Cascadian Subduction Zone Earthquakes

“January 26, 1700 – why don’t I know that date?”

A young girl asked that question – Bonnie Henderson, author of The Next Tsunami, told the audience during a recent talk in Eugene – upon first learning about the last great Cascadia earthquake, which was an estimated M9.0.

Wait – how do they know that? It was some 40 years before the Russians reached North America and about 70 years before the Spanish and English explored the Pacific Northwest coast. Nobody was keeping written records back then.

Well, they’ve also estimated the time it happened – around 9 p.m.

Here’s how they figured it out.

 

Map of Kyushu, showing tsunami sites in January 1700.  Source #1 below

Map of Kyushu, showing tsunami sites in January 1700. Source

The “orphan tsunami” of 1770

Tsunami wave trains began to hit the eastern coast of Japan’s Honshu Island at midnight on the night of January 26-27, 1770 (locally, it was the hour of 9 on the 8th day, Genroku 12, Younger Brother of the Earth, in the Year of the Rabbit).

We know the precise time because it was during the shogunate. Records were kept by local officials and sent to Edo (modern Tokyo) in order to collect aid for the damage the tsunamis caused. In some affected areas, prominent people noted the event in their journals.

Only one official called it a “tsunami.” Japan is no stranger to earthquakes and tsunamis, and many noticed that no shaking had accompanied these waves. Most people referred to them as as “unusual waves,” “high tide” and the like.

They certainly weren’t small ones.

Seismologist have visualized the event this way:
 

In Kuwagasaki (PDF), 13 houses were destroyed by the waves and more burned down in the resulting fires. No lives were lost – people fled to the high ground. People were also fortunate in Tsugaruishi (PDF), where the water ran inland over a mile, washing away houses and entering the village.

Rice paddies were damaged at Otsuchi (PDF), along with two houses and two salt-evaporation kilns. The flood stopped before it reached the main street. A 20th-century historian mentioned this flooding in an earthquake catalog that eventually led to recognition of the 1770 “orphan tsunami” and identification of its parent.

People died at Nakaminato (PDF), a port city at the mouth of the Naka River, but this probably was because a storm complicated things. High waves that were caused probably the ebbing tsunami flood kept a big rice barge from entering the port. This went on all day. The weather was fine until evening, when a storm came up and blew the barge onto the rocks. Two crew members were lost, as was the entire cargo of rice (28 tonnes).

At Miho (PDF), the waves repeated rose and fell. The headman advised his villagers to evacuate, although the tsunami there caused no damage.

In Tanabe (PDF), “unusual seas” flooded farmland and got into the castle moat as well as a government storehouse.

In most cases, tsunami damage and estimated wave heights were similar to those in the same regions after the M9.5 Chile earthquake in 1960.

Twentieth-century historians and geologists eventually tracked down this orphan’s Big Daddy, using the Japanese records and Cascadian field evidence.

Cascadian earthquakes

Cascadia is the region west of the Cascades, comprising parts of three US states and a Canadian province – northwestern California, western Oregon and Washington, and southwestern British Columbia.

Just offshore, the Juan de Fuca seafloor plate and the North American continental plate are jousting, and Juan is losing. The heavier seafloor is subducting underneath the continental plate.

These massive forces involved are ultimately responsible for Cascadia’s current natural beauty, as well as its hazardous volcanoes and earthquakes.

Most of the earthquakes here are shallow, M7.5 or less. They happen because of movement along local faults and only cause serious damage every few decades.

Earthquakes of up to M7.5 also happen at very deep levels, 18 miles or so, below Cascadia. They occur as the downgoing slab of seafloor breaks apart. Energy from these quakes is dispersed over a wide area, so they’re much less damaging than if they happened near the surface.

Until plate tectonics came along in the late 20th century, scientists believed that shallow and deep earthquakes were all the seismic risk that Cascadia faced. Then, with plate tectonics established, earth scientists recognized that there is a subduction zone just offshore and that several very large earthquakes have happened here over geologic time.

Subduction zone earthquakes happen when energy that has built up between two tectonic plates moving against one another is suddenly released.
 

In 2004 and again in 2011, the whole world saw how bad such an earthquake and the resulting tsunami can be.

The amount of energy released during an earthquake like this depends on how big an area breaks. It is estimated that an area as long as the state of California slipped during the Cascadian subduction zone quake in 1700. That’s a lot of energy.

The shaking from such a quake goes on for several minutes, of course, since such a huge area of ground is on the move. Smaller, wooden structures can stand up to such shaking, but the duration sets up a resonance with the ground that can bring down larger structures like office buildings and bridges.

While geoscientists were coming to the awful realization that the Cascadian region faces a much worse seismic hazard than they had thought, historians on the Pacific’s Japanese shores were finding clues in old records that led them to the “orphan tsunami” of 1700.

In the late 1990s, with the additional help of Native American oral traditions that clearly describe the effects of a massive tsunami, people put it all together.

A subduction zone earthquake did indeed hit Cascadia at around 9 p.m. local time on January 26, 1700.

That’s a date that should indeed receive special recognition throughout the region, because such an earthquake is eventually going to happen here again.

The next big quake(s)

In 2005, the Cascadia Region Earthquake Workgroup (CREW) put together a scenario (PDF) for a M9 Cascadia subduction zone earthquake that happens in mid-July, when the weather is hot and dry. I added a note to their general description:

Different parts of Cascadia will have different experiences.

  • Coastal communities will be subjected to strong shaking, landslides, and tsunamis. Buildings, roads, bridges and utility lines will suffer varying amounts of damage. Some will be destroyed. Extensive injuries and fatalities are likely. Within minutes, a tsunami will arrive, making it essential that residents and visitors understand the need to head for higher ground or inland as soon as the shaking stops. Coastal Highway 101 will be impassable over large stretches, and landslides through the Coast Range will sever highway travel between the coast and inland areas. Destruction of roads, runways, ports, and rail lines will leave individual cities isolated. Residents and visitors will have to do much of the work of rescuing those trapped in the rubble and will be responsible for the immediate clean-up and organization to distribute relief supplies.
  • Along the I-5/Hwy 99 corridor [which includes Eugene, where I live], utilities and transportation lines in some areas could be disrupted, perhaps for months. This particular type of earthquake is especially hazardous to tall buildings, which could lead to significant fatalities in downtown are a s . Buildings that would be unscathed in a more typical 30-second quake might be severely damaged after several minutes of shaking. Long bridges and utility lines are also at risk, which could create serious long- term economic losses. Landslides could block east- west travel through the Cascades. As the the center of our regional transportation network, closures at any point here could have far-reaching consequences.
  • East of the Cascades, communities can expect a lower level of shaking. Even so, they will feel economic effects from the regional damage and will be important staging points for recovery efforts in Cascadia.

At the time this scenario was devised, geologists believed such devastating quakes happen roughly every 500 years. However, new interpretation of the evidence (which was gathered approximately 50 years ago) suggests that the interval might be more like 300 years.

When will the Big One happen again?

Earth scientists set the 500-year interval based on, among other things, cores of seafloor mud. These showed that mud flows (called turbidity currents) had repeatedly happened offshore, descending into submarine canyons. The flows met at the open seafloor where tributary canyons converged.

Scientists realized that these big flows happened because the ground was shaking. The mud flows provided useful evidence about how frequently major movement like that has occurred down through geologic time. However, researchers are now realizing that things aren’t that simple.

According to the abstract of a new paper by Atwater et al.:

A stratigraphic synthesis of dozens of deep-sea cores, most of them overlooked in recent decades, provides new insights into deep-sea turbidites as guides to earthquake and tsunami hazards along the Cascadia subduction zone, which extends 1100 km along the Pacific coast of North America. The synthesis shows greater variability in Holocene stratigraphy and facies off the Washington coast than was recognized a quarter century ago in a confluence test for seismic triggering of sediment gravity flows…The fuller synthesis…shows distinct differences between tributaries, and these differences suggest sediment routing for which the confluence test was not designed. The synthesis also bears on recent estimates of Cascadia earthquake magnitudes and recurrence intervals. The magnitude estimates hinge on stratigraphic correlations that discount variability in turbidite facies. The recurrence estimates require turbidites to represent megathrust earthquakes more dependably than they do along a flow path where turbidite frequency appears limited less by seismic shaking than by sediment supply. These concerns underscore the complexity of extracting earthquake history from deep-sea turbidites at Cascadia.

In plain English, science reporters say that the “findings reinforce the idea that big quakes occur in the Pacific Northwest — but raise doubts about how well researchers understand past seismic activity.”

The evidence may now be interpreted in a way that shows Cascadia to be prone to slightly smaller magnitude earthquakes (around M8) limited to the southern end of the subduction zone (off the Oregon coast) every 240 years or so, rather than a big one every 500 years or so.

Magnitude 8 is still a lot of pasta, as this video shows.
 

Complicating matters, if I understand this right, is that central Oregon may be locked, tectonically speaking. Instead of storing up energy for a “big snap,” that energy might be getting released slowly via persistent fault creep.

Disaster resilience

The closer you look at something through the eyes of science, the more complicated it sometimes gets. Emergency planners, however, can’t wait for the scientific process.

Since it has been 314 years since the last Cascadia subduction zone quake, they need to know now whether they should expect a M9 every 500 years or a series of M8 quakes every 240-300 years.

Of course earth scientists can’t tell them that with kind of accuracy. The reinterpretation only appeared a few months ago. Experts are still discussing it.

In the meantime, it doesn’t hurt to prepare for a worst-case scenario.

In her Eugene talk, writer Bonnie Henderson described how the state of Oregon did just this with their planning for a coastal tsunami. They had originally set evacuation zones for an “average-sized” tsunami that was some 30 feet in height. Eventually planners decided to prepare for the worst case – an expected 100-foot-high tsunami wave.

Waves 100 feet high Coos Bay, Florence, Newport…it’s a nightmare, but one where those involved are already planning to wake up from.

The CREW writers say:

A Cascadia earthquake will seriously affect our region, but it won’t destroy us. We will rebuild our cities, our neighborhoods, and our businesses. The time it takes us to recover will depend depend largely on what preparations we make before the earthquake.

It will take years to recover from a Cascadia subduction zone earthquake. The tsunami that follows may damage not just us, but Alaska, Hawaii, Japan, and other Pacific Rim economic powers. The resources of the US, Canada, Japan, and other nations will be used to rebuild damaged areas, affecting the world economy.
After other disasters like hurricanes, communities have used the event to embark on a new plan for the future. We can prepare for this earthquake, recover from it, and build a future Cascadia that is still the place in which we want to live, work, and play.

I have never seen a better summation of the nature of human resilience in the face of natural disaster. If the Big One comes while I’m still round, I hope I can live up to that native Western strength, become a resource for others in need, and help move Oregon on and into a better future.
 
 
 


 
Sources:

  1. The Orphan Tsunami of 1700. Brian Atwater et al., US Geological Survey/University of Washington
  2. Cascadia Subduction Zone Earthquakes: A Magnitude 9 Earthquake Scenario. (PDF) Cascadia Region Earthquake Workgroup.
  3. Native American Legends. US Geological Survey
  4. Seabed samples cast doubt on earthquake risk for Pacific Northwest. Mark Zastrow

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