Seismic velocity variations along the rupture zone of the 1989 Loma Prieta earthquake, California

Abstract

We revisit the rupture zone of the 1989 Mw6.9 Loma Prieta earthquake, central California, by developing high‐resolution three‐dimensional (3‐D)Vp and Vp/Vs models. We apply the simul2000 inversion method and algorithm to a set of “composite” events, which have greater number of picks per event and reduced random picking errors compared with traditional master events. Our final P‐wave velocity model generally agrees with previous studies, showing a high velocity body of above 6.7 km/s in the southeast rupture zone of the main shock. The 3‐DVp/Vs model, however, has different features, with low Vp/Vs in the upper crust and high Vp/Vs anomalies in deeper layers of the rupture zone. We interpret the low Vp/Vs at shallow depths to be granitic rocks, whereas at greater depths the areas of higher Vp/Vs(around 1.725–1.75) presumably are mafic rocks. The resulting 3‐D velocity model was used to improve absolute locations for all local events between 1984 and 2010 in our study area. We then applied a similar event cluster analysis, waveform cross‐correlation, and differential time relocation methods to improve relative event location accuracy. Over 88% of the seismicity falls into similar event clusters. A dramatic sharpening of seismicity patterns is obtained after using these methods. The medians of the relative location uncertainties calculated by using the bootstrap approach are 5 m for horizontal and 8 m for vertical. Differential times from cross‐correlation are used to estimatein situnear‐sourceVp/Vsratio within each event cluster. The high‐resolutionVp/Vs method confirms the trend of the velocity variations from the tomographic results, although absolute values are slightly different.