December Quake’s Effect on San Andreas Fault Estimated

December’s San Simeon earthquake has probably moved up the eventual date of a quake on the San Andreas fault in the Parkfield area in Central California, according to scientists who have studied stress levels since the temblor.

In a report prepared for an April meeting of the Seismological Society of America in Palm Springs, Malcolm Johnston of the U.S. Geological Survey said: “The San Simeon earthquake placed significant additional shear stress” on a portion of the San Andreas fault that sees fairly frequent seismic activity.

But in an interview, Johnston said any stress that the San Simeon quake had put on the fault was minor, compared with the stress created by the movement of the Pacific and North American tectonic plates.

The San Andreas fault marks the boundary between the two plates.

Johnston suggested that the magnitude 6.5 quake near San Simeon might have advanced an eventual quake on the San Andreas fault near Parkfield by a few months.

What the scientists are not able to say is when the quake will occur.

They know stress is accumulating, but they do not know how much is necessary to trigger a quake.

Jeanne L. Hardebeck, a scientist with the U.S. Geological Survey and principal author of a report on the San Simeon quake that will appear next month in the journal Seismological Research Letters, said she agreed with Johnston.

She said the quake might have reduced the time before the next quake near Parkfield by “less than six months.”

Another geological survey scientist, Bob Simpson, cautioned that the stress increase would be true only of the 30 miles of the San Andreas fault around Parkfield, but that both north and south of that area, stress on the fault would actually have been slightly eased, and the time until the next San Andreas quake in those areas would be slightly longer.

Simpson also said his calculations of stress on the Hosgri fault near the Diablo Canyon nuclear plant showed that, inland from Diablo Canyon along the fault, the San Simeon quake had increased the stress. But offshore in the Pacific Ocean, it had eased it.

Both Simpson and his geological survey colleague, Ross Stein, have concluded that the 1983 quake near Coalinga, magnitude 6.4, reduced stress temporarily along the San Andreas fault near Parkfield.

Both have written that the reduction in stress probably had much to do with invalidating a prediction made by scientists in 1985 that there was a 95% chance of a magnitude 6 earthquake occurring near Parkfield by 1993.

No such temblor has happened.