BOREHOLE SEISMIC IMAGING AND MONITORING OF THE SAN ANDREAS FAULT USING AN 80 LEVEL 3C 4,000 FT LONG CLAMPED SEISMIC ARRAY AT SAFOD

The San Andreas Fault (SAF) at Parkfield California was historically mapped as a single right-lateral strike-slip fault. The 2004 M6 earthquake occurred 11 years later than predicted and ruptured in the opposite direction than anticipated showing that earthquake prediction is a science with large uncertainties.

Seven months after the 2004 earthquake, Paulsson, Inc. deployed an 80-level, 3C, 240 channel, 4,000 ft long seismic array to a depth of 9,000 ft in the SAFOD (San Andreas Fault Observatory at Depth) main hole next to the SAF, to monitor and map small and large earthquakes for 13 days. Paulsson recorded many hundred events sized M-5, and smaller, and as large as M3, using our borehole array equipped with 15 Hz omni geophones. USGS in the same period recorded 47 events using surface and near surface seismic sensors.

From the Paulsson borehole seismic data, we found that the SAF is composed of three active faults, each separated by about 3,000 ft or 1 km. Furthermore, the one fault which shows surface ruptures near the borehole is not the main subsurface rupture. The triple-fault model of the SAF in the Parkfield area may explain the sequence of 12, 24, or 36 years of periodic M6 earthquakes.

By analyzing the location of the hypocenters of the largest 100 earthquakes recorded by the Paulsson array, we achieved a clear picture of the SAF’s complex structures and dynamics. First, the SAF at the SAFOD site is composed of three active faults, SAF1, SAF2, and SAF3 from SW to NE with SAF1 being the fault traditionally labeled as SAF. During the monitoring period, there was a trend of the earthquakes migrating from SAF1 to SAF2, and to SAF3, which indicates that the move of SAF1 triggers SAF2, which in turn triggers SAF3, to move one after another, not only in the SAF’s strike direction, but dipping downwards.

The successful monitoring of the SAF provides strong evidence that one must use large arrays of borehole 3C vector sensors, e.g. geophones or accelerometers, to monitor and understand subsurface dynamic processes such as active earthquake faults as well as fluid injections and extractions.

Acoustic sensors are not capable of detecting, monitoring and mapping complex structures such as earthquake fault zones or complex events such as injection or extraction of fluids including CO2 for CCS or H2O for fracturing the formation for Enhanced Geothermal Systems (EGS).