The electrical conductivity structure between the transitional (near SAFOD) and locked (SE of Cholame) segments of the San Andreas Fault, including the source region of the non-volcanic tremors

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The DeepRoot magnetotelluric (MT) experiment near the San Andreas Fault (SAF) Observatory at Depth (SAFOD) revealed a steeply dipping upper crustal high electrical conductivity zone flanking the seismically defined SAF to the NE, widening into the lower crust where it appears to be connected to a broad anomaly in the upper mantle. Becken et al. (2007a) suggested that the high conductivity represents a deep-rooted channel for crustal and/or mantle fluid ascent. Deep (~20-40 km), non-volcanic seismic tremors (NVT) have only been observed in an area ~40 km SE of the SAFOD where they are thought to originate from oscillation of fluids. The presence or absence of NVT appears to coincide with the transition of the SAF from being locked (Cholame) to intermediate creep (SAFOD) and could reflect significant structural changes affecting the deep hydraulic system along this portion of the SAF which in turn could be detectable with MT. Here, we propose an experiment to image an entire segment of an active plate boundary with a network of MT stations, from the Pacific into the Great Valley, crossing the NVT region beneath the SAF near Cholame. These results are important for the site characterization of the SAFOD as along-strike variations of the conductivity structure may reflect deep structural changes controlling the still debated mechanical state of the SAF.