February 28, 2008
Science
Daily
On January 26, 1700, at about 9 p.m. local time, the Juan de Fuca
plate beneath the ocean in the Pacific Northwest suddenly moved, slipping some
60 feet eastward beneath the North American plate in a monster quake of
approximately magnitude 9, setting in motion large tsunamis that struck the
coast of North America and traveled to the shores of Japan.
Since then, the earth beneath the
region – which includes the cities of Vancouver, Seattle and Portland -- has
been relatively quiet. But scientists believe that earthquakes with magnitudes
greater than 8, so-called “megathrust events,” occur along this fault on average
every 400 to 500 years.
To help prepare for the next megathrust
earthquake, a team of researchers led by seismologist Kim Olsen of San Diego
State University (SDSU) used a supercomputer-powered “virtual earthquake”
program to calculate for the first time realistic three-dimensional simulations
that describe the possible impacts of megathrust quakes on the Pacific Northwest
region. Also participating in the study were researchers from the San Diego
Supercomputer Center at UC San Diego and the U.S. Geological Survey.
What
the scientists learned from this simulation is not reassuring, as reported in
the Journal of Seismology, particularly for residents of downtown
Seattle.
With a rupture scenario beginning in the north and propagating
toward the south along the 600-mile long Cascadia Subduction Zone, the ground
moved about 1 1/2 feet per second in Seattle; nearly 6 inches per second in
Tacoma, Olympia and Vancouver; and 3 inches in Portland, Oregon. Additional
simulations, especially of earthquakes that begin in the southern part of the
rupture zone, suggest that the ground motion under some conditions can be up to
twice as large.
“We also found that these high ground velocities were
accompanied by significant low-frequency shaking, like what you feel in a roller
coaster, that lasted as long as five minutes – and that’s a long time,” said
Olsen.
The long-duration shaking, combined with high ground velocities,
raises the possibility that such an earthquake could inflict major damage on
metropolitan areas -- especially on high-rise buildings in downtown Seattle.
Compounding the risks, like Los Angeles to the south, Seattle, Tacoma, and
Olympia sit on top of sediment-filled geological basins that are prone to
greatly amplifying the waves generated by major earthquakes.
“One thing
these studies will hopefully do is to raise awareness of the possibility of
megathrust earthquakes happening at any given time in the Pacific Northwest,”
said Olsen. “Because these events will tend to occur several hundred kilometers
from major cities, the study also implies that the region could benefit from an
early warning system that can allow time for protective actions before the brunt
of the shaking starts.” Depending on how far the earthquake is from a city,
early warning systems could give from a few seconds to a few tens of seconds to
implement measures, such as automatically stopping trains and
elevators.
Photo: Scientists used a
supercomputer-driven "virtual earthquake" to explore likely ground shaking in a
magnitude 9.0 megathrust earthquake in the Pacific Northwest. Peak ground
velocities are displayed in yellow and red. The legend represents speed in
meters per second (m/s) with red equaling 2.3 m/s. Although the largest ground
motions occur offshore near the fault and decrease eastward, sedimentary basins
lying beneath some cities amplify the shaking in Seattle, Tacoma, Olympia, and
Vancouver, increasing the risk of damage. (Credit: Kim Olsen, SDSU)
Added
Olsen, “The information from these simulations can also play a role in research
into the hazards posed by large tsunamis, which can originate from such
megathrust earthquakes like the ones generated in the 2004 Sumatra-Andeman
earthquake in Indonesia.” One of the largest earthquakes ever recorded, the
magnitude 9.2 Sumatra-Andeman event was felt as far away as Bangladesh, India,
and Malaysia, and triggered devastating tsunamis that killed more than 200,000
people.
In addition to increasing scientific understanding of these
massive earthquakes, the results of the simulations can also be used to guide
emergency planners, to improve building codes, and help engineers design safer
structures -- potentially saving lives and property in this region of some 9
million people.
Even with the large supercomputing and data resources at
SDSC, creating “virtual earthquakes” is a daunting task. The computations to
prepare initial conditions were carried out on SDSC’s DataStar supercomputer,
and then the resulting information was transferred for the main simulations to
the center’s Blue Gene Data supercomputer via SDSC’s advanced virtual file
system or GPFS-WAN, which makes data seamlessly available on different –
sometimes distant – supercomputers.
Coordinating the simulations required
a complex choreography of moving information into and out of the supercomputer
as Olsen’s sophisticated “Anelastic Wave Model” simulation code was running.
Completing just one of several simulations, running on 2,000 supercomputer
processors, required some 80,000 processor hours – equal to running one program
continuously on your PC for more than 9 years!
“To solve the new
challenges that arise when researchers need to run their codes at the largest
scales, and data sets grow to great size, we worked closely with the earthquake
scientists through several years of code optimization and modifications,” said
SDSC computational scientist Yifeng Cui, who contributed numerous refinements to
allow the computer model to “scale up” to capture a magnitude 9 earthquake over
such a vast area.
In order to run the simulations, the scientists must
recreate in their model the components that encompass all the important aspects
of the earthquake. One component is an accurate representation of the earth’s
subsurface layering, and how its structure will bend, reflect, and change the
size and direction of the traveling earthquake waves. Co-author William
Stephenson of the USGS worked with Olsen and Andreas Geisselmeyer, from Ulm
University in Germany, to create the first unified “velocity model” of the
layering for this entire region, extending from British Columbia to Northern
California.
Another component is a model of the earthquake source from
the slipping of the Juan de Fuca plate underneath the North American plate.
Making use of the extensive measurements of the massive 2004 Sumatra-Andeman
earthquake in Indonesia, the scientists developed a model of the earthquake
source for similar megathrust earthquakes in the Pacific Northwest.
The
sheer physical size of the region in the study was also challenging. The
scientists included in their virtual model an immense slab of the earth more
than 650 miles long by 340 miles by 30 miles deep -- more than 7 million cubic
miles -- and used a computer mesh spacing of 250 meters to divide the volume
into some 2 billion cubes. This mesh size allows the simulations to model
frequencies up to 0.5 Hertz, which especially affect tall buildings.
“One
of the strengths of an earthquake simulation model is that it lets us run
scenarios of different earthquakes to explore how they may affect ground
motion,” said Olsen. Because the accumulated stresses or “slip deficit” can be
released in either one large event or several smaller events, the scientists ran
scenarios for earthquakes of different sizes.
“We found that the
magnitude 9 scenarios generate peak ground velocities five to 10 times larger
than those from the smaller magnitude 8.5 quakes.”
The researchers are
planning to conduct additional simulations to explore the range of impacts that
depend on where the earthquake starts, the direction of travel of the rupture
along the fault, and other factors that can vary.
This research was
supported by the National Science Foundation, the U.S. Geological Survey, the
Southern California Earthquake Center, and computing time on an NSF
supercomputer at SDSC.
Adapted from materials provided by University of
California - San Diego.
http://www.sciencedaily.com/releases/2008/02/080226144524.htm