Highlights
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.
