By Elizabeth VanBoskirk and Linda Rowan
As anyone who has visited or lived in the Pacific Northwest knows, there is a plate-crunching collision going on beneath our feet that has produced volcanoes, earthquakes, landslides, and tsunami.
The Juan de Fuca plate is moving beneath the North American plate to form the Cascadia subduction zone. The plates converge at a rate of 30–40 mm/year, about the same rate your finger nails grow.
The subduction zone has generated very large earthquakes in the past, creating tsunamis that have flooded the Pacific Northwest coast and sometimes even Japan.
The strong ground-shaking from an earthquake can trigger landslides on hillslopes and cause liquefaction where there are saturated sediments.
In the Pacific Northwest geoscientists have relied upon the sleuthing of the sediments to find a record of each earthquake and tsunami wave over several thousands of years.
The timing between these Cascadia earthquakes is one about every of 300 to 1,100 years, with a 500-530-year average recurrence interval in Washington and 240-year interval in southern Oregon.
Cascadian earthquakes can vary in size from M 7.5 to M 9.1. It has been about 300 years since the last large earthquake (about a magnitude 9 event), so another one may be in our near future.
Japan has the densest earthquake monitoring network in the world and the recurrence interval for a M 9 earthquake is 300 to 1,200 years. The last large earthquake was in northeastern Japan 1,100 years ago, although the Tohoku-Oki earthquake of 2011 resulted in tragic consequences.
Figure 1: Basic Subduction Zone /Cross–section of the Cascadia Subduction Zone
Geoscientists cannot yet predict earthquakes, but they can observe how Earth deforms over time using an array of instruments including seismometers, strainmeters, and Global Positioning System (GPS) receivers.
On the West Coast, as well as across the U.S., the Plate Boundary Observatory (PBO) provides GPS receivers and borehole seismometers and strainmeters to observe Earth motions.
The goal of this network is to understand how plates move, how earthquakes and tsunamis are generated, and how volcanoes work.
Knowing more about how the Earth moves can help society to prepare and respond to sudden changes.
Figure 2: Map of PBO coverage
PBO is one of three components of EarthScope, a National Science Foundation funded project. Two other parts are the San Andreas Fault Observatory at Depth and the U.S. Transportable Array.
GPS is a U.S. system of about 30 satellites orbiting the Earth, provide time and location to receivers on the ground and satellites in space.
PBO sites consist of high–end receivers and large, noise reducing antennas drilled into solid rock. These stations provide positions that are accurate to about a millimeter compared to a handheld device that may be accurate to about a meter at best.
Over time, surface deformation can be observed with the PBO data.
Recently, GPS sites have been upgraded to “real-time” and the data can be transmitted to a researcher within a second.
Potentially, real-time data can help with early warning of earthquakes, tsunamis, and volcanic eruptions to allow people to get out of harm’s way and shut down critical infrastructure before something fails.
Figure 3: PBO GPS site on Mt. St. Helens
PBO borehole strainmeter are instruments placed at the bottom of 400 to 800 foot wells filled in with cement after the instrument is aligned at the bottom. They observe stresses and strains within it.
They are designed to detect changes in the surrounding bedrock as small as one ten-millionth of the width of a human hair, too small to be detected by the GPS sites.
Strainmeters act like seismometers too observing earthquakes, and long-term strain accumulation.
In the Pacific Northwest, seismometers and strainmeters observe Episodic Tremor and Slip events (ETS). These consist of swarms of hundreds of tiny, imperceptible earthquakes that persist for weeks.
Tremors migrate hundreds of miles laterally, for example from Olympia, WA, to Vancouver Island, Canada. Over time, this large area of slow slip releases the amount of energy equivalent to a large magnitude earthquake without anyone feeling any part of it, basically they pose no direct hazard.
ETS events are sometimes called silent events because they are not “heard” by seismometers. They occur along all segments of the Cascadia subduction zone, with recurrence intervals of 10 to 20 months.
Geoscientists estimate how much strain may be building up along the fault and how this build–up or release of strain may alter the possibility of larger earthquakes.
Figure 4: ETS events mapped using the Pacific Northwest Seismic Network interactive mapping tool (www.pnsn.org)
GPS and borehole can detect ETS events very well. Combining this data with seismic observations of regular earthquakes allows us to understand the timing and magnitude of geologic hazards related in our subduction zone.
These research endeavors help society deal with the risks of geologic hazards at a major plate boundary.
UNAVCO authors:
Elizabeth VanBoskirk works as a Borehole Strainmeter Field Engineer based in Portland, OR
Linda Rowan works with Education and Outreach and is based in headquarters in Boulder, CO
References & educational links:
Pacific Northwest Seismic Network
www.pnsn.org
Plate Boundary Observatory
www.pbo.unavco.org
www.earthscope.org/pbo
USGS
www.usgs.gov
Oregon State University (OSU)
www.activetectonics.coas.oregonstate.edu/Cascadia_turbs.htm