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Ready To Rumble

"Cascadia subduction zone tsunamis could conceivably cause the loss of tens of thousands of lives on the Pacific Northwest coast of North America. Paleoseismic and other data support Cascadia earthquakes with moment magnitudes of >9, rupture lengths of >1000 km and recurrence of 400-600 years; the last event was 301 years ago, so the conditional probability of another occurring in the next 100 years is high."

—From a 2001 paper by George R. Priest, Oregon Department of Geology and Mineral Industries, Newport, Oregon

On January 26, 1700, a megathrust earthquake along the Cascadia fault of North America generated a tsunami that propagated havoc along Japan’s Honshu Island. The waves came in from the deep at more than 10 meters — destroying coastal villages, sinking ships and sweeping untold numbers of villagers into the sea.

Historians in Japan recorded the event, but the witnesses in North America handed down oral stories of the giant winter waves that appeared from nowhere and swallowed villages. Those foggy myths made it to the ears of 20th-century American geologists who combined them with tree rings and weird layers of sand to support a theory that the Cascadia fault had let loose with a tremendous earthquake between August 1699 and May 1700.

It was all theory until 1996, when geologists in Japan found written reports of a large tsunami event along the east coast of Honshu Island. In 2003, seismologists Kenji Satake, Klein Wang and Brian Atwater published a paper proving the Cascadia fault had produced a 9.0 earthquake in 1700.

With that mystery answered, another one arose: How likely is that to happen again, and what would be the consequences for the Pacific Basin?

Similar to the megathrust fault in Sumatra that caused so much destruction throughout South Asia and East Africa, the Cascadia fault is a subduction zone where two massive tectonic plates meet in slow collision. From Cape Mendocino in northern California to Vancouver Island in British Columbia, the Juan de Fuca plate is sliding under the North America plate and building up tremendous pressure.

The Cascadia is a relatively quiet fault, with most of its seismic history buried in time and under many fathoms of ocean. Seismologists regard the Cascadia as equally silent and potentially deadly, and so in coastal towns from Eureka to Washington there are tsunami evacuation signs posted on every other street corner. On local television in coastal areas, hourly PSAs answer the musical question, "Where you gonna run to now?" by suggesting high ground at least 100 feet above sea level. Public safety officials are more than a little concerned for what could happen along the Cascadia fault. Which leads to another question: "What about us south of the Point Conception line?"

Eric Geist is a geologist with the United States Geological Survey in Menlo Park who is more than a little wary of Cascadia, but does not have enough information to compute what a megathrust event up there would do down here: "Unfortunately, it’s difficult to say because we are just starting to learn about great earthquakes along the Cascadia subduction zone," Geist said. "The best we can do is to look at megathrust earthquakes of the past."

On May 22, 1960, a magnitude 9.5 earthquake along the Peru-Chile trench generated a tsunami that struck Hilo, Hawaii, 15 hours later, killing 61 people, destroying more than 500 homes and causing $24 million in damage. The tsunami was felt as far north as Crescent City, California, but damage there was minimal.

Four years later, Crescent City took a direct hit from a tsunami spawned by the magnitude 9.2 Great Alaska earthquake. Four hours and 2,000 miles away from Anchorage, a series of four waves drained Crescent City Harbor, rushed over the breakwater and swept commercial fishing boats 30 blocks inland. The tsunami killed 11 people and started a fire that raged for three days.

With models made from those two Pacific megathrust events, Geist can estimate when a Cascadia-generated tsunami would arrive here: "There are timetables available online, but the first tsunami waves would probably come in at about one to two hours after the earthquake."

Geist cannot say just how powerful those tsunamis will be: "Because Southern California is not directly across from a Cascadia rupture, the tsunami waves will be relatively less. Sri Lanka and eastern India, as well as Sumatra itself, are situated directly across from the rupture area, and they were the hardest hit. Similarly, Northern California and Crescent City were in line with focused waves from the 1964 Alaska earthquake. However, most of California is at an oblique angle to Cascadia, and the little research we have about such events suggest that oblique angle lessens the impact of seismic waves."

Seismologists cannot say what the consequences of such an event will be for Southern California, but here is a rough caveat: If you hear of a massive seismic event occurring along the Pacific Northwest coast, resist the temptation to run down to the beach to pick free lobster or get the money shot for CNN. If you must, find a cliff at Palos Verdes or a vantage point high in the Malibu Hills. Anything from 0 to 100 feet above sea level is the danger zone.


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