Subduction zone earthquakes off Oregon,
Washington more frequent than previous
estimates
5 August 2016
A new analysis suggests that massive earthquakes
on northern sections of the Cascadia Subduction
Zone, affecting areas of the Pacific Northwest that
are more heavily populated, are somewhat more
frequent than has been believed in the past.
The chance of one occurring within the next 50
years is also slightly higher than previously
estimated.
The findings, published this week in the journal
Marine Geology, are based on data that is far more
detailed and comprehensive than anything prior to
this. It used measurements from 195 core samples
containing submarine landslide deposits caused by
subduction zone earthquakes, instead of only
about a dozen such samples in past research.
The work was done by researchers from Oregon
State University, Camosun College in British
Columbia and Instituto Andaluz de Ciencias de la
Tierra in Spain. The research was supported by
the National Science Foundation and the U.S.
Geological Survey.
The Cascadia Subduction Zone runs from northern
California to British Columbia, and scientists say it
can be roughly divided into four segments. There
have been 43 major earthquakes in the past 10,000
years on this subduction zone, sometimes on the
entire zone at once and sometimes only on parts of
it. When the entire zone is involved, it's believed to
be capable of producing a magnitude 9.1
earthquake.
It's been known for some time, and still believed to
be accurate, that the southern portions of the
subduction zone south of Newport, Oregon, tend to
rupture more frequently - an average of about every
300-380 years from Newport to Coos Bay, and
220-240 years from Coos Bay to Eureka, California.
The newest data, however, have changed the
stakes for the northern sections of the zone, which
could have implications for major population
centers such as Portland, Tacoma, Seattle and
Vancouver, B.C.
A section of the zone from Newport to Astoria,
Oregon, was previously believed to rupture on
"These new results are based on much better data average about every 400-500 years, and that
than has been available before, and reinforce our average has now been reduced to 350 years. A
section further north from Astoria to Vancouver
confidence in findings regarding the potential for
Island was previously believed to rupture about
major earthquakes on the Cascadia Subduction
every 500-530 years, and that average has now
Zone," said Chris Goldfinger, a professor in the
been reduced to 430 years.
College of Earth, Ocean and Atmospheric
Sciences at OSU, and one of the world's leading
experts on tectonic activity of this subduction zone. The last major earthquake on the Cascadia
Subduction Zone - pinpointed in time because it
caused a tsunami that raced all the way across the
"However, with more detailed data we have also
changed somewhat our projections for the average Pacific Ocean to Japan - occurred in January,
1700, more than 315 years ago.
recurrence interval of earthquakes on the
subduction zone, especially the northern parts.
"What this work shows is that, contrary to some
The frequency, although not the intensity, of
earthquakes there appears to be somewhat higher previous estimates, the two middle sections of the
Cascadia Subduction Zone that affect most of
than we previously estimated."
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Oregon have a frequency that's more similar than
different," said Goldfinger, who directs the Active
Tectonics and Seafloor Mapping Laboratory at
OSU.
Based on these findings, the chances of an
earthquake in the next 50 years have also been
slightly revised upwards. Of the part of the zone off
central and northern Oregon, the chance of an
event during that period has been changed to
15-20 percent instead of 14-17 percent. On the
furthest north section of the zone off Washington
and British Columbia, the chance of an event has
increased to 10-17 percent from 8-14 percent.
land. New technology is helping researchers to
actually simulate these underwater landslides,
better understand their behavior, and more
accurately identify the "turbidite" or sediment layers
they leave behind.
The large amounts of additional data, researchers
say, has helped refine previous work, fill holes in
the data coverage, and also to rule out other
possible causes of some sediment deposits, such
as major storms, random landslides or small local
earthquakes.
More information: Chris Goldfinger et al, The
importance of site selection, sediment supply, and
The study also increased the frequency of the most hydrodynamics: A case study of submarine
massive earthquakes, where the entire subduction paleoseismology on the northern Cascadia margin,
zone ruptures at once. It had previously been
Washington USA, Marine Geology (2016). DOI:
believed this occurred about half the time. Now, the 10.1016/j.margeo.2016.06.008
data suggest that several partial ruptures were
more complete than previously thought, and that
complete ruptures occur slightly more than half the
time.
Provided by Oregon State University
"Part of what's important is that these findings give
us more confidence about what's coming in our
future," Goldfinger said.
"We believed these earthquakes were possible
when the hypothesis was first developed in the late
1980s. Now we have a great deal more certainty
that the general concern about earthquakes caused
by the Cascadia Subduction Zone is scientifically
valid, and we also have more precise information
about the earthquake frequency and behavior of
the subduction zone."
Based in part on the growing certainty about these
issues, OSU has developed the Cascadia Lifelines
Program, an initiative working with Pacific
Northwest business and industry to help prepare for
the upcoming subduction zone earthquake, mitigate
damage and save lives. Many other programs are
also gaining speed.
The new measurements in this research were
made with cores that showed the results of massive
amounts of sediments released by subsea
landslides during a subduction zone earthquake - a
catastrophic event beneath the sea as well as on
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APA citation: Subduction zone earthquakes off Oregon, Washington more frequent than previous
estimates (2016, August 5) retrieved 18 June 2017 from https://phys.org/news/2016-08-subduction-zoneearthquakes-oregon-washington.html
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