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Andrew Alden

M8 Earthquakes Off Sumatra

By April 11, 2012

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Late in the afternoon of 11 April (local time), a magnitude 8.6 earthquake struck northern Sumatra, well offshore. A second event of magnitude 8.2 struck just two hours later. For the moment, it's being called an aftershock. These very large earthquakes produced only small tsunami waves, and tsunami watches in the region have expired without significant incidents.

So far so good. In Banda Aceh, so thoroughly devastated in the 2004 earthquake, there was widespread evacuation and panicky behavior but little damage.

What catches the attention of seismologists is that these two earthquakes appear to be the largest strike-slip events ever recorded. That is, the motion of the rocks on either side of the fault was side-to-side rather than one side moving beneath/above the other side. (See "Fault types in a nutshell.") This kind of earthquake motion does not push the seafloor up or down to any great degree, therefore the tsunami risk is low. Expect a flurry of research papers about this earthquake pair in a few months. But already you can sense the conversation around the University of London in this post from Pete Rowley's lithics blog.

More:
USGS special page on the M8.6 shock
Back-projection models of the mainshock
USGS special page on the M8.2 shock
Back-projection models of the aftershock
What are earthquakes?

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