When ever I have wondered if, in my lifetime, I might experience a huge subduction-zone earthquake, my answer would have been—well, depends on whether I am living in the Pacific Northwest the next time Cascadia ruptures, which could be during my lifetime (and if I were to live it out in Seattle), but (I tell myself) the last one was in AD 1700, “only” 310 years ago, and it was a big one—about 9.0, so it takes time to build up enough strain to make another one that big. So I’ve figured in my reasonable but unrigorous way that it’s unlikely…though possible, and if it happens soon, it would be smaller than an 9.0. But if I were designing a nuclear power plant... go with the worst case.
Never in my wildest dreams or nightmares did I expect to experience a giant earthquake on some other subduction zone. A word about size --in the formal moment-magnitude scale, which basically measures the total energy released during the event, earthquakes larger than 8 are called “great earthquakes.” In the 7’s they are “large earthquakes,” and perhaps because ones of 9 or bigger are so rare (I suppose), they didn’t get a formal name, though nowadays people sometimes use “giant.”
I guess we came pretty close to having the great-earthquake experience while working in the Kurils in summers of 2006 and 2007, sandwiching two great (but not giant) earthquakes in the intervening winter months…camping at some spots that got obliterated during the ensuing tsunamis. So much for expert knowledge.
But each of these really great or giant events (bigger than the Kurils, the ones that are 8.9 or bigger, let's say) happens on one day in one year, over some finite stretch of coastline, and globally only a handful of times in a century. So that’s a pretty low chance you’ll experience it, unless you live on a zone ready to pop. And so far, the scientific question for any spot of whether the pop will be “great” or “giant” isn’t well constrained, though giants can’t recur in one place as often as greats, because they release much more of the accumulated energy. In fact, the energy released by just one giant event, Chile 1960, released as much energy as all of the magnitude 8 through 9 events IN A CENTURY [see to left, from IRIS].
If the pressure cooker blows off all its steam, the steam must build up again. OK, analogy more appropriate for volcanoes, but you get the idea. In earthquake lingo it’s called “stick-slip” behavior – the fault is stuck for some time, and then slips quickly, generating an earthquake and, if under water, a tsunami.
Northern Honshu “knew” the potential for a great earthquake-tsunami pair like this because, contrary to the reports, there _is_ such an event in the “historical record.” What the reporters mean to say is that there hasn’t been an event of this scale in Japan in the “instrumental record” (where actual size/energy release can be quantified). There is, however, at least one great or giant earthquake and tsunami in Japan’s historical record, known as the “Jogan earthquake” (and tsunami), which occurred in A.D. 869; the deposit is found under a historic ash layer from 915 A.D. (see below, from April 2010 field trip). Yes, although Japan’s written historical record is not continuous, it includes much older events than many regional historical catalogues such as the Pacific Northwest, which only goes back into the 19th century, and earlier a few European visitors for short intervals. There were people in the Pacific Northwest before that, but they didn’t keep a calendrical record, though they did tell stories consistent with earthquakes and tsunamis.
My own understanding of this part of the coast of Japan started with a trip to see the record of the A.D. 869 Jogan tsunami on the Sendai plain--one of the regions so dramatically inundated on 11 March 2011. Sendai is a broad flat plain dominated by rice fields, and including an airport, where videos showed a brown, sludgy-like tsunami seeming to ooze across the surface (see above). I was out on this plain in April 2010, on a field trip during a tsunami conference. We were taken to the Sendai plain to examine the evidence for the Jogan tsunami and also to see the counter-tsunami measures for that region – informational signs, evacuation signs,
and the idea that storm mitigation [offshore riprap, low seawall, forest designed to stop salt air from the rice fields] might help mitigate the tsunami. Farther inland, a monument marked where the Jogan tsunami is thought to have reached. The geologic evidence comprises a sand layer within a loamy peaty soil and underlying a dated volcanic ash from A.D. 915 (see above).
I discuss more about trying to figure out how big this A.D. 869 tsunami and earthquake were in my blog on Nuclear Earthquakes. But it seems that no one expected a magnitude 9 here, from any kind of record, historical or not. Live and learn. The very hard way.