‘Shadows’ in Spotlight

In a scientific field filled with uncertainty, earthquake experts think they have a pretty good handle on where quakes are most likely to occur.

Now three U.S. Geological Survey scientists have begun identifying places in the Bay Area and, with less certainty, parts of Southern California, where the probabilities may be low. They hope this information will lead to a better understanding of where and when earthquakes will strike.

Bill Ellsworth, Bob Simpson and Ruth Harris have been mapping areas of reduced earthquake activity that they call “stress shadows” resulting from the 1906 San Francisco earthquake and the 1857 Ft. Tejon earthquake, north of Los Angeles.

They think these areas reflect strain reductions resulting from the temporary relief of stress by the great quakes.

A stress shadow is defined by Ellsworth as an area where a quake has “relaxed the strain, not just along the fault, but within the surrounding Earth’s crust, thereby making it harder for another earthquake to happen.”

His colleague, Harris, views these vicinities as “set back from their previous state. We’re not going to have an earthquake there until we get back to where they were before the last big quake occurred.”

While the effect in some cases may last a century or more, it is only temporary, the scientists say. It wears off as years pass and gradually disappears.

For small to moderate quakes, such as the 4.7 temblor that occurred May 21 near San Jose, the effects of stress release are neither large nor long-lasting. But for the great quakes, it is a different story.

The scientists acknowledge that their studies in the Bay Area over the last decade are more comprehensive than in Southern California, where their research began only in 1991. Their maps of where stress shadows lie are more definite in the north.

It is a matter of record that the Bay Area had 16 earthquakes of magnitude 6.0 or greater in the 70 years preceding the 1906 San Francisco quake.

But, aside from a 1911 earthquake of magnitude 6.5 south of San Jose, the Bay Area had no earthquake as powerful as magnitude 5.7 until the 1979 Coyote Lake quake near San Jose, which measured 5.9.

Maps prepared by Simpson and Harris show a stress shadow created by the 1906 San Francisco quake that extended through all the principal faults of the Bay Area--the San Andreas, the Calaveras, the Hayward and the Rodgers Creek. Only the San Andreas south of Monterey Bay had increased stress.

Today, however, the shadow has shrunk considerably. Quake activity in recent years has gradually crept northward in the East Bay along the Calaveras and Hayward faults. Other evidence of the shrinking of the shadow came in 1989 with the magnitude 7.1 quake at Loma Prieta, near the San Andreas fault in the Santa Cruz Mountains.

The shadow of reduced quake activity still remains along much of the San Andreas north of Monterey Bay, including the San Francisco area, the researchers say, and possibly along the northern segments of the Hayward fault under heavily populated Oakland and Berkeley.

But the scientists believe that an earthquake starting along southern stretches of the Hayward fault might propagate into these northern segments.

“The main point,” Ellsworth said, “is that the very low rate of large earthquakes that has persisted in the Bay Area since 1906 shows definite signs of ending.”

The scientists believe that the stress shadow on the northern Hayward fault will disappear “in the next decades,” if it has not done so already, and that it may vanish along the Rodgers Creek fault, north of the Bay Area, between 2030 and 2100.

In Southern California, the issue of where a shadow may persist is even more clouded, in part by the more common existence of deeply buried thrust faults.

Maps show that between 1857--the date of the Ft. Tejon quake--and 1907, most significant Southern California quakes took place south and east of the 1857 rupture, which extended from Parkfield to the Cajon Pass.

However, between 1907 and 1957, several desert quakes, and particularly the 1952 quake of magnitude 7.7 near Tehachapi, extended quake activity to the north and east of the 1857 rupture.

Between 1957 and 1996, many more quakes have occurred to the southeast of the rupture, although there remains a paucity of large quakes in certain areas along the San Andreas adjacent to the 1857 rupture.

The trend, Ellsworth says, suggests “the gradual disappearance of the 1857 shadow.” And with that, of course, would come a gradual heightening of earthquake risk.

Remarks Harris: “We sure would not want to say that some of the faults are still safer because of the great 1857 earthquake. We can say that that was the case until 1907 . . . and maybe even 10 years ago, but not now.

“We are currently calculating the effects of 1857 on specific ‘anticipated’ . . . earthquakes in the Los Angeles Basin, but we have not yet completed that study, which is to be done by the end of this year.”

The situation is also more complex in Southern California, Simpson points out, because the faults go in many different directions. In the Bay Area, in contrast, all the major faults are more or less northwest-southeast tending, and the structure of quake activity is simpler.

Strain studies after both the 1992 Landers-Big Bear earthquakes and the 1994 Northridge quake indicated that while stress had been decreased slightly in certain areas by those quakes, it was also increased in others.

One such study suggested that the Northridge quake occurred in an area of increased stress resulting from the 1971 San Fernando temblor.

Ellsworth observes, “The public does seem to understand that when an earthquake happens, it should make the chances of another at the same spot lower. Consequently, the one question that we are often asked immediately following almost any earthquake is if the event that just occurred reduced the chances of another one.

“In general, the answer is no, since the events in question aren’t large enough to have regional consequences. But our research is showing that for the big ones, [like San Francisco and Ft. Tejon], the answer is yes.”

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Stressed Out

In the general areas of California’s last great earthquakes- the 1906 San Francisco and the 1857 Ft. Tejon- the rate of large tremblors diminished sharply for many years thereafter. This phenomenon, which scientists call a “stress shadow,” represents an apparent relaxation of fault stress in these areas.

Three scientists with the U.S. Geological Survey have now mapped the regions- especially distinct in the Bay Area- where they say quakes remain less likely.*

In large parts of the Bay Area, a shadow of diminished quake activity persists 90 years after the 1906 San Francisco quake. On maps, blue lines indicate areas of reduced stress. Red lines show increased stress after 1906 quake, left, and as scientists think it is today.

* As usual in such matters, anomailies in quake patterns may exist.