Earthquakes and Seismology

Susan E. Hough

After a spate of large, damaging earthquakes in late 1999 left both seismologists and the public reeling, the year that followed seemed pleasantly dull by comparison. The year 2000 was not without large seismic events, however. In fact, five quakes of magnitude 7.8 to 8.1 occurred — a number of earthquakes that were significantly larger than those in 1999. But all struck in relatively lightly populated regions of the South Pacific and South Indian Ocean, including three over the course of two days near Papua New Guinea. A magnitude-8.0 event near Sumatra, Indonesia, was the most deadly earthquake of 2000, causing at least 103 fatalities and more than 2,000 injuries.

A good deal of seismological research last year focused on issues raised by three of the 1999 earthquakes. The unprecedented volume of high-quality data produced by the magnitude-7.7 earthquake in Chi-Chi, Taiwan, on Sept. 20, 1999, is allowing seismologists to investigate the complexity of earthquake rupture processes and ground motions in far more detail than has previously been possible. These studies will ultimately improve our ability to design structures to withstand strong shaking, a legacy of hope resulting from the tremendous foresight and investment of the Taiwanese government.

[To left: An intriguing study by Gao and others (Nature, v. 406, p. 500-504) found that the remotely triggered seismicity may continue for some time. This graph shows declustered seismicity in the western United States from Jan. 1, 1998, back to Jan. 1., 1997.]

Two other 1999 events — the earthquakes in Izmit, Turkey (Aug. 17) and at Hector Mine, Calif. (Oct. 16) — posed interesting challenges last year for developing theories of earthquake interactions and “stress triggering.” The Izmit disaster, and the subsequent magnitude-7.2 earthquake in the nearby city of Düzce on Nov. 12, occurred in a region identified as high risk for large seismic events, based on basic tenets of static stress transfer (Stein and others, Geophysical Journal International, v. 128, 1997, p. 594-604). During 2000, researchers have sought to explain the two-month lag between the Izmit and Düzce events. Their studies have generally focused on post-seismic relaxation processes, whereby the deep crust and upper mantle respond to a large earthquake in the crust over months to years. In some cases, this process augments the instantaneous stress transfer associated with a large event. The earthquakes in Turkey provided an apparent confirmation of developing theories; they also raise concern for faults near Istanbul, which are now more stressed.

By comparison, the Hector Mine earthquake appeared to pose a challenge for the stress-triggering premise, as simple theories of static stress transfer cannot easily explain how this event could occur just seven years after (and in close proximity to) the magnitude-7.3 earthquake in Landers, Calif. Once again, researchers are focusing on post-seismic relaxation processes, in this case as a mechanism to trigger a subsequent event in a region where stress was not necessarily increased immediately after a first mainshock.

In general, the issue of earthquake interactions remained at the forefront of seismological research in 2000. This research focused on both static and on dynamic stress changes associated with the seismic waves from large events. Dynamic stress changes are generally thought to be the cause of so-called “remotely triggered earthquakes” — subsequent events that take place at great distances (from a few hundred kilometers up to 1,000 kilometers) and from earthquakes greater than magnitude 7.

The Hector Mine earthquake, which was similar to Landers but with opposite directivity, provided a highly fortuitous “repeat experiment,” resulting in further evidence that dynamic stress changes are responsible for remotely triggered events. An intriguing study by Gao and others (Nature, v. 406, p. 500-504) found that remotely triggered seismicity may continue for some time: Throughout California, seismicity rates remained elevated for a full five years following the Landers earthquake. Remotely triggered earthquakes were also identified in other parts of the world, including in Greece following the Izmit event (Brodsky and others, Geophysical Research Letters, v. 27, p. 2741-2744).

Seismology is a data-driven science, so research efforts in 2000, as always, focused on recent earthquakes that generated high-quality data. However, a study by Kenner and Segall (Science, v. 289, p. 2329-2332) yielded intriguing new insights into the behavior of the New Madrid Seismic Zone, which last produced large earthquakes in 1811-1812. The two scientists developed a mechanical model for the New Madrid Seismic Zone that reconciled a long-standing paradox: how there could be a high rate of large earthquakes over the last few thousand years when the regional strain rate is very low. Additionally, other researchers showed that the process of post-glacial rebound might explain why the New Madrid Seismic Zone became active only in very recent times (Grollimund and Zoback, Geology, v. 29, 2001, p. 175-178). These studies allow scientists not only to understand the complex physical processes associated with the New Madrid Seismic Zone, but also to better evaluate the hazard posed by future earthquakes in the region.

A number of other studies in 2000 focused on improving hazard estimates for future earthquakes. A report by the Southern California Earthquake Center presented 13 coordinated studies on site response in the greater Los Angeles region and helped identify so-called “hot spots” where significantly amplified shaking is expected (Field and others, Bulletin of the Seismological Society of America, 90B, p. S1-S31).

Sophisticated computer modeling techniques and state-of-the-art data were used to predict ground motions in other regions as well, including Santa Clara (Silicon Valley), Seattle and areas of Japan.  These and other studies, such as one by Koketsu and Kikuchi (Science, v. 288, p. 1237-1239), have typically focused on one of the most difficult and important remaining issues in ground-motion seismology: the prediction of shaking in complicated sedimentary basins and valleys. Via studies like these, we will acquire a broad range of seismological knowledge on earthquake sources, wave propagation and crustal structure. This knowledge can benefit society.

Hough is a research geophysicist with the U.S. Geological Survey in Pasadena, Calif.
E-mail: hough@seismo.gps.caltech.edu.

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