Non-technical Summary

Magnetotelluric data were collected on the Parkfield and Cholame segments of the San Andreas Fault in Central California in 1997. The goal of the survey was to image the electrical resistivity structure of the fault zone. Since the electrical resistivity in the earth is very sensitive to the presence of interconnected fluids, this is parameter can help delineate the distribution of fluids within the fault zone. The data have been interpreted and show a fluid rich fault zone extending to a depth of 2-4 km on each profile beneath Middle Mountain, Parkfield.

Investigations undertaken

Broadband magnetotelluric data were collected on 3 new profiles across the San Andreas Fault in Central California between September 23 and October 29 1997. The goal of the survey was to extend the measurements made in 1994, and to see if the low electrical resistivity observed close to the 1966 hypocenter is typical of the entire Parkfield segment. 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.

Two of the profiles crossed Middle Mountain on either side of the profile collected in 1994 (Unsworth et al, 1997). The third profile crossed the San Andreas Fault in the Cholame Valley, some 5 km northwest of Highway 46. Each profile consisted of continuous profiling close to the fault (measurements every 100 meters) and single MT sites away from the fault zone. The location of the profiles is shown in Figure 1. The data from 1994 and 1997 magnetotelluric surveys have been combined and analyzed using a combination of forward modeling and inversion studies.

Results

Figure 2 shows electrical resistivity models for the 3 profiles on Middle Mountain at Parkfield. These models were obtained using the 2-D magnetotelluric inversion code developed by Randy Mackie, and represent the maximum depth extent of the fault zone conductor. Other inversions were performed to determine the minimum depth extent of the conductor, which was found to be around 1500-2000 meters. Figure 2 also shows a preliminary interpretation of the fault zone structure. The low resistivity, fluid rich, fault is present along the length of the seismically active Middle Mountain. The active creeping trace is consistently located on the east side of this conductor. Note also that the fault zone conductor appears to deepen to the north as the creeping central segment is approached. To the east of the SAF the Franciscan formation (KJf) has a low but variable resistivity, that is presumably due to it's high fluid content. The depth at which these fluids enter the San Andreas fault remains unclear.

The data collected in Cholame Valley have also been inverted and the resulting resistivity model is shown in Figure 3. Note the shallow, 1 km deep, basin in the Cholame Valley and the resistive Salinian granite (Kg) to the west of the San Andreas Fault. Other models suggest that a weak fault zone conductor may lie between the Cholame and Parkfield segments of the San Andreas fault in the region of the en echelon offset. The maximum depth extent of this zone is constrained to be 2 km. Figure 4 shows a comparison of the resistivity model with the seismic data collected during the mini-SOSIE experiment (Shedlock et al, 1991). Note that the base of the 1 ohm-m zone correlates well with the base of seismic reflections, showing that basement rocks dip west towards the Cholame segment.

These results have confirmed that in the upper 4 km the San Andreas Fault at Parkfield has a lower resistivity than the Cholame and Carrizo segments to the south. This can interpreted as implying a higher fluid content in the seismically active Parkfield segment. In the coming year this grant will support analysis of MT data that will be collected on the creeping segment near Hollister in 1999. This will allow us to compare the electrical structure of the full range of fault behavior, from the locked Carrizo segment to the creeping segment at Hollister.

References

Shedlock, K.M., T.M. Brocher and S.T. Harding, Shallow structure and Deformation along the San Andreas Fault in Cholame Valley, California, based on High-Resolution Reflection Profiling, J. Geophys. Res., 95, 5003, 1991.

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