We applied an Empirical Green's Function method to recordings of teleseismic body waves from the April 21, 1997 Santa Cruz Island earthquake. The far-field moment rate functions reveal that rupture occured on a shallow dipping fault plane (dip 17 degrees). The main rupture consists of 6 large subevents and covers a total area of about 5000 km2. While the hypocenter is fixed at 33 km depth, the main slip is located above 20 km depth within the accretionary wedge. The source duration is about 70 sec and the average rupture velocity is found to be 1.9 km/sec. This event caused a local tsunami and although it is not possible to determine whether it was caused by tectonic faulting or submarine shaking the fact that most of the moment release occured at shallow depths is consistent with either of these possibilities, and points to the utility of the procedure outlined here in potentially rapid characterization of tsunami potential.
We performed an inversion for fault slip distribution for the 1992 Landers and the 1994 Northridge (Fig. 13.1) earthquakes using a pre-calculated catalog of theoretical SoCal Green's functions described in Dreger and Helmberger (1993) and Dreger (1994). The method is based on a non-negative least-square inversion for finite source parameters given a source mechanism. Smoothing and moment minimization constraints are applied. Although we used only few TERRAscope broadband stations, the slip distributions provide a good level of fit to data. It is possible to resolve the fault plane ambiguity and to obtain the average slip and fault dimension. In addition, comparison of obtained slip distributions for the Landers and Northridge earthquakes to those from other near-field studies (Cohee et al., 1994; Wald et al., 1996) shows that we are able to map the slip location well. This positive correlation implies that we should be able to use these regionally derived slip maps to produce near-source shake maps in near-realtime and is the focus of a study currently funded by PGE/PEER.
Cohee, Brian P., et al. "Slip distribution of the 1992 Landers earthquake and its implications for earthquake source mechanics", Bull.Seism.Soc.Am., 84, 692-712, 1994.
Dreger, D. S. "Empirical Green's function study of the January 17, 1994 Northridge, California earthquake", Geophys.Res.Lett., 21, 2633-2636, 1994.
Wald, David J., et al. "The slip history of the 1994 Northridge, California, earthquake determined from strong-motion, teleseismic, GPS, and leveling data", Bull.Seism.Soc.Am., 86, 49-70, 1996.