ELECTRICAL-CONDUCTIVITY VARIATIONS AROUND THE PALMDALE SECTION OF THE SAN-ANDREAS FAULT ZONE

Our analysis of a decade of observations from the Palmdale California telluric array is presented. The array is a system of large-scale telluric field measurements used for the passive monitoring of electrical conductivity variations over a large region centered over the San Andreas Fault Zone northeast of Los Angeles. Since 1985, systematic changes in the relationships between the telluric field measurements around the array have been observed. These changes amounted to 1% to 2% by the time the array was closed down in 1989, with the fields measured along the fault and northeast of the fault showing signal decreases relative to the fields southwest of the fault. Because of the possible implications of these changes for conductivity variations related to tectonic activity on the San Andreas, we have attempted to son out the geography of the zones that could cause the changes. The simplest, though nonunique, interpretation of the observed variations in the telluric relationships is a steady increase in the conductivity of the lower crustal region of the fault zone. From two-dimensional models of the telluric response around the array, we infer that an increase of about 60 milli-Siemens in the conductance of the lower crustal fault zone could account for the changes seen over the period 1985 to 1989. Assuming changes were in a kilometer-wide zone and that the lower crustal fluids were very conductive, this could represent a strain of about 0.4 x 10(-6). The surface strain would be much smaller and would be lost in surface strain noise. Because of the possibility that fluid pressure changes are involved, these small strains could still be significant for earthquake cycle phenomena. Alternatively the observed changes could have been produced by increases in the conductivity of the upper crust of the Mojave Desen, northeast of the fault, or by decreases in the conductivity of the upper crust in the San Gabriel Mountains southwest of the fault. To produce the observed telluric changes, however, the additional conductivity and the associated strain changes would have to be much larger. The strain needed to accommodate the increased conductivity on the Mojave side would have to be of the order of 6 x 10(-4), which seems unrealistically high. Smaller strains could account for conductivity changes in the San Gabriel Mountains, but these strains would have to be compressive. Water-level changes are not considered to be a possible cause of the conductivity changes because the measurement scale is much larger than the typical scale of water level changes, and no seasonally correlated changes in the telluric fields were observed.