Non-technical Summary

Magnetotelluric data were collected on the creeping segment of the San Andreas Fault near Hollister, California. The goal of the survey was to image the electrical resistivity structure of the fault zone. Since the electrical resistivity of the earth is very sensitive to the presence of interconnected fluids, this parameter can help delineate the distribution of fluids within the fault zone. The new data will allow the electrical resistivity of the seismically active, creeping segment to be compared with both the Parkfield and Carrizo segments. Since these segments exhibit very different seismic behavior, this comparison will shed light on the role of fluids in the earthquake cycle.

Investigations undertaken

Broadband magnetotelluric data were collected on two new profiles across the creeping segment of the San Andreas Fault near Hollister, California. The goal of the survey was to extend the measurements made in 1994 at Parkfield and Carrizo Plain (Unsworth et al, 1999) to a fault segment that is actively undergoing creep. This will allow the electrical structures of creeping and locked fault segments to be compared. Under a cooperative research program, a Department of Energy grant funded the data collection and this U.S.G.S. grant is supporting data analysis. The location of the two profiles is shown in Figure 1. The northern profile crosses the San Andreas and Calaveras Faults near Paicines. The southern profile is located in Bear Valley. A detailed seismic study of this area by Thurber et al, (1997) will allow a joint interpretation of electrical and seismic properties.

Results

The data collected in 1999 on the creeping segment is heavily contaminated with electrical noise from powerlines, cathodically protected gas pipes, water pumps and also the Bay Area Rapid Transit (BART). To remove these effects, the remote reference data simultaneously recorded at Parkfield will be needed. This work is in progress and should be completed by summer 2000.

An initial model for the southern profile in Bear Valley is shown in Figure 2. It should be noted that this model is very preliminary in nature. It was generated with data that were not processed with remote reference data. In addition, it was generated by inverting just the transverse magnetic (TM) mode data in which currents flow across the San Andreas Fault. Detection of a low resistivity wedge in the fault zone, as imaged at Middle Mountain, Parkfield (Unsworth et al, 1999; Unsworth et al, 2000) requires the careful interpretation of both the TM and TE (transverse electric) mode data.

References

Thurber, C., S. Roecker, W. Ellsworth, Y. Chen, W. Lutter and R. Sessions, Two-dimensional seismic image of the San Andreas fault in the Northern Gabilan Range, central California: Evidence for fluids in the fault zone, Geophys. Res. Letters, 24,1591-1594, 1997.

Unsworth, M.J., P.E. Malin, G.D. Egbert and J.R. Booker, Internal Structure of the San Andreas fault at Parkfield, California, Geology, 25, 359-362, 1997.

Publications