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Earthquake Signals Tectonic Plate Ripping Apart

Sumatra 2012

A map of the Indian Ocean region shows boundaries of Earth’s tectonic plates in the area, and the epicenters (red stars) of two great earthquakes that happened April 11, 2012. CREDIT: Keith Koper, University of Utah Seismograph Stations.

Three papers in this week’s issue of the journal Nature present startling new findings about an earthquake that struck the Indian Ocean off the coast of Sumatra last April.

As reported by Andrea Mustain at Our Amazing Planet, three features of the quake made it unusual:

First, it was extremely powerful—at magnitude 8.7, it was the sixth most powerful ever recorded. Second, it struck in the middle of a tectonic plate and not along a plate boundary. Third, when the quake zipped along the initial fault and ran into faults intersecting it at right angles, those intersecting faults ruptured too.

Mustain interviewed Thorne Lay, a professor at the University of California, Santa Cruz and co-author of one of the papers:

Lay said that, typically, when earthquakes spread to connecting faults, the rupture rips along faults that branch away from the initial fault like the branches of a river. These earthquakes raced along in a grid-like pattern, making 90-degree turns along faults that resemble a lattice.

“Here, they really do seem to go along perpendicular faults, and we haven’t seen anything like that with a big earthquake,” he said.

So why did this earthquake behave so oddly? Scientists believe they are witnessing the Indo-Australian plate ripping in two, forming a new plate boundary, a process that has already been playing out for millions of years.

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