Soundly asleep in their beds, the residents of Yucca Valley and Big Bear Lake were not the only ones jolted by last month's early morning earthquakes. Scientists met with surprises, too.
"When I drove down there, I fully expected to find that a major east-west fault line called the Pinto Mountain fault had ruptured," says LBL geologist Pat Williams, who has been studying ancient fracture patterns along the Hayward fault (see Currents, June 29, 1990). Instead, the largest earthquake to occur in the contiguous 48 states in the last 40 years broke through four separate, lesser-known north-south fault lines in the Mojave Desert. "It let me know again that the Earth will always surprise you," Williams says.
The magnitude 7.4 Yucca Valley quake, named the Landers earthquake for the nearest, hardest-hit town, ruptured at 5 a.m. on June 28. A second one, 6.5 on the Richter scale, hit about three hours later, 30 kilometers west at Big Bear Lake, a ski resort in the San Bernardino Mountains. News reports that day said the two quakes were "unrelated," but reports the next day said they were related. Williams explains that although technically speaking, the second may have been too far away to be called an aftershock, the two earthquakes were clearly related in that the Landers earthquake lowered the compressional stress on the Big Bear fault and allowed it to rupture.
It also lowered the compressional stress across the central portion of the lower San Andreas fault. "It didn't go off, but the period of worry is not over," says Williams, who believes that the southern end will be the next big section of the San Andreas to rupture. He was concerned that slippage on the four Yucca Valley faults would set up a continuation north toward Barstow and south to the San Andreas via the Joshua Tree structure that slipped in April. If the San Andreas fault were to break as far north as the Cajon Pass (where Interstate 15 pierces the San Bernardino Mountains to connect Palmdale and San Bernardino) and south to the Salton Sea (which lies south of the Mojave Desert), Williams predicts the quake would be a magnitude 8.0 -- the Big One the experts are expecting.
The Landers earthquake is providing scientists with a wealth of information. "The Yucca Valley zone is very wide and complex, with lots of large and lots of small displacements," Williams says.
The fault rupture that produced the earthquake had little trouble stepping across the gaps that separated its four separate fault lines -- the Johnson Valley, Homestead, Emerson, and Camp Rock faults. Williams says in the past similar steps in many "dormant" faults have been considered probable barriers to slip. For example, a step near San Leandro has been considered a barrier to the rupture of the full length of the Hayward fault. "This conclusion," he says, "will now be viewed with added caution."
Williams and Earth Sciences technician Preston Holland flew to the area the day of the quake, rented a car, and crisscrossed the fault zone, providing the U.S. Geological Survey with reconnaissance mapping of a 29-kilometer area -- or about a quarter of the fault zone -- to guide their interpretation of aerial photographs. Often, the LBL geologists found two major boundary fractures, between 10 and 30 meters apart, with many diagonal faults in between.
"This is something many of us have seen before, but never on such a large scale," Williams says. "We'll learn a lot about the possible range of surface patterns, which should have application to the zoning of faults."
Holland was struck by the similarity between a fault structure he saw near Landers -- in which the fault takes a side-step to create a pull-apart depression and a compression ridge -- and an ancient structure revealed by a trench in Fremont where the LBL geologists are studying the Hayward fault.
"To see a surface break through that kind of feature gives us a better idea of what we might be looking at in the trenches," Holland says. "We've already started to interpret some of the features we see in the trenches a little differently based on what we saw at Landers."
A theory that was posed in the aftermath of last month's earthquakes was that a new San Andreas fault could be opening up from the Salton Sea north to central California. Williams says instead that the recent activity has linked the Gulf of California with slip systems east of Mount Whitney, the 1872 site of one of California's three historic magnitude 8.0 earthquakes.
Williams' theory is that the strain is heading toward the back side of the Sierras, northeast into central Nevada, where it may affect active geothermal fields. The earth's movement could enhance the production of geothermal fluids by contributing to a thinning of the crust and allowing the fluids to circulate through complex fractures.
A huge strain response on a scale never seen before occurred during the 24 hours following the Landers earthquake. At the Pinion Flats Observatory researchers from UC San Diego observed a massive redistribution of strain deep in the earth's crust. For the first time, Williams says, scientists will be able to study not just how the upper 12 kilometers of brittle crust reacts, but the response of the deep crust beneath it.
Seismic activity in California continues at a pace not unexpected
in the aftermath of such major shocks. Compared to the average
yearly rate of two magnitude 5.0 earthquakes in southern
California, there have already been at least 10 since June 28.
One that has caused some concern occurred a few days afterward
near Lathrop Wells on U.S. 95, about 10 miles from the potential
nuclear waste burial site at Yucca Mountain. Another 5.0 quake
just this Saturday shook the large east-west Garlock fault, which
forms the Mojave's northern boundary.
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