A key objective of the San Andreas Fault Observatory at Depth (SAFOD) project is to drill through the rupture patch of a small repeating earthquake. The size of the rupture patch is on the order of tens of meters; hence knowing its location to a comparable degree of accuracy is vital to SAFOD's success. The work we are carrying out is critical to this final phase of SAFOD drilling, and requires reaching a level of location accuracy for the target earthquakes of about 10 meters. We are approaching this task by taking advantage of a borehole string, to be installed at the end of Phase II drilling, that will bring earthquake sensors very close (~100 m) to the target zone. We will then set off 6 shots of about 50-100 lbs. near temporary earthquake recording sites during a ~9 to 12 month period of concurrent recording by our temporary array, a borehole string in the SAFOD Pilot Hole, and the Phase II SAFOD borehole string. The resulting combined active and passive data set will provide a potent means for calibrating and improving the seismic wavespeed models that will enable us to achieve the 10 m accuracy required for the Phase III drilling. Finally, we will continue to analyze data collected after Phase III drilling is completed in order to refine and extend our models of San Andreas fault zone structure.
