Chris Rowan is a geologist specialising in the dark arts of paleomagnetism, and getting people to pay him to travel to exotic destinations for fieldwork. Having drilled up New Zealand during his PhD, and South Africa in his first post-doc, he now works at the University of Edinburgh.Japan was hit by a couple of large-ish earthquakes yesterday. According the USGS moment tensor solutions, the first magnitude 6.6 was caused by the rupture of a normal (extensional) fault just off the west coast of Japan at a depth of about 8 km. About 12 hours later, a magnitude 6.8 occurred in the same region, but at a much greater depth of 360 km or so, which means that it has to have been within the cold, westward dipping slab of Pacific plate that is being subducted beneath the east coast of Japan (at that depth, normal mantle is much too hot and weak for brittle fracturing to occur).
Note that the quake that caused all the damage (some photos from BBC news) was actually the weaker of the two; the other one was so deep that the seismic energy had probably mainly dissipated by the time it got to the surface.
Perhaps the most worrying news was the report that the quake caused a fire at a large nuclear power plant, and a leak of water containing "a small amount of radioactive material" (whatever that means). This does not fill me with optimism for the future, because these quakes were nothing to do with the subduction megathrust off the east coast of Japan, which is capable of producing much larger earthquakes. Whatever your wider thoughts on the general safety/desirability of nuclear power, not building them next door to giant earthquake-generating faults is surely a bit of a no-brainer.
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Comments
Isn't that actually a reverse fault moment tensor? (It's early in the morning here, and I confess to always needing to review which beach ball is which before talking about them in class. But someone who works with them all the time taught me a trick for remembering: reverse fault moment tensors look like cats' eyes; cats always do the reverse of what you want them to do.)
The reverse solution is interesting tectonically (I think), because it's behind the arc, in an area that is extensional in many places (and presumably was in Japan in the past). But I've noticed that a lot of the moderate-sized earthquakes on the west coast of Japan have reverse slip; I wonder if the Sea of Japan is actually closing?
(And yes about the nuclear power plant. It is worrisome. M 6.6 is big enough to do a lot of damage, especially where buildings are susceptible to shaking. But one would hope that a nuclear power plant in Japan would not be one of those highly susceptible buildings.)
Posted by: Kim | July 17, 2007 09:20 AM