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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.
