane is fixed in the inversion, it is chosen to be compatible with teleseismic waveforms and the distribution of aftershocks. Our estimates of the spatial and temporal distribution of slip will enhance studies of fault segmentation and earthquake recurrence (King et al., 1990; Working Group on California Earthquake Probabilities, 1988), which depend on reliable estimates of the rupture dimensions and amplitude of slip. Furthermore, the variation in rake angle as a function of position along strike and downdip on the fault plane is critical to analyses of the complicated fault interactions within the Sargent-San Andreas system (Dietz and Ellsworth, 1990; Michael et al., 1990; Olson, 1990; Schwartz et al., 1990; Seeber and Armbruster, 1990). The method we employ is that of Hartzell and Heaton (1983), which has been shown to provide valuable insight into the rupture history of other California earthquakes (Hartzell and Heaton, 1983; Hartzell and Heaton, 1986; Hartzell, 1989; Wald et al., 1990), as have other finite fault approaches (Olson and Apsel, 1982; Archuleta, 1984; Beroza and Spudich, 1988). In addition to providing an 1540 R U P T U R E MODEL OF THE LOMA PRIETA E A R T H Q U A K E 1541 estimate of the rupture history for individual earthquakes, these studies also provide new insight into the general characteristics of the rupture process that are common to many events. After studying slip models from several earthquakes, Mendoza and Hartzell (1988) suggested that large gaps in aftershock pat terns often coincide with the regions of relatively large slip. From the distribution of slip, we can also place constraints on the location and depth extent of significant energy release and characterize the distribution of stress changes on the faults. These results provide a start ing point for calculating ground motions for future events comparable to size to the Loma Prieta earthquake. Such ground motion calculations are important for augmenting the sparse data base of near-source strong-motion recordings of crustal ear thquakes having magnitudes of 7 or larger. The Loma Prieta ear thquake was well recorded at both local strong-motion and teleseismic broadband stations. The strong-motion velocity recordings used here are dominated by energy in the range of 1 to 5 sec, while the broadband teleseismic recordings range from 3 to 30 sec. This wealth of data provides an opportunity to compare rupture models that are derived independently from either the strong-motion or the teleseismic waveforms with models derived from the combined data sets and over a wide range of frequencies. Our results provide insight into the limitations and constraints provided by previous studies that have less extensive data sets. DATA Ground motions from the Loma Prieta ear thquake were recorded over a wide range of frequencies and distances, from high-frequency waveforms on local accelerometers and regional seismic networks to very low frequencies observed in teleseismic surface waves and geodetic line length changes. Unfortunately, deterministic waveform inversion of high-frequency motion (> 3 Hz) requires an accurate and detailed knowledge of the wave propagation in the geologically complex structure in the Loma Prieta region. Furthermore, inversion of high frequencies requires a proliferation of free variables that significantly increase the computation t ime and decrease the stability of the inversion process. Therefore, we have chosen to concentrate our study on the lower frequency part of the rupture history. Near-source low-pass-filtered strong-motion and teleseismic body waves seem to be the most suitable data sets to study the general characteristics of the slip history. Geodetic data can also provide important constraints on an ear thquake slip distribution model. Unfortunately, we were not able to obtain enough geodetic data at the time of this study to justifyits inclusion in the formal inversion process. Teleseismic The teleseismic stations chosen for this study are listed in Table 1. The data are digital recordings obtained from the Chinese Digital Seismograph Network (CDSN), GEOSCOPE and Incorporated Research Institution for Seismology (IRIS) broadband components, and Global Digital Seismograph Network (GDSN) intermediate-period components. These stations provide a uniform azimuthal coverage of the focal sphere and also contain several near-nodal observation for both P and SH source radiation( Fig. 1). In this analysis, instrument responses were deconvolved from the original recordings to obtain true ground velocities. 1542 D. J. WALD E T A L . TABLE 1 TELESEISMIC STATIONS Distance Phases Station ( ° ) Azimuth Backazirnuth Used AFI 69.2 232.6 40.8 P, SH ARU 86.9 359.7 0.4 P, SH CAY 70.8 98.6 307.6 P, SH COL 31.8 339.4 138.5 P HIA 77.9 324.0 45,9 P HON 35.0 253.5 55.2 P HRV 38.5 65.7 279.2 P MDJ 76.0 305.2 51.3 P NNA 64.8 130.1 321.5 P, SH OBN 85.1 11.9 343.0 P, SH PPT 60.5 210.6 25.2 P, SH RPN 65.2 167.7 349.0 P, SH SCP 34.1 67.8 278.3 P SSB 84.6 34.7 319.8 P TOL 84.3 43,0 314.8 P, SH WFM 38.5 65.6 279.1 P