New Data Show L.A. at Higher Quake Risk

Downtown Los Angeles is drifting toward Pasadena at a rate of one inch every five years, squeezing the land between in a massive vise that could cause more earthquakes than had been expected in the region, JPL scientists using new satellite measurements report today.

Although quakes triggered by the powerful compression are likely to be significantly smaller than those associated with the nearby San Andreas fault, they will probably do just as much damage because they will occur in more heavily populated areas, the researchers report in the journal Geology.

In the long term--over a period of millennia--the compression will produce a new mountain range in Los Angeles just south of the San Gabriel Mountains, they said.

The newly measured rate of Los Angeles’ drift toward the mountains, about 20% of the rate of slippage along the San Andreas fault, “is relatively high for that short stretch of ground,” said Caltech geologist Thomas Heaton. “It’s clear that it has important consequences for earthquake hazard in the region,” he added.

Researchers have known for more than a decade that the Los Angeles Basin is slowly being squeezed as the massive tectonic plate that contains the western coast of California slips northward toward Alaska. In the past, that squeezing produced the San Gabriel Mountains.

It was also an underlying cause of the magnitude 6.6 Sylmar earthquake in 1971 and the magnitude 6.7 Northridge temblor in 1994, according to Donald Argus and his JPL colleagues. More such quakes can be expected in the future, the scientists said, although it is not yet possible to guess at their frequency.

But it has been difficult to get good measurements of the modern rates of compression.

That problem is quickly being overcome with the use of the new Global Positioning Satellite system, which allows extremely accurate measurement of any point on the earth’s surface.

Seismologists have been establishing a system of positioning sensors throughout Southern California. At the time of the Northridge quake, there were only four permanent stations in California. Today, there are nearly 120 and researchers hope to double that number within the next couple of years.

The new measurements from JPL are among the first fruits of that system. The results are from 60 such sensors whose motions have been charted for three to five years.

The team reports that the compression is occurring in a broad band from three to 25 miles wide that stretches through the Ventura Basin, the San Fernando Valley, the east San Gabriel Valley and the area between downtown Los Angeles and Pasadena.

“This probably means that there is a greater likelihood of a quake inside this [band] than at other places outside of it--with the caveat that this is over the next several hundred years,” Argus said Sunday.

San Diego researchers had reported last year that some of the compression in the Los Angeles Basin is being relieved by a slight lengthening of the basin along an east-west axis. In other words, they concluded that Los Angeles is getting shorter and wider.

But the new, more accurate data indicates that less east-west lengthening is taking place to relieve pressure. In that case, the pressure can only be relieved by the ground bulging upward, leading to earthquakes and mountain-building.

Two years ago, researchers reported that a large part of the San Fernando Valley had thrust upward almost half a foot over two years.

In that case, the movement was not accompanied by earthquakes. Next time, researchers say, residents may not be so lucky.