New Monitoring Stations
Detect 'Silent Earthquakes' In Costa Rica
ScienceDaily - After installing an extensive network of
monitoring stations in Costa Rica, researchers have detected slow
slip events (also known as "silent earthquakes") along a major fault
zone beneath the Nicoya Peninsula. These findings are helping
scientists understand the full spectrum of motions occurring on the
fault and may yield new insights into the events that lead to major
earthquakes.
A slow slip event involves the same fault motion as an earthquake,
but it happens so slowly that the ground does not shake. It can be
detected only with networks of modern instruments that use the
Global Positioning System (GPS) to measure precisely the movements
of the Earth's crust over time.
Susan Schwartz, a professor of Earth and planetary sciences at the
University of California, Santa Cruz, leads a team that has
installed a permanent network of 13 GPS monitoring stations and 13
seismic stations on Costa Rica's Nicoya Peninsula.
"At least two slow slip events have occurred beneath the Nicoya
Peninsula since 2003," Schwartz said. "When we recorded the first
one in 2003, we had only 3 GPS stations. By 2007, we had 12 GPS
stations and over 10 seismic stations, so the event that year was
very nicely recorded."
The National Science Foundation (NSF) has funded the work by
Schwartz and others to install monitoring equipment in Costa Rica.
Schwartz, who directs UCSC's Keck Seismological Laboratory, has been
working in the region since 1991. At the annual meeting of the
American Association for the Advancement of Science (AAAS) in
Chicago, she will describe results from the past decade of
fault-zone monitoring in Central America.
"The newest discovery is the occurrence of these slow slip events.
But there has been a decade of focused effort in this area that has
significantly advanced our knowledge of the Central America
seismogenic system," Schwartz said. "Initially, we focused on areas
of the fault that are locked up, which slip in an earthquake. The
slow slip is occurring in regions that are not strongly locked, and
a big question is whether that is loading the locked area, making it
more likely to break, or relieving stress on the fault."
Schwartz said she does not think slow slip events significantly
increase the likelihood of a major earthquake on a locked portion of
the fault. She noted, however, that scientists are still at an early
stage in terms of understanding the implications of different kinds
of fault motion and translating that information into earthquake
hazard assessments.
Flanked by active tectonic margins on both the Pacific and Caribbean
coasts, Costa Rica is one of the most earthquake-prone and
volcanically active countries in the world. Just off the west coast
is the Middle America Trench, where a section of the seafloor called
the Cocos Plate dives beneath Central America, generating powerful
earthquakes and feeding a string of active volcanoes. This type of
boundary between two converging plates of the Earth's crust is
called a subduction zone--and such zones are notorious for
generating the most powerful and destructive earthquakes.
The slow slip phenomenon was first observed at subduction zones
where hundreds of GPS and seismic instruments are deployed: the
Cascadia fault zone (off the coast of Washington and British
Columbia) and Japan's Nankai Trough. At these and most other
subduction zones, the part of the plate boundary where earthquakes
originate, called the seismogenic zone, lies beneath the ocean. But
in Costa Rica, the seismogenic zone runs right beneath the Nicoya
Peninsula.
"It's a perfect opportunity to study the seismogenic zone using a
network of land-based instruments," Schwartz said.
The 2007 slow slip event in Costa Rica involved movement along the
fault equivalent to a magnitude 6.9 earthquake. But it took place
over a period of 30 days rather than the 10 seconds typical for an
earthquake of that size, and such slow motion does not radiate the
seismic energy associated with normal earthquakes. The instruments
did pick up seismic tremor, however, which Schwartz likened to a lot
of very small earthquakes. Tremor activity is also associated with
slow slip events in Japan and Cascadia, but there are some
differences in Costa Rica, Schwartz said.
"Costa Rica has a different type of subduction zone from the
well-studied ones in Japan and Cascadia," she said. "One thing that
makes it interesting is that the temperature is much cooler at the
depth range where slip occurs, and that is helping us work out the
role of fluids in generating slow slip."
Ultimately, the goal of this research is not only a better
understanding of subduction zones, but also better assessments of
earthquake hazards. Schwartz said her Costa Rican colleagues have
been working to educate the population of Nicoya about earthquakes
and related hazards. With a growing population along the coast, the
region faces a potential tsunami threat as well as the possibility
of a major earthquake, she said. |
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