Pacific Northwest has History of Giant Quakes like Japan’s Recent 9.0

Northern California has a history of quakes as large as the giant 9.0 that occurred in Japan in March 2011. That’s according to Delta Independent Science Board (ISB) Member Brian Atwater, whose expertise includes studying ancient earthquakes and tsunamis.

Atwater points to earthquake history inferred from North American geology and Japanese documents. Together this evidence tells of a giant earthquake in A.D. 1700 along North America’s largest active fault outside Alaska - a fault called the Cascadia Subduction Zone that extends 700 miles from Vancouver Island in British Columbia to Cape Mendocino in Humboldt County. (A subduction zone is when one tectonic plate sinks beneath another). How this quake and its predecessors were discovered can be seen in The Orphan Tsunami of 1700, a book he prepared with five coworkers from the United States and Japan.

“There are many different sources of earthquakes in western North America,” Atwater said. “The single biggest ones are the gently sloping faults that convey oceanic plates beneath the continent. The Cascadia subduction zone is one of these faults.” The Cascadia fault has much in common with the one that produced the giant earthquake and devastating tsunami in Japan on March 11, Atwater says. The similarities include long times between giant earthquakes - so long that people tend to forget about them.

The giant 9.0-magnitude earthquake off the coast of northern Honshu is the largest to hit that part of Japan in more than 1,000 years. Not since A.D. 869 has a comparable tsunami extended 2-6 miles inland.

In a 2007 paper co-authored with a Japanese geophysicist, Atwater said that histories of earthquakes and tsunamis, inferred from geological evidence, aid in anticipating future catastrophes. Under favorable circumstances, such paleoseismology can serve as a long-term advisory that helps people prepare for unusually large earthquakes and tsunamis. The paper’s prime example is Cascadia.

“The term ‘plate tectonics’ didn’t appear in print until 1969,” Atwater said. “Without the idea of plate tectonics, it was hard to even imagine the possibility of a 9-magnitude earthquake at Cascadia.” But around 1980, questions began to be asked whether such Cascadia earthquakes could happen, and by the late 1980s, geologists had shown that they had - most recently in the decades between 1680 and 1720. A few scientists who were following this North American research asked Japanese earthquake and tsunami historians whether Japanese written records from those decades tell of a tsunami from far away - one unaccompanied by shaking felt in Japan. “The historians replied they had one tsunami in 1700 they'd been looking for a home for, for a long time,” he said.

“We owe quite a bit of our tsunami preparedness/earthquake preparedness to this help received from Japan,” Atwater said. “We couldn’t tell by the geology here whether we were dealing with magnitude 8’s or 9’s. The Japanese thought 9 - a magnitude of 8 doesn't displace enough seawater. Their houses washed away in 1700 with our earthquake and the resulting tsunami.”

 


“We owe quite a bit of our tsunami preparedness/earthquake preparedness to help received from Japan.”

--Brian Atwater, Delta Independent Science Board member


 

Atwater and his colleagues gave the Japanese link a final test: they revisited earthquake-killed trees to learn whether they'd died in 1700. “The trees told us they’d lived through the 1699 growing season and were dead by the start of 1700,” he said.

It’s hard to imagine an earthquake of magnitude 9 that is represented on a map by its so-called epicenter, usually depicted as a dot with concentric circles. Atwater encourages replacing “epicenter” with a term that is less misleading, such as “rupture starting point.”

Atwater offered two analogies to help explain “rupture starting point.” In the first, you’re driving along the highway and a big truck from the other direction kicks up a pebble that cracks your windshield. The pock mark just shows where the crack began.

In the second analogy, a dropped match initiates a house fire. Exactly where the fire starts (analogous to the rupture starting point) is usually less important than the extent of the burn (the fault-rupture area).

To make the giant Japanese earthquake in March 2011, the fault rupture grew from such a starting point to attain an estimated length of about 250 miles and a width that’s commonly estimated to be 100 miles, Atwater said. “Having that much of one rock mass shift over the other makes for a lot of shaking. And that same shift sets off the tsunami by displacing the sea floor.”

As a geologist thinking long term, on time scales of millennia, Atwater thinks more in terms of earthquake forecasting than earthquake prediction. He likens earthquake forecasting to a long-range weather forecast that covers decades. He cites forecasts of Bay Area earthquakes from well-known faults like the San Andreas and Hayward.

Earthquake forecasts in California are based, in part, on written and geological histories.

The source area of the 1700 Cascadia earthquake, though as little as 200 miles from Sacramento, is close enough for seismologists to take Cascadia earthquakes into account as they try to forecast future shaking in the Delta. The forecasts give greater weight, however, to quake sources in the Bay Area and beneath the Delta itself. The forecasts figure in the Delta Plan, which Atwater has been reviewing as a Delta ISB member.