Western Perspectives

Massive earthquake threatens Pacific Northwest

By , Communities Digital News

Author: annotated USGS data displayed using Google Earth
Author: annotated USGS data displayed using Google Earth

SALEM, Ore., March 17, 2014 — A large 6.8 magnitude earthquake with dozens of aftershocks struck off the Northern California coast 50 miles west of Eureka on March 10, 2014. It didn’t create a tsunami or widespread damage, so few in the region paid much attention to it. They should have.

Unbeknown to most people in the Pacific Northwest, they are in grave danger. The Pacific Northwest is 71 years overdue for an immense 8.4 or larger earthquake, and tens of thousands of lives are at risk.

Last week’s earthquake battered a geologically unstable place called the Mendocino Triple Junction. It is where three of earth’s tectonic plates collide. They are twisting around each other like a giant bottle cap screwing tight onto a well-shaken bottle of soda pop that’s ready to explode.

The 6.8 earthquake wasn’t in a populated area. Another quake of that size hit Kobe, Japan in 1995. That one killed over 6,400 and caused $100 billion in property damage. That’s small compared to what the northwest faces.

The triple junction is a geologically scary place. It will be the origin point for a mighty tear northward along the 680-mile long Cascadia Subduction Zone. It’s happened before. When it happens again, it will be the largest modern-day earthquake in the contiguous United States.

There have been only two comparable earthquakes in recent history. One was the 2011 Tohoku earthquake that damaged most of Japan’s east coast, killed 19,000 and destroyed the Fukushima Daiichi Nuclear Power Plant. The other was the 2004 Sumatra–Andaman earthquake that killed an estimated 230,000 in the Indian Ocean basin.

Newly assembled research shows the Pacific Northwest is 71 years overdue for its own major earthquake. The danger lurks just offshore under the Pacific Ocean.

Introducing Cascadia

USGS: 15 largest earthquakes ever recorded in the lower-48 U.S. states

In terms of property damage and human loss of life, earthquakes are historically the single most cataclysmic events on earth. Shown above, the last Cascadia earthquake in 1700 sits atop the lower-48 leader board.

A “great quake” is in a special destructive class all its own; it is the worst of the worst. According to Stanford seismologist Gregory Berozo, a great quake has a Richter magnitude of 8.5 or higher. Earth experiences only 1 great quake, by Beroza’s definition, every 8 years.

Only one known great quake, a Cascadia, has ever struck the continental United States.

Cascadia last battered the Pacific Northwest 314 years ago, at 9 p.m. on January 26, 1700. The date and time are precisely known because the tsunami it generated was well documented over 5,000 miles away in Japan.

Cascadia in 1700 is tied for earth’s 4th largest measured earthquake. The largest was the 9.5 Valdivia earthquake of 1960 in southern Chile.

The Cascadia quake didn’t get attention because it occurred 105 years before Lewis and Clark. Evidence of its massive effect on Pacific Northwest shorelines were not discovered until the 1980s.

The 2nd largest earthquake ever recorded was the Great Alaskan Earthquake, on March 7, 1964. That one had a magnitude of 9.2, raised some areas near Kodiak by 30 feet, and lowered areas southeast of Anchorage by 8 feet. It killed 139 people. The second most powerful quake recorded in the contiguous United States, at least 33 times smaller than the Cascadia quake, was California’s 7.9 magnitude Fort Tejon quake in January 1857. That one was in the hills just north of present day Los Angeles.

Since 1700, 42 feet of accumulated downward stress has built up along the Cascadia subduction zone. Complicating matters, the Pacific plate has chugged 104 feet to the northwest at about 4 inches a year, while the San Juan de Fuca plate is pushing under North America at 1.5 inches a year. It causes a twisting, pinching motion around the south end of Cascadia at the Mendocino Triple Junction.

What happened on Japan’s northeast coast in 2011 and Sumatra in December 2004, will soon be repeated by Cascadia in the United States.

After that, life in the Pacific Northwest will be altered forever.

Long History of Cascadia Earthquakes

DOGAMI: Oregon Resiliency Plan

Cascadia has averaged an earthquake every 243 years over the last 10,000 years. There have been 41 monster quakes along the Cascadia Subduction Zone in that period. The last one was 314 years ago

The Pacific Northwest is 71 years overdue for its next massive earthquake.

Scientists discovered the earthquakes from studies of ocean and lake sediment core samples taken all along the Oregon and Washington coasts. They reveal Cascadia “inundations.” “Turbidites” is the term geologists use to describe large seabed landslides and ocean-side lake beds inundated by tsunamis debris.

DOGAMI: Oregon Resiliency Plan

Cascadia quakes are categorized into two general types:

  1. Full-length rupture (680 miles)
  2. Partial rupture (<680 miles)

The partials usually occur in the southern section and range in magnitude from 8.2 to 8.6. Full-length Cascadias start at 8.8 and then get super big. By Cascadia standards, the 9.0 earthquake of 1700 is classified as “medium”. According to the experts, Cascadia sizes range up to “extra-extra-large”.

The largest natural disaster in U.S. history is Galveston Island’s September 8, 1900 Great Storm, where 6,000 died and 3,600 structures were destroyed by 145 mph hurricane-driven waves that over-swept the island. The next Cascadia will dwarf it.

Oregon alone will see upwards of 10,000 dead.

Tsunami Planning

DOGAMI: Oregon tsunami evacuation map for Cannon Beach, Oregon

Special colored overlays for Google Maps are used to identify coastal tsunami hazard areas:

  • Orange: Distant tsunami (non-Cascadia)
  • Yellow: Local tsunami (Cascadia)
  • Green: No hazard

The State of Oregon’s Department of Geology and Mineral Industries (DOGAMI) is the leader in Pacific Northwest earthquake preparedness. Since 2011 it has built tsunami hazard maps for the entire Oregon Coast.

Maps can be explored with a fascinating interactive online northwest oceanography tool called the Nanoos Visualization System (NVS) using its Pacific Northwest Tsunami Evacuation Zones product. It’s intended to help coastal citizens plan their individualized personal tsunami escape routes.

A distant tsunami is patterned after the 1964 Prince William sound 9.2 quake 1,200 miles away that caused considerable damage in Oregon.

DOGAMI has assembled a lot of scientific research on Cascadia since the great Sumatra and Japanese subduction zone quakes. Their tsunamis caused 240,000 deaths and tremendous destruction.

One of DOGAMI’s important early outcomes is Special Paper 43 – “SIMULATING TSUNAMI INUNDATION AT BANDON, COOS COUNTY, OREGON, USING HYPOTHETICAL CASCADIA AND ALASKA EARTHQUAKE SCENARIOS“.

The result of Special Paper 43 was development of scientific techniques for calculating tsunami evacuation zones. Those provide municipalities badly needed information necessary for disaster and city planning.

The effect of Cascadia on Oregon

DOGAMI: Oregon Resiliency Plan

Cascadia Projection vs. the March 2011 Japanese earthquake and tsunami

Reacting to the 2011 Japanese earthquake, the State of Oregon commissioned a study of Oregon earthquake and tsunami preparedness for a 9.0 Cascadia earthquake. It is called the Oregon Resiliency Plan. The final report was published in February 2013.

Its chilling findings apply only to Oregon:

  • 1,250 to 10,000 killed
  • Severe shaking lasting 5-6 minutes
  • Tens of thousands of structures destroyed
  • $32 billion in property damage
  • Parts of northwest Oregon near Astoria could drop 4-8 feet in elevation
  • Tens of thousands left homeless
  • Severe infrastructure damage causing permanent economic slowdown

DOGAMI: Oregon Resiliency Plan

Strong ground motion of one foot or more affects everywhere in western Oregon, according to the DOGAMI report.

And that is just Oregon. Cascadia will affect the entire Pacific Northwest from Vancouver, British Columbia to northern California.

Conclusions

The next Cascadia earthquake will be the most destructive natural disaster ever to strike the United States. Its effects on the state of Oregon alone will be immense.

According to geologists, there is a 37 percent chance of a Pacific Northwest mega-quake in the next 50 years.

Add in earthquake and tsunami devastation from Northern California, Washington state and southern British Columbia, and there will be a swath of death and destruction of unimaginable proportion.

Every municipality, city planner, builder and citizen in the Pacific Northwest inland 100 miles to the crest of the Cascade Mountains should study the Oregon Resiliency Plan. Tsunami evacuation routes in all coastal community should be well publicized. It’s a matter of life and death.



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Steve Davidson is a columnist, wilderness landscape photographer and former teacher, software developer and lecturer who writes about science, technology and the environment.

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