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Proceedings of the National Academy of Sciences of the United States of America
detecting thin conductive zones in the lower crust. Normally, it is difficult to see such zones in the lower crust because of the higher conductivity of the upper crust. However, near the coast there is a lot of extra electric current coming from the ocean. These currents gradually leak into the mantle, and they will use conductive zones in the lower crust to do so. One can detect this leakage because the ocean current has a different phase than the normal continental currents, and the leakage of the ocean current will change the phase of the currents left in the upper crust. Fig. 1 shows an array of MT stations across California from Parkfield to the Basin and Range. Fig. 2 shows the MT phase as a function of period for the transverse magnetic (TM) mode (electrical field perpendicular to the coast line) across this array. One can see a big change of phase between Parkfield and the Great Valley and between the Great Valley and the Sierra Nevada, which identified two lower crustal leakage zones (18). The Parkfield measurements were on the northeast side of the SAF, so the leakage was not involved with the fault zone.
After the Loma Prieta earthquake we made MT measurements (19) across the fault zone in this area as shown in Fig. 1. In Fig. 3 we show the TM phase for two stations (2 and 1) which have the earthquake zone between them. To model the phase difference at the longest periods, we had to put a lower crustal leakage zone underneath the fault zone. We cannot tell
FIG. 1. Map showing the major tectonic provinces of California and the locations of several MT stations. H, Hollister; F, Fresno; P, Palmdale; PK, Parkfield; TL, Tulace Lake; EX, Exeter; MK, Mineral King; OV, Owens Valley; EV, Eureka Valley. (Upper Inset) Data profiles collected around the Loma Prieta Fault zone. (Lower Inset) Palmdale telluric array. Source: ref. 21.