Science News
... from universities, journals, and
other research organizations
Earthquake-Triggered Landslides Pose Significant Hazard for Seattle, New
Study Details Potential Damage
Oct. 21, 2013 — A new study
suggests the next big earthquake on the Seattle fault may cause devastating
damage from landslides, greater than previously thought and beyond the areas
currently defined as prone to landslides. Published online Oct. 22 by the Bulletin of the Seismological Society of America (BSSA), the research offers a framework for simulating hundreds of earthquake
scenarios for the Seattle area.
"A major quake along the Seattle fault is among the worst case
scenarios for the area since the fault runs just south of downtown. Our study
shows the need for dedicated studies on seismically induced landsliding"
said co-author Kate Allstadt, doctoral student at University of Washington.
Seattle is prone to strong shaking as it sits atop the Seattle Basin --
a deep sedimentary basin that amplifies ground motion and generates strong
seismic waves that tend to increase the duration of the shaking. The broader
region is vulnerable to earthquakes from multiple sources, including deep
earthquakes within the subducted Juan de Fuca plate, offshore megathrust
earthquakes on Cascadia subduction zone and the shallow crustal earthquakes
within the North American Plate.
For Seattle, a shallow crustal earthquake close to the city would be
most damaging. The last major quake along the Seattle fault was in 900 AD, long
before the city was established, though native people lived in the area. The
earthquake triggered giant landslides along Lake Washington, causing entire
blocks of forest to slide into the lake.
"There's a kind of haunting precedence that tells us that we should
pay attention to a large earthquake on this fault because it happened in the
past," said Allstadt, who also serves as duty seismologist for the Pacific
Northwest Seismic Network. John Vidale of University of Washington and Art
Frankel of the U.S. Geological Survey (USGS) are co-authors of the study, which
was funded by the USGS.
While landslides triggered by earthquakes have caused damage and
casualties worldwide, they have not often been the subject of extensive
quantitative study or fully incorporated into seismic hazard assessments, say
authors of this study that looks at just one scenario among potentially
hundreds for a major earthquake in the Seattle area.
Dividing the area into a grid of 210-meter cells, they simulated ground
motion for a magnitude 7 Seattle fault earthquake and then further subdivided
into 5-meter cells, applying anticipated amplification of shaking due to the
shallow soil layers. This refined framework yielded some surprises.
"One-third of the landslides triggered by our simulation were
outside of the areas designated by the city as prone to landsliding," said
Allstadt. "A lot of people assume that all landslides occur in the same
areas, but those triggered by rainfall or human behavior have a different
triggering mechanism than landslides caused by earthquakes so we need dedicated
studies."
While soil saturation -- whether the soil is dry or saturated with water
-- is the most important factor when analyzing the potential impact of
landslides, the details of ground motion rank second. The amplification of
ground shaking, directivity of seismic energy and geological features that may
affect ground motion are very important to the outcome of ground failure, say
authors.
The authors stress that this is just one randomized scenario study of
many potential earthquake scenarios that could strike the city. While the
results do not delineate the exact areas that will be affected in a future
earthquake, they do illustrate the extent of landsliding to expect for a
similar event.
The study suggests the southern half of the city and the coastal bluffs,
many of which are developed, would be hardest hit. Depending upon the water
saturation level of the soil at the time of the earthquake, several hundred to
thousands of buildings could be affected citywide. For dry soil conditions,
there are more than 1000 buildings that are within all hazard zones, 400 of
those in the two highest hazard designation zones. The analysis suggests
landslides could also affect some inland slopes, threatening key transit routes
and impeding recovery efforts. For saturated soil conditions, it is an order of
magnitude worse, with 8000 buildings within all hazard zones, 5000 of those
within the two highest hazard zones. These numbers only reflect the number of
buildings in high-risk areas, not the number of buildings that would
necessarily suffer damage.
"The extra time we took to include the refined ground motion detail
was worth it. It made a significant difference to our understanding of the
potential damage to Seattle from seismically triggered landslides," said
Allstadt, who would like to use the new framework to run many more scenarios to
prepare for future earthquakes in Seattle.
Story Source:
The above story is based on materials provided by Seismological Society of America, via EurekAlert!, a
service of AAAS.
Note: Materials may be edited for content and length. For further
information, please contact the source cited above.
Journal Reference:
1. Allstadt, K., Vidale,
J.E., and Frankel, A. A scenario study of seismically induced
landsliding in Seattle using broadband synthetic seismograms and accounting for
site amplification. Bull Seism. Soc. Am., 2013
Seismological Society of America (2013,
October 21). Earthquake-triggered landslides pose significant hazard for
Seattle, new study details potential damage. ScienceDaily. Retrieved October
24, 2013, from http://www.sciencedaily.com /releases/2013/10/131021211710.htm?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+sciencedaily%2Fearth_climate%2Fgeography+%28ScienceDaily%3A+Earth+%26+Climate+News+--+Geography%29
Note: If no author is given, the source
is cited instead.
No comments:
Post a Comment