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.
"These new results are based on much better data than
has been available before, and reinforce our confidence in findings regarding
the potential for major earthquakes on the Cascadia Subduction Zone," said
Chris Goldfinger, a professor in the College of Earth, Ocean and Atmospheric
Sciences at OSU, and one of the world's leading experts on tectonic activity of
this subduction zone.
"However, with more detailed data we have also changed
somewhat our projections for the average recurrence interval of earthquakes on
the subduction zone, especially the northern parts. The frequency, although not
the intensity, of earthquakes there appears to be somewhat higher than we
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,
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 average about every 400-500 years, and that
average has now been reduced to 350 years. A section further north from Astoria
to Vancouver Island was previously believed to rupture about every 500-530
years, and that average has now been reduced to 430 years.
The last major earthquake on the Cascadia Subduction Zone -
pinpointed in time because it caused a tsunami that raced all the way across
the Pacific Ocean to Japan - occurred in January, 1700, more than 315 years
"What this work shows is that, contrary to some
previous estimates, the two middle sections of the Cascadia Subduction Zone
that affect most of 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.
The study also increased the frequency of the most massive
earthquakes, where the entire subduction zone ruptures at once. It had
previously been believed this occurred about half the time. Now, the data
suggest that several partial ruptures were more complete than previously
thought, and that complete ruptures occur slightly more than half the time.
"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
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 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
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.