The Trembling Earth

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Reblog (not mine): Pete Rowley of “Lithics” writes about the M8.6 near Sumatra

I’ve got tons of stuff I should be doing, but I can’t resist the allure of discussion of the fascinating pair of earthquakes that happened last night in the Indian Ocean. I don’t have much of anything to add to the existing discussion, and I need to, like, get to work, so this is a “reblog” of the most thorough description I’ve seen of the setting of these quakes, by Pete Rowley on his “Lithics” blog. Note that there were two, both were massive (8.6 and 8.2), and they had a “foreshock” on January 10–sameish place, similar focal mechanism. Both of these are larger than any strike-slip earthquake we’ve recorded before, and the larger one makes it into the top 10 list of all time in any setting. It’s additionally fascinating (read: perplexing, at the moment) that they happened in oceanic crust, which is generally considered too thin to support massive strike-slip earthquakes. This will be an absolutely fascinating earthquake sequence to learn from in the coming hours, days, and weeks.


Earlier I posted an info bulletin about this morning’s Banda Aceh earthquake.  Rather than muddle it with more and more stuff, I thought it might be better to include this update as a separate post, as it is more of a discussion than a news piece in any case.

The truth is that this earthquake is properly strange.

The part of the Indian Ocean in which this earthquake occurred has two very different types of geologic structure very close to each other; there is the Ninetyeast ridge – a volcanically produced range,  and a destructive margin subducting the Indian plate eastwards under the Pacific margin. It is important to note (in the context of this earthquake at least) that the sea floor under the Ninetyeast ridge was originally produced by standard constructive margin seafloor spreading.

These two structures are shown quite nicely in this image taken from GeoMapApp, with…

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