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| 50 m sea floor slip Tohuku Japan, drill project |
Yesterday there was a talk at "Heartland University" (not real name) by some distinguished lecturer. It was a pretty good talk, but there were problems. He went on and on about heat flow and that you get seismic slip on subduction megathrusts only between about 100 or 150 deg C at the top and 350 deg C at the bottom. He said the upper part above the limit undergoes aseismic creep. I asked the first question and it was why they said there was 50 m of slip at trhe sea floor for the March 11 M9 Tohoku quake (Honshu) mentioned in a mass email I had received about participating in drilling this subduction thrust. He admitted that this was true. I said that it is not possible for there to be aseismic creep at the full slip rate and then turn around and have large slip (see Tony's earlier email and our discussion a couple weeks ago about Hayward fault). I also pointed out that the southern 100 km of the main Central thrust (I think) in the Himalaya is aseismic: the GPS shortening and the small seismicity is beneath the high Himalaya of Nepal, but the Great quakes rupture the whole thing. I don't know if the link will work; if not, it is below, or search on: On Google the correct link is #3. Figure 1 in this report shows a large area of 50 m slip near the upper edge of the thrust (near the sea floor). That is why the tsunami was so large. What I was really concerned about was his strong suggestion that you could use heat flow and temperature and fault dip and historical seismicity to predict the largest megathrust earthquake. The implication was that Costa Rica, and Central America trench in general, only has low M8 quakes. Well, that worked really well for the nuclear power plant and sea wall designs for Honshu. And let us not forget that before Geologist (note the capitalization) Brian Atwater found evidence in Cascadia from tsunami deposits and drowned forests that seismologists (small "s") thought that Casacadia could not have Great quakes; that the whole subduction zone was characterized by aseismic slip. Maybe next time earth scientists (and nuclear power plant designers) should learn from their mistakes! (there used to be a lot of jokes and cartoons in the oil industry about Geologists vs. geophysicists...thought I would try and revive this; it's been too quiet on this board lately). Chris
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