Descr: 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 very high degree of accuracy is vital to SAFOD's success. Results from our previous work in this area, which involved the analysis of data from the deployment of dense, temporary networks of seismic stations (PASO-UNO and PASO-DOS) in the vicinity of the drill site, show that while the epicenter of the target patch is consistent with the results of a previous study, the depth is significantly shallower than anyone previously supposed; different enough to require rethinking the original drilling plan. We estimate that the uncertainty in depth currently is on the order of 200-m, which, while an improvement over previous efforts, is still larger than what would be required to successfully guide the later phases of SAFOD drilling. Sh! ortly after Phase I drilling, until about 12 months after completion of Phase II drilling, we propose to install and operate a network (PASO-TRES) that reoccupies 25 of the PASO-DOS sites. In addition to recording ambient seismicity simultaneously with the instruments in the drill hole and permanent stations in the area, we will record signals from 10 small (~2 lb) shots and 12 large (~100-200 lb) shots in order to calibrate wavespeeds and estimate absolute location accuracy. We anticipate that the combination of PASO-TRES data, both active and passive, with those from the permanent stations near the drill site and downhole sensors, will, when subjected to the same analysis techniques as those we have employed on PASO-UNO and PASO-DOS, allow us to reduce the uncertainties in hypocenter location to the ~10-m level required to direct the multilateral coring of Phase III drilling.
