At around 9pm on the 26th January 1700, the Cascadia subduction zone – a shallowly dipping thrust fault that runs more than 1000 km north from Cape Mendocino in Northern California to the vicinity of Vancouver Island, ruptured in an estimated magnitude 9 earthquake. No Europeans were there to witness the shaking and the inundation that followed, as the Oregon and Washington coasts were engulfed by tsunami more than 10 m high. But indigenous people were, and some of their oral accounts, of this earthquake and similar ones before it, still survive. The tsunami crossed the Pacific basin to Japan, where it was recorded as an ‘orphan’ tsunami (one that was not preceded by a large local earthquake). The Japanese records are how we precisely know the day and rough time of the rupture.
More recent records from Japan – pictures and video of the tsunami generated by the 2011 Tohoku earthquake – provide a sobering vision of what the Cascadia megathrust might do to the Pacific Northwest when it next wakes up. 317 years have passed, and we can’t know precisely how many more might pass before the next big earthquake. Records of giant submarine landslides triggered by large subduction zone ruptures, preserved in sediments off the Cascadia margin, show that on average, a big rupture like the 1700 event occurs every 500 years or so. But the Earth is not clockwork, and the gap between two individual earthquakes can vary significantly from the long-term average. All we know is that it will happen at some point in the next few hundred years, and we had best get prepared.
This is not exactly happy knowledge. Nonetheless, having that knowledge is still something to be thankful for, when we consider the all-too-common alternative: us going about our rapid human business, unaware that the slow geological workings of the planet beneath our feet are turning, building up to a disaster we never see coming until it is upon us. But for Cascadia, dogged and careful scientific detective work* over the past thirty years means that we are in the relatively happy position of comprehending the threat before it takes us by unpleasant surprise.
It could still be a tragedy – even the most recent assessments make it clear that there is still plenty of work to do to prepare the region for the day when the ‘years since last rupture’ counter flips back to zero. But knowledge is power, and in this case it is life-saving power.
*much of it, I feel compelled to point out in these interesting times, funded by the USGS and other US government agencies.