Wait for Quake No Reason to Quiver

It has been about 325 years since an earthquake of magnitude 7.0 or higher hit the Inland Empire area. So that means the next big quake is “overdue” in the region east and south of the Cajon Pass, right?

Not necessarily, say most of the prominent researchers who in recent years have studied the two major faults in the Inland Empire.

Those faults are the San Andreas, which runs from the Cajon Pass to Bombay Beach on the Salton Sea, and the San Jacinto, which runs from its junction with the San Andreas near Cajon Pass to the Imperial Valley.

The reason, they explain, is that research indicates no pattern of quakes in the region, although the very long-term average is about 225 years. Hundreds of years can pass without a “big one,” and then there might be three within 30 years. What scientists call “recurrent averages” are based on the evidence from trenching along faults and calculations of their slip rates going back thousands of years, not simply the written records of the last 200. Paleoseismic records developed from carbon dating are also used.

A slip rate is usually stated in annual terms, but that does not mean the fault slips every year. It means that in, say, 100,000 years, the fault has slipped so much, and the rate is divided by 100,000 to show an average.

Tom Fumal, a researcher with the U.S. Geological Survey in Menlo Park, says, “I would not use the word ‘overdue’ to describe a quake. Earthquakes do not have a due date, and the average time is variable between limits. So I wouldn’t say a big quake [in the Inland Empire] is overdue. But the probability is high.”

One thing is clear: What will happen in the future is based to some extent on the past. Quakes, like volcanic eruptions, happen in the same areas time and again, though the intervals can be very long in human terms.

Sally McGill, geology professor at Cal State University San Bernardino, points out that in 1994, a working group of scientists under the auspices of the Southern California Earthquake Center estimated a probability of a quake of at least magnitude 7.3 along the San Andreas fault between the Cajon Pass and the San Gorgonio Pass at 28% before the year 2024.

At the same time, McGill said, the working group estimated a probability of 37% for a quake of at least 6.7 on the San Jacinto fault in the San Bernardino-Riverside area by 2024.

But the fact that nearly a decade has passed since those probabilities were released means the chances of such quakes by that time are reduced.

A 7.3-magnitude quake would be substantially larger than any that have occurred in the Los Angeles Basin since the city’s founding in 1781.

McGill called the San Bernardino Valley “one of the areas of highest earthquake hazard in the state, although perhaps not the highest.”

Some scientists believe that distinction should be reserved for the East Ventura Basin straddling Ventura and Los Angeles counties.

But, McGill added, the fact that there has been no major quake south and east of Cajon Pass since about 1680 “in my mind makes [the Inland Empire] the most likely to have a big quake next.”

However, Ray Weldon II, a professor at the University of Oregon who has studied seismicity in the Inland Empire, cautions that techniques of dating past quakes are too uncertain to even be sure of the date 1680 for the last big one there.

“I’d say 1690, give or take 20 years,” he said.

Weldon noted that Caltech’s Kerry Sieh and Lisa B. Grant, a professor at UC Irvine, have argued that a megaquake in about 1480 may have ruptured the San Andreas all the way from Parkfield in central California to the Salton Sea.

And, he added, some researchers have concluded that a magnitude 7.0 quake on the San Andreas in 1812 “extended from Palmdale to somewhere close to San Bernardino and maybe from Ft. Tejon to the San Gorgonio Pass.”

Grant said scientists often need to be modest in their assessments.

The San Andreas fault, as it passes through the San Gorgonio Pass into the Coachella Valley, splits into “several different traces there,” she said. “That is the area we know the least about.

“The 1680 quake was fairly sizable.... But how big was that quake and how regularly do such ruptures occur? That we don’t really understand.”Katherine Kendrick, a U.S. Geological Survey scientist who has her principal office in Riverside, said, “It is difficult without a lot more investigation to say what the hazards are in the Inland Empire.”

She noted the scientists have two primary ways of researching earthquake records -- either calculating the slip rate by measuring offsets of streams, rock formations and other geological features, or through paleoseismic investigation, using radiocarbon dating to try to determine the intervals between large quakes.

But, she noted, because the Inland Empire contains large desert areas, there is not much material for carbon dating, and in any case, such dating “is not more reliable than within 100 years” and can easily mix big quakes when they occur relatively close together in distance and time.

There have been some large recent quakes nearby, such as the 7.3 Landers temblor in 1992 and the 7.1 Hector Mine in 1999, but both were in the Mojave Desert, outside the area generally considered the Inland Empire.