Hi All. Not to long ago someone asked a question about the Loam Prieta quake and questioned whether or not it occurred on the San Andreas Fault. Here is a answer although I’m sure it’s not the only one. Take Care…Don in creepy town.

Jachens and Griscom [1995] produced a comprehensive interpretation of the epicentral region of the Loma Prieta earthquake based on magnetic and gravity data and constrained by geologic and seismic information.

Their modeling experiments show a San Andreas fault system that deviates significantly from a simple vertical boundary between two major plates. Aftershocks indicate that the deeper parts of the fault surface in the epicentral region dip steeply to the southwest, but potential-field data indicate that in the shallow subsurface magnetic sediments of Pliocene age are thrust northeastward beneath rocks on the northeast side of the San Andreas fault.

A few kilometers south, the San Andreas is nearly vertical in the shallow subsurface, changing to a southwest dip farther south. Jachens and Griscom, [1994] concluded that most Loma Prieta aftershocks occurred along three major faults, the San Andreas, Zayante-Vergeles, and Sargent faults, and that the widely dispersed pattern of epicenters was the result of non-vertical and spatially variable surfaces on these faults.