Cascadia Subduction Zone: Unprepared and Liquefaction

Call me paranoid, but when I see a 7.8 earthquake in Indonesia, and the news recalls the 8.9 magnitude earthquake that triggered the deadliest tsunami in history in 2004 killing more than 200,000 people, I’m reminded that we live in the shake zone of earthquakes and tsunamis, the Cascadia Subduction Zone. It doesn’t help when The New Yorker Magazine tells us that the “Really Big One” is coming and we’ll be able to surf to Idaho soon.

Last year, OPB-TV won awards for their “Unprepared” television series and documentary on the historical “big one” coming to the Pacific Northwest. It led to discussions around the state of Oregon involving geologists, seismologists, and area experts, all asking if we are prepared and what are we going to do or not do about it. They talked about the state of our bridges, schools, and the impact of liquefaction on our ports, home to fuel tanks, some almost 100 years old, that could rupture, dump into our precious waterways, and burn for ages. It was a wake-up call for all of us.

As a rock lover, I started questioning the ground under my feet. According to FEMA’s Earthquake Risk and Cascadia Region Earthquake Workgroup (CREW) and their educational Cascadia Subduction Zone Earthquakes 9.0 Magnitude Scenario (PDF), while I’m personally outside of the tsunami zone, besides being cut off from the rest of the world, the thing to fear most is: Liquefaction.

Tilted Victorian Home in San Francisco due to liquefaction - Photograph by G.K. Gilbert of the U.S. Geological Survey

Liquefaction is the process in which soil, often thought to be firm and solid, is “reduced” by earthquake shaking. While most commonly associated with saturated soils, liquefaction occurs in dry soils where there is space between the particles. Take a jar and fill it full of flour or grains. Tap it against the counter and you will see the level drop. Depending upon the space and shape of the grains, it might drop a little or a lot. That’s liquefaction in action.

Much of the area in Washington County and Portland metro is loose soil. According to the University of Washington Civil and Environmental Engineering Department’s report on soil liquefaction:

When liquefaction occurs, the strength of the soil decreases and, the ability of a soil deposit to support foundations for buildings and bridges is reduced…

Liquefied soil also exerts higher pressure on retaining walls, which can cause them to tilt or slide. This movement can cause settlement of the retained soil and destruction of structures on the ground surface.

Increased water pressure can also trigger landslides and cause the collapse of dams.

In this video on Japan Earth Moving – Liquefaction, it shows various film clips of liquefaction as well as water lines breaking during an earthquake. You can see where the soil has dropped or shifted to push objects up as the ground form changes.

In another video on extreme soil liquefaction, people jump up and down on soil that shifts and moves under them in a construction site. The bulldozer scoop pushes on the soil to show the movement, then digs out a scoop of dry soil. (Note, a few “language” words used in the filming.)

In another YouTube video you may watch yourself, news reports of the damage after the recent Christchurch, New Zealand, earthquake show liquefaction as cars are trapped in the road after it liquefied under their tires and more.

In “How Scientists Know When the Last Big Earthquake Happened Here” for OPB-TV, they explain how scientists and researchers came together to determine the last major earthquake that devastated much of the Pacific Northwest.

In the 1980s and 1990s, scientists conducting fieldwork found evidence that coastal lands in the Northwest dropped suddenly and were inundated by tsunami waves and mud. All along the Oregon and Washington coast, the rapid advance of seawater had buried delicate marsh plants that were still alive and killed Sitka spruce, leaving behind rotting stumps. Rot-resistant western red cedar survived longer and their remnants became known as “ghost forests.”

These sudden changes in the coastal landscape pointed heavily to an earthquake and tsunami.

How could scientists know for sure?

American researchers used carbon dating on the spruce, peat and fossilized plants. Sitka spruce as far apart as southern Washington and Northern California died from the high waters during the same few decades — sometime between 1695 and 1720. Their rings were wide right up through the last ring, proving that a sudden event rather than slow sea level rise was the culprit.

The American scientists published a letter in the journal, “Nature,” to summarize their findings and the meticulous records found in the United States and Canada.

In 1997, scientists used dendrochronology — a method of dating trees by comparing ring patterns to samples of known age — on the ghost forest trees. They were able to determine the trees had stopped growing after completing the 1699 growing season, dating their death to somewhere between September 1699 and May 1700, further confirming the date independently of Japanese records.

More evidence is on the ocean floor. Paleoseismologists have dated core samples of debris from earthquake-induced landslides. This data indicate a large earthquake with a lot of shaking was going on during this period — and that it was happening off the Oregon coast.

The scientists estimate that over the past 10,000 years, 9.0 magnitude earthquakes in the Cascadia Subduction Zone happen every 526 years, and that the Pacific Northwest is past due.

If you haven’t watched “Unprepared,” the documentary on whether or not Oregon is prepared for the next big earthquake is available to watch online free. It also reruns often on OPT-TV.

After more research, I learned that my wood-frame home built on level ground with no slopes in the immediate vicinity isn’t as safe as I thought. On the saturated clay and hummus soil, it could drop several inches to feet depending upon the length of the shake, just as that jar will compress its contents the more you tap down on the counter. Those with brick and concrete homes and buildings are less flexible. The shake could cause the building’s stiff materials to crack and break, combined with liquefaction, crumble. Makes you think, doesn’t it.

You can test your own home or office or school. OPB offers an interactive tool called “Aftershock.” Enter your address and it reports on the modeling of what the impact would be like after a 9.0 magnitude earthquake. It is currently limited to Oregon only. It was created during a weekend “Storytelling with Data build-a-thon” by Hack Oregon and Agora Journalism Center of the University of Oregon. Data comes from the Oregon Department of Geology and Mineral Industries as well as other geological and safety agencies.

Earthquake Aftershock Report on Rice Northwest Museum of Rocks and Minerals 2016

The report estimates the earthquake to be very strong and ground deformation levels to include liquefaction.

The quake causes soils here to turn liquid, like wet sand. Underground pipes are damaged and evacuation routes could be compromised. Nearby houses are damaged, and the ground is permanently deformed. Rebuilding here will be difficult.

Experts project it could take several months to restore your community to its normal function based on damage to pipes, infrastructure, and the transportation corridors needed for recovery efforts.

The Oregonian also worked with local geology and educational institutions to create an interactive map that asks, “How vulnerable are Portland’s buildings?” Enter your addresses and the map will give you a general recommendation on the date the structure was built and the likelihood it complies with today’s earthquake standards.

The Oregon Department of Geology and Mineral Industries offers Interpretive Map Series (IMS) Publications for Oregon, a collection of interpretive maps covering Oregon’s geology. The collection includes inventories of landslides, Missoula Floods impact, tsunami maps, and general earthquake and related geological hazard maps.

For more information on earthquakes in the Washington/Oregon area along the Cascadia Subduction Zone, we’ve put together a list.

The more I learn about rocks, the more I’ve come to appreciate the geology under my feet and how it impacts our lives. Earthquakes, tsunamis, floods, landslides, volcanoes, all disrupt human life but also teach us more about geology and natural sciences. It takes a rock lover to appreciate the wonder of that destructive force.

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