Scientists Fear Southland Quake With Double Punch

It would be a rare event--perhaps occurring every 1,000 to 3,000 years--but a group of scientists believe that a Southland earthquake could simultaneously rupture the San Andreas fault and the thrust faults along the San Gabriel Mountains.

This would be equal to the 1857 Fort Tejon quake, the Northridge, the San Fernando, the Sierra Madre and the Upland quakes all rolled into one--a real “Big One.”

It would combine a strike-slip quake, the horizontal movements associated with the San Andreas fault, with the vertical thrust movements associated with the 1994 Northridge quake.

This scenario is suggested in an article in the July issue of the journal Geology, written by seven scientists--two Americans, a Russian and four Mongolians.

They draw parallels between the 1957 Gobi-Altay earthquake, which had a magnitude of about 8, in Mongolia--where a strike-slip quake did trigger virtually simultaneous rupture on nearby thrust faults--with Southern California.

“The local tectonic setting of the 1957 Gobi-Altay earthquake closely resembles that of the San Gabriel Mountains,” the scientists write. “A sharply defined strike-slip fault, analogous with the San Andreas fault, follows the northern margin of a high mountain, Ih Bogd. . . .

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“A basin, analogous to the Los Angeles basin, lies south of Ih Bogd; the boundary between the basin and the high range is marked by a system of thrust faults, like those at the southern edge of the San Gabriel Mountains. . . .

“The high terrain in both regions . . . has formed where jogs or bends [occur] in the predominantly strike-slip regime. . . . Thus, in both areas the two systems of faulting are intimately related.”

In interviews, both American authors, Kenneth W. Hudnut of the U.S. Geological Survey’s Pasadena office and Peter Molnar of MIT, cautioned that a simultaneous quake would not occur very often.

Both also conceded that there is no proof that such a quake has ever occurred in Southern California, although they say there may be indications of some thrust fault slip in the magnitude 8 Fort Tejon quake.

Molnar said that an estimate in the article that thrust slip occurs in perhaps 10% of great earthquakes along the San Andreas “is a crude number” and not exact.

The article says great San Andreas quakes occur “roughly once per century,” but Hudnut said that in Southern California’s portions of the fault they happen only between 200 and 300 years.

In Northern California, the scientists noted, the 1989 Loma Prieta earthquake had thrust components very close to the San Andreas, although the magnitude of that quake, 7.1, was not large enough to be comparable to the giant quake that the scientists are discussing in the article.

There has been considerable speculation among earthquake scientists that the series of eight significant earthquakes that have occurred along the San Gabriels since 1970 could presage an eventual San Andreas quake.

But Molnar said the kind of quake the scientists discuss in their article would probably happen instantaneously, with a rupture on the San Andreas triggering ones on the lesser faults.