Active and Passive Seismic Imaging of the San-Andreas-Fault System at SAFOD

High-quality active and passive seismic data have been acquired in the vicinity of the San-Andreas-Fault system within EarthScope project SAFOD (San-Andreas-Fault-Observatory-at-Depth). We have processed parts of the available seismic data sets using newly developed techniques in order to derive a high-resolution image of the subsurface in the vicinity of the fault system. On one hand we applied Fresnel-Volume-Migration to the SAFOD2003 reflection seismic data set. Along the 50 km long profile we obtained a structural image of the region around the fault system from the surface down to about 10 km depth. The San-Andreas-Fault (SAF) appears as a strong subvertical reflector which can be followed from its surface trace down to about 4 km depth. At this depth it converges to a second similarly strong subvertical reflector which correlates with the Buzzard-Canyon-Fault (BCF) approximately 2 km southwest of the SAF. Also the Waltham-Canyon-Fault (WCF) about 9 km northeast of the SAF shows up clearly down to a depth of 5 km. Furthermore a bunch of subvertical reflectors appears at shallow depths over a distance of 10 km southwest of the SAF and another prominent reflector is visible approximately 15 km southwest of the SAF from almost the surface down to a depth of 10 km. Both latter features have no yet known surface expression, however a comparison with geodynamic modelling results shows that the existence of such faults is probably realistic. On the other hand we applied our novel migration-type location algorithm to passive seismic data recorded with an 80-level-3C-receiver array in the SAFOD main hole. We have located a number of local events in the vicinity of the fault system. A modification of the location algorithm also allowed the precise location of target events, which are the subject of the drilling activities in phase 3. The events are interpreted in terms of their relationship to the fault system structure. We have combined our results (target events, structural image) with additional available information (seismicity, borehole logs, shallow seismic images, etc.). Altogether this combined image provides a basis for a combined interpretation of the structure and the earthquake dynamics of this mega-shear zone on different scales and in particular in the vicinity of the SAFOD borehole.