4.7 Quake May Be Only a Precursor, Scientists Believe

Scientists with the U.S. Geological Survey said Tuesday that there is about a 30% chance that the 4.7-magnitude earthquake that rattled the Imperial Valley on Monday afternoon will be followed by a stronger quake within the next week.

But it will probably do little damage in the sparsely populated region, they said.

Lucile Jones, a seismologist in the Geological Survey’s Pasadena office, said there is no reason to believe that the recent swarm of earthquakes near the southern tip of Salton Sea is likely to trigger a major quake on the San Andreas Fault. The quakes have been occurring in a region called the Brawley Seismic Zone, and the largest quake on record in that area is a 5.7 temblor in 1981.

Scientists are monitoring the region along the southern leg of the San Andreas closely because of concerns that smaller quakes in the area could trigger a catastrophic quake on that long-dormant leg of the great fault. But the most recent swarm of earthquakes is not considered a likely candidate because they are too far from the San Andreas Fault.

Fault’s Location

“There’s something of a question as to where the San Andreas ends,” Jones said. The southern end of the fault is probably at least 10 miles north of the Brawley Seismic Zone, she added.

“If it extended down through the Salton Sea, this would be quite close to it, and we would really be worried,” she said.

Based on historical experience, if the 4.7 quake is a foreshock to a larger temblor, the main quake should occur no more than five miles from the epicenter of the 4.7 temblor--and probably much closer, Jones said. That would place the main shock--if it is yet to come--a safe distance from the San Andreas.

The quakes, which she described as “a very active sequence,” are occurring in a region known for earthquake swarms characterized by scores of small quakes that would be hardly noticeable even in the immediate vicinity and some that range up to magnitude 5. However, since the area is so sparsely populated, even the larger quakes usually produce very little, if any, damage.

The Brawley Seismic Zone is a bit of an oddity among fault systems in that it is believed to be a “spreading center” where the Earth’s crust is literally being torn apart as two giant tectonic plates slowly separate. Tectonic plates--the huge slabs of crust that support the continents and the oceans--crunch together in some parts of the globe, pull apart in others and slide under one another in still others.

In California, that process produces the San Andreas Fault because the Pacific Plate is sliding slowly north past the North American Plate.

Most spreading centers are under the oceans, but the Brawley Seismic Zone has been covered with sediment from the Colorado River and is thus located on land, making it a rare spreading center.

Although it is not expected to produce a great quake, residents of wide areas of Southern California should expect a little shaking for a while yet.