I was at Lamont-Doherty working on Ross Sea Antarctica when the quake occurred, so went by the seismology building where Art Lerner-Lam was being interviewed by CBS radio. I've been to that are on a field trip during 1990 run by the consultants doing work related to Diablo Canyon nuclear power plant.

First, the M3+ quakes on the San Andreas fault farther south are no longer on the map, so probably did not happen.

There is still a M4.4 offshore, but it has not been checked by a seismologist. As Don said, it is a thrust earthquake. The fault strikes (is aligned) WNW-ESE according to the focal mechanism, and the aftershocks. Since both nodal planes have the same strike, I can not tell if the dip is to the NNE or the SSW (but could if I had these in 3D). This earthquake represents NNE-SSW contraction. This is a typical direction for the central and southern California margin.

The San Simeon fault comes ashore just west of the quake. But, the quake was not on the San Simeon fault-because the San Simeon fault strikes NNW-SSE and is a right-lateral fault, active in the 1-3 mm/yr range (the San Andreas fault at this latitude is active at about 33 mm/yr). This thrust earthquake is perfectly consistent with right-lateral slip on the San Simeon fault. The San Simeon fault is just part of the San Gregorio-San Simeon-Hosgri fault system. The San Simeon steps over offshore south of San Simeon area to the Hosgri fault. The Hosgri fault extends over 100 km SSE. Its seafloor trace is located a few km west of Diablo Canyon nuclear power point, and it may dip E beneath the plant. When PG&E found out that this fault existed beneath their partly built (?) plant they had to redesign/rebuild it. I think this cost about $2 billion, which they passed on to their rate payers.

So, did this quake advance or delay a quake on the San Simeon Hosgri fault? I don't know-they will model this in the months to come. But, if a 100 km chunk of fault failed, it would be a M7+ (maybe 7.5). If the average slip during such a quake was, say, 2 m, at 2 mm/yr the recurrence interval would be 1000 years-so big quakes on these faults are not that common.

The PG&E consultants (e.g., Bill Lettis and others) interpret the ranges in the area to be bounded by reverse faults. It is likely that a large part of one of these range-bounding faults failed in this quake-by comparison to Northridge it would be about a 15 km length-I guess I could measure the aftershocks on the map if I could get to this page....Chris