A broadband seismological investigation of the 1989 Loma Prieta, California, earthquake: Evidence for deep slow slip?

Abstract

Analysis of regional and teleseismic broadband body waves and long-period surface waves from the 1989 Loma Prieta earthquake demonstrates that a fairly simple average fault model explains most characteristics of seismic wave radiation over a broad period range (1 to 300 sec), but there are some systematic period-dependent characteristics that are not yet understood. Determinations of the preferred point source focal mechanism are consistent for different seismic waves, as long as optimal source-time functions and depths are chosen for each wavetype. The major double couple orientation compatible with the entire period range has a strike = 128 +_ 3 °, dip = 67 _+ 5 ° , and rake = 133 _+ 8% While this source mechanism consistency is encouraging and indicates that the average source process is quite simple, there are some systematic discrepancies in the source parameter estimates from different period waves. The source duration estimated from the principal body-wave ground motions is 8 + 2 sec, compatible with the duration of near-field strong-motion vibrations, but the surface waves indicate total durations of 18 to 30 sec, depending on the earth model selected for computing propagation corrections. The model dependence of the surface-wave results suggests that it may be possible to reconcile the duration estimates with improved earth models, but the discrepancy is quite large. There is some complexity in the source-time function, comprised of three subevents at different depths, but this complexity is only manifested in short-period signals and does not explain the duration discrepancy. Changes of fault geometry or slip direction during rupture, as suggested by recent finite-fault inversions, are unlikely to cause a bias in source duration estimates. The centroid depth determined from our body-wave analysis is 10 to 12 km, consistent with the finite-fault models, which indicate rupture extending from 5 to 18 km. Surface-wave centroid depth estimates (15 to 30 km) vary substantially for different choices of global Q model and lithospheric structure in the source region, but they do tend to be deeper than the body-wave results. It is again not clear whether this depth discrepancy is due to the source process or inadequate earth models. The body-wave and surface-wave seismic moment estimates are also not fully consistent; the moment from our body-wave inversions is 2.4 _+ 0.3 x 1019 N-m, while our surface-wave analysis gives 3.2 _+ 0.5 x 1019 N-m. It is not known whether reasonable changes of the earth models can bring the moment estimates into full agreement. The systematic discrepancies in source duration, centroid depth, and seismic moment, despite the consistency in source orientation, raise the possibility of a deep, relatively slow co-seismic slip component, but trade-offs with modeling assumptions preclude us from confirming this hypothesis at present.