Ruth A. Harris and Robert W. Simpson of the USGS have applied stress-transfer analysis to the 1992 magnitude 7.5 Landers earthquake, which originated near Palm Springs, Calif. They find that the Landers rupture partially "unclamped" the San Andreas fault near San Bernardino, east of Los Angeles. This unclamping brought the date of the next earthquake--expected to be up to a devastating magnitude 8--about 14 years closer than would otherwise have been the case. Because a precise seismic history for the San Bernardino segment is lacking, Harris and Simpson have not translated this estimate into a probability forecast. Even without their analysis, however, the area has been rated as among the most hazardous in the U.S. It has a 60 percent chance of experiencing a damaging earthquake before the year 2024, according to the Working Group on California Earthquake Probabilities.
The Landers earthquake also increased the stress in a zone extending northeastward to the Mojave Desert--an area struck by the October 16 magnitude 7.1 earthquake. This so-called Hector Mine earthquake, which caused little damage thanks to its remote location, further demonstrates the influence of transferred stresses on future ruptures, Stein says.
The stress-transfer approach is valuable but incomplete, according to geophysicist Steven M. Day of San Diego State University. A fully adequate description of earthquake behavior, he says, needs to incorporate dynamic processes that are ignored in the static stress-transfer models. The actual shaking of the earth ahead of an advancing rupture, for example, may permit the rupture to extend for a greater distance than the static models would predict, thus unleashing a more powerful earthquake. Day believes that it may be premature to use the results of stress-transfer analysis for the routine estimation of seismic hazards.
Still, Stein and his group are busy thinking about what may happen next on the North Anatolian fault. According to their preliminary calculations, the Izmit earthquake has increased stresses on the Yalova segment of the fault, which runs westward across the floor of the Sea of Marmara, southeast of Istanbul. Consistent with this finding, the rate of small earthquakes under the Sea of Marmara has increased markedly since the Izmit temblor. An earthquake on the Yalova segment could devastate Istanbul. "If you look at the records for the 1,000-year-old Hagia Sophia mosque," Stein says, "you'll see that it's a seismometer--they've had to rebuild it over and over again. It's not rocket science to say that Istanbul is at risk."