The quake gap: waiting for magnitude 7.

The quake gap: Waiting for magnitude 7

The aftershocks grew progressively weaker during the five days following eastern California's 6.1 magnitude earthquake on July 21 -- signaling an end to the week-long swarm of seismic activity in the Chalfant Valley. Besides the main shock, there had been three quakes above magnitude 5 -- a 5.5 magnitude foreshock and 5.2 and 5.1 magnitude aftershocks. (According to seismologists, none was related to the 5.9 magnitude quake that struck near Palm Springs on July 8 or the 5.3 quake that hit the ocean near Oceanside on July 13.)

Now, the Chalfant Valley might remain quiet for weeks, months or even years. But before another 30 years go by, it's likely to get a much bigger quzke -- a ground-splitting magnitude 7 quake, seismologists say.

They base this prediction on the valley's location along a 300-mile, north-south line, stretching from northern Nevada to east-central California, in which magnitude 7 earthquakes have occurred periodically since at least 1872. The Chalfant Valley, 10 miles north of Bishop, is the largest link in the line that so far has not been fractured by a magnitude 7 quake. Because the valley lies along the western side of the White Mountains, it has come to be known as the White Mountain gap in the earthquake chain.

Alan Ryall of the Center for Seismic Studies in Arlington, Va., first identified this gap in the mid-1960s, while working at the University of Nevada at Reno. The line, known as the central Nevada seismic zone, begins near Winnemucca, Nev., where a magnitude 7.8 quake hit in 1915, and stretches south to the southern Owens Valley in eastern California, where a quake estimated at magnitude 8 struck in 1872, Ryall says. The Owens Valley quake cut a north-south escarpment nearly 50 miles long--and in some places 10 feet high--between Big Pine and Lone Pine.

The White Mountain gap lies just north of this escarpment and south of the Cedar Mountains of Nevada, where a 7.2 quake occurred in 1932. North of the Cedar Mountains is the Dixie Valley, where a 7.1 earthquake struck in 1954.

The swarm of quakes that shook the Chalfant Valley in late July was enough to damage most homes in the area but not enough to split the ground to match the rest of the line. "It would take a magnitude 7 quake to fill the gap," Ryall says.

Additional evidence that a major earthquake may be imminent is the White Mountains' steep rise from Chalfant Valley, according to Edward J. Corbett, research seismologist at the University of Nevada at Reno. The mountains rise nearly 10,000 feet within 10 miles of the valley floor, which demonstrates that the area has experienced significant seismic activity throughout the last 3 million years.

Some seismologists find further evidence for a quake prediction in the fact that seismic activity during the past few decades has tended to circle the center of the White Mountain gap. This suggests the pattern of a "Mogi Doughnut," a ring of seismic activity that often precedes a great quake. But the Mogi Doughnut theory may not apply, Corbett says, because it was developed for Japan, where earthquakes are caused by one of the earth's plates pushing under another. In eastern California, earthquakes occur in the "basin and range" faults that lie alongside the mountain ranges, Corbett says.

None of this evidence helps seismologists pin down exactly when the magnitude 7 quake will occur. "We just know there is an obvious gap in the historic pattern of seismicity in the western Great Basin," Ryall says. "And in this area, there has been a pattern of seismic activity that in other areas has preceded large quakes."