In 923 C.E., a massive earthquake -- estimated at magnitude 7.5 -- struck the Pacific Northwest.
Marine terraces -- stairstep-like formations often found on oceanic shorelines -- show evidence of a sudden uplift. Tree-ring data, from stumps and logs entombed in mud, appear to be the remnants of a forest swamped by a massive tsunami, and dendrochronological techniques date their burial to that specific year.
When I first heard about this, from a paper last week in the journal Geology, I immediately assumed the culprit had been the massive (and scary) Cascadia Subduction Zone, about which I wrote here at Skeptophilia a couple of years ago. This fault line, considered one of the most potentially dangerous seismic zones in the world, is capable of producing megathrust earthquakes, where a piece of a plate is driven underneath another. When they slip it's often catastrophic, pushing the overriding plate upward by as much as several meters. When this happens underwater, it's even worse; the volume of water that displaces causes enormous tsunamis, that can travel all the way across oceans while losing very little of their energy -- with horrifying results.
So the 923 temblor seemed to fit the bill. A dreadful tsunami, and shoreline uplift by over eight meters. But I was wrong about its cause.
This quake occurred because of a pair of faults that bisects the Seattle/Tacoma metropolitan area itself.
If you live in the Seattle area and are thinking, "Oh, great, one more natural disaster in the making to worry about," the good news is that this fault -- or, more accurately fault zone -- doesn't appear to slip very often. In the eleven-thousand-odd year geological record, the 923 quake was the only apparent major failure. But it always bears keeping in mind that unlike many other natural disasters, such as volcanic eruptions, hurricanes, and tornadoes, we still don't have a reliable way of forecasting earthquakes. Seismologists can say, "This fault has a fifty percent chance of failing in the next twenty years," but anything more precise -- when exactly it'll slip, the magnitude of the earthquake that will result, how much damage it will cause -- is beyond our current science.
"We just don’t know what the recurrence interval for these big quakes is," said Elizabeth Davis, geologist at the University of Washington, who led the study. "For a fault that has had so much attention, there's so much we still don't know."