Three-Dimensional Seismic Velocity Structure and Precise Earthquake Relocations in the Salton Trough, Southern California

I present a new 3D seismic velocity model and high-precision earthquake relocations between 1981 and 2010 near the Salton Trough and the San Jacinto fault zone. The simul2000 tomography algorithm is applied to derive the 3D V-P and V-P/V-S models using first arrivals of a composite event data set. The final VP model correlates with geological features at shallow depths and is more consistent with the refraction studies than the starting 3D regional-scale model. The V-P/V-S model shows strong variations in the upper crust and is dominated by relatively low values (below 1.7) in the middle crust except for the Salton Trough and the Imperial Valley. The newly resolved model is used to relocate all the seismicity in the study area. I then apply the similar-event cluster analysis and differential-time relocation approach based on waveform cross-correlation data to the 3D relocated events. A dramatic sharpening of seismicity patterns is obtained after using these methods. The new locations are more tightly clustered than the previous catalogs mainly due to the different absolute locations. The depth distribution along a basal surface may suggest the existence of a detachment fault from the north end of the Salton Trough to United States-Mexico border. The fine-scale velocity structure and precise earthquake relocations will be helpful for investigating the fault geometry, seismicity, stress interaction, and other seismological studies for southern California.