SAN FRANCISCO — Earthquake experts delivered a double jolt to Southern California Tuesday with warnings that the San Gabriel Valley may be building toward a major earthquake and that the fault running beneath Hollywood and Santa Monica is more active than had been thought.
None of the scientists predicted an imminent earthquake, and the full significance of the research is not yet known. But the dual findings underscore the fact that the Los Angeles Basin is a highly complex--and very active--seismic zone.
The findings, by two teams of scientists from Caltech and the U.S. Geological Survey, were reported Tuesday during the winter meeting of the American Geophysical Union.
One team reported that a pattern of earthquakes over the past four years suggests that the Sierra Madre Fault may be due for an earthquake that could be greater than magnitude 7. That fault runs along the foot of the San Gabriel Mountains and passes beneath Pasadena, La Canada and several other communities.
The other team disclosed evidence that the Santa Monica-Hollywood Fault System, which many had thought had not been active in the past 3 million years, actually ruptured the surface beneath what is now Beverly Hills within the last few tens of thousands of years, and possibly more recently. That indicates the fault is more active than scientists had thought.
State officials who are familiar with the research said the findings so far are insufficient to provide a basis for predicting the time and size of any earthquake. The scientists did not disagree.
"We don't have a clear indication of when the earthquakes will occur," said state geologist Jim Davis.
The suggestion that the Sierra Madre Fault may be building toward a large earthquake was based on a pattern of six earthquakes of greater than magnitude 4.5 that have struck near Pasadena in the past four years, including a magnitude 5.8 jolt in June. No quakes of that size had been recorded in that area in the preceding 50 years.
Lucile Jones, a seismologist with the Geological Survey, said those earthquakes formed a pattern that is particularly troubling. Most earthquakes in California hit very close to the surface, but these were relatively deep. They were centered nine to 10 miles below the surface.
Furthermore, the quakes have been moving generally in a northerly direction. The first was the 1987 Whittier Narrows quake, magnitude 5.9, followed by the 4.7 Upland quake, June 26, 1988; the Pasadena quake, 4.9, Dec. 3, 1988; the Montebello quake, 4.5, June 12, 1989; another Upland quake, 5.2, Feb. 28, 1990, and the Sierra Madre quake, 5.8, June 28, 1991.
Jones said the pattern suggests that the deeper crust is sliding beneath the San Gabriel Mountains, but the upper crust is locked and accumulating strain.
If so, that strain could result in a large earthquake.
"These types of clusters have been seen before," she said. "Sometimes, they have been followed by a large earthquake. But sometimes, they haven't."
However, the long period of seismic inactivity before 1987 is particularly troubling because it means that strain has been accumulating in that region for some time. The recent deep earthquakes may have relieved the strain far below the ground, but the surface rocks are still "highly stressed," Jones said.
Jones said she would not be surprised to see that stress relieved through a large earthquake on the Sierra Madre Fault.
Other scientists working on the project are Hiroo Kanamori, director of Caltech's Seismological Laboratory, and Egill Hauksson, also of Caltech.
Across town, seismologists have long known about the Santa Monica-Hollywood Fault System, which runs east-west along the southern margin of the Santa Monica Mountains. Until now, however, the fault had been thought to be a "blind thrust fault," a type that does not rupture the surface and thus is hidden under the ground. And there was no evidence that the fault had moved within the last 3 million years.
Such a long period of inactivity would suggest that the fault may no longer be much of a hazard, but James F. Dolan and Kerry E. Sieh of Caltech shattered that illusion Tuesday.
By examining old topographic maps and aerial photographs that predate the development of that region, the two scientists found evidence of earthquake "scarps"--ridges on the surface of the ground that were created by movement along a fault.
After locating the scarps on the old photos and maps, Dolan and Sieh walked through the area looking for banks and ridges that bear the signature of earthquake scarps.
"I have spent a lot of time walking around the area, and in many places you can see the original scarps" created by fault movement, said Dolan. Sieh said the scarps stand out because they are not parallel to the streets, are of irregular heights and can't be explained by natural processes such as erosion.
To the untrained eye, a scarp looks like any other small hill, but "if you have the magic eyes of a geologist it's very obvious," Sieh said.