High-resolution electromagnetic imaging of the San Andreas fault in central California

Although there is increasing evidence that fluids may play a significant ro le in the earthquake rupture process, direct observation of fluids in activ e fault zones remains difficult. Since the presence of an electrically cond ucting fluid, such as saline pore water, strongly influences the overall co nductivity of crustal rocks, electrical and electromagnetic methods offer g reat potential for overcoming this difficulty. Here we present and compare results from high-resolution magnetotelluric (MT) profiles across two segme nts of the San Andreas Fault (SAF) which exhibit very different patterns of seismicity: Parkfield, which has regular small earthquakes and creep event s, and in the Carrizo Plain, where the fault is seismically quiescent and a pparently locked. In both surveys, electric fields were sampled continuousl y, with 100 m long dipoles laid end-to-end across the fault. From 100 to 0. 1 Hz the data from both profiles are consistent with a two-dimensional (2-D ) fault-parallel resistivity model. When both transverse electric and magne tic (TE and TM) mode data are included in the interpretation, narrow (simil ar to 300-600 m wide) zones of low resistivity extending to depths of 2-4 k m in the core of the fault are required at both locations. However, at Park field the conductance (conductivity thickness product) of the anomalous reg ion is an order of magnitude larger than at Carrizo Plain, suggesting much higher concentrations of fluids for the more seismically active Parkfield s egment. We also image structural differences between the two segments. At C arrizo Plain, resistive, presumably crystalline, rocks are present on both sides of the fault at depths below 3-4 km. In particular, we clearly image resistive basement extending similar to 10 km or more east of the SAF, bene ath the Elkhorn Hills and Temblor Range. At Parkfield the situation is quit e different with a resistive block of Salinian granite west of the fault an d an electrically conductive, presumably fluid rich Franciscan complex to t he east. It is possible that these structural differences control the diffe rence in mechanical behavior of the fault, either directly by affecting fau lt strength or indirectly by controlling fluid supply.