Tense moments between two quakes.

Tense moments between two quakes

Two earthquakes and a swarm of aftershocks struck California's Imperial Valley last week in a bout of crustal rearrangements that caused some scientists to worry that the so-called "big one' might follow on the heels of these smaller quakes. And this week, a quake registering at least 7.4 on the Richter scale of magnitude occurred in the Gulf of Alaska.

The larger of the California quakes, measuring 6.3 on the Richter scale, was centered on the Superstition Hills fault about 90 miles east of San Diego. The first earthquake, a 6.0 in magnitude, hit 11 hours earlier and 6 miles to the east, along an unnamed fault that trends northeast from the Salton Sea toward the southern branch of the San Andreas fault.

Scientists believe that the first quake triggered the second one. While this pattern is unusual, it is not unprecedented in the Imperial Valley.

These and most other earthquakes in the region result from the movement of two great crustal blocks that slowly slip past one another. As the northwest-moving Pacific plate slides against the North American plate, this motion is absorbed by the intricate faults in California and off its coast. Most of the strain from this plate motion is stored in the well-known San Andreas fault.

Though the earthquakes occurred on faults that lie 20 miles west of the San Andreas, scientists who monitored the aftershocks of the first quake became concerned when the cluster of temblors began to head toward the San Andreas. "We were definitely worried about the possibility of it going north and we were keeping a very close watch,' says Lucile Jones, a seismologist with the U.S. Geological Survey (USGS) is Pasadena.

By all estimates the southern section of the San Andreas fault is just waiting to break. Instead of sliding peacefully past one another, the two sides of the fault have spent 300 years locked together, storing up the potential energy equivalent to a magnitude 8 earthquake. Seismologists estimate a 50 percent probability that the fault will break in the next 20 years.

While Jones and others watched the aftershocks to see if they would unlock the San Andreas, the northeast-moving aftershocks turned around and headed back toward the southwest. "Then the [magnitude] 6.3 [earthquake] occurred on the Superstition Hills, and we had aftershocks in the Superstition Hills, and we sort of breathed a sigh of relief and said, "It looks like it's going south.'' Jones told SCIENCE NEWS.

The second California quake did not catch seismologists totally by surprise. Last year, Robert Wesson and Craig Nicholson of the USGS headquarters in Reston, Va., reported that this section of the Superstition Hills fault had remained noticeably quiescent in the last 20 years. "In a sense we predicted the position of the earthquake,' says Nicholson. "But we had no indication of how soon or how late such an earthquake might occur.'

The Alaska quake shook the ground for a full minute and sent thousands of people fleeing from low coastal areas. It did not trigger a major tsunami, or giant sea wave, as had been feared at first.