how wide the zone was, but if it were 5 km wide its conductivity would be about 300 times greater than the average lower crust conductivity. The MT data from stations 4 and 3 also showed leakage, but the other stations, which were further to the southeast, had less leakage. Our old telluric data from Hollister did not show leakage across the SAF. On this basis we think the leakage was created by the Loma Prieta earthquake. (A lot of fluid flow was seen at the surface at Loma Prieta after the earthquake.) It is hard to believe that fluids could have gone down into the lower crust on a short time scale, so probably the fluids were there before the earthquake, but not well connected. In that case the fluid pressure would be close to the rock pressure, and if these fluids can get better connected they may play a role in modifying the fluid pressure in the fault zone in the upper crust, which is a factor in the strength of the fault zone.
At Loma Prieta we did not have MT measurements on a long time scale, but at Palmdale (Fig. 1) we had telluric measurements (20) for 11 years (1979–1990). Starting at about 1985, changes in the telluric relationships took place across the array (Fig. 4). Since two dipoles have to be used as references, the relative changes across the array are not unique, but, using the smallest changes that fit the data, we see a lowering of the telluric signals on the dipoles on the fault (A and C), and northeast of the fault (B), compared with the dipoles southwest of the fault (on the ocean side). The variations relative to dipole D from 1984 to 1989 were −0.6%, −1.3%, and −1.9% for dipoles A, B, and C and −0.1% and 0.2% for dipoles F and H. A doubling of the lower crustal conductivity in a 5-km-wide zone would create these changes. The increase of the conductivity is probably due to a better connectivity of the lower crustal fluids. If this keeps up, there is the possibility that the lower crust will influence the upper crust fault zone fluid pressure, but at this stage we cannot say too much.
1. Lachenbruch, A.H. & Sass, J.H. (1980) J. Geophys. Res. 85, 6185–6222.
2. Zoback, M.D., Zoback, M.L., Mount, V.S., Suppe, J., Eaton, J.P., Healy, J.H., Oppenheimer, D., Reasenberg, P., Jones, L., Raleigh, C.B., Wong, I.G., Scotti, O. & Wentworth, C. (1987) Science 238, 1105–1111.
3. Byerlee, J.D. (1990) Geophys. Res. Lett. 17, 2109–2112.
4. Rice, J.R. (1992) in Fault Mechanics and Transport Properties in Rocks, eds. Evans, B. & Wong, T. (Academic, New York), pp. 475–503.