Even though a magnitude-8.6 undersea earthquake that occurred far west of Sumatra on 11 April caused no damage and triggered no tsunami, it was a record-setter nonetheless. For one thing, it was the largest temblor ever measured far from the boundary of a tectonic plate, researchers report online today in Science . Also, the so-called intraplate quake is the largest ever measured for a fault zone for which the two sides of the fault slide horizontally past each other, a la the San Andreas fault, rather than having one side of the fault shoved beneath its neighbor. The epicenter of the quake (denoted with a star, above) was located about 400 kilometers southwest of the magnitude-9.1, tsunami-spawning temblor that occurred off the northwestern coast of Sumatra on 26 December 2004 (areas affected by largest slippage of that quake are depicted in red, orange, and yellow at upper right; northwestern tip of Sumatra depicted in gray silhouette). Over the course of about a minute and a half, this spring's temblor ruptured 500 kilometers of three separate but related faults, all of which were highly stressed. A lot of that stress was shifted to the area by the December 2004 quake, the researchers suggest. The faults unzipped at a relatively slow but steady 2.5 kilometers per second, the researchers say, with much of the slippage along the faults taking place more than 25 kilometers below the seafloor—a depth that helped contribute to the quake's great magnitude.
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