GEOPHYSICS: Sensing Supershear

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Science  05 Sep 2008:
Vol. 321, Issue 5894, pp. 1272d
DOI: 10.1126/science.321.5894.1272d

Recent observations, supported by experiments, have indicated that some earthquake ruptures transiently exceed the local speed of sound along the fault zone. This “supershear” can explain enhanced shaking from these quakes; thus, supershear ruptures are critical in assessing seismic risks. Many of the details of how ruptures accelerate to above the sound speed and then decelerate, in some cases repeatedly, as a rupture progresses are unclear, as most supershear ruptures have been inferred by data inversions. Vallée et al. were able to observe these dynamics more directly in the 2001 Kokoxili earthquake (M = 7.8)—which ruptured 400 km along the Kunlun fault in northern Tibet—thanks to an array of seismometers in Nepal that were nearly parallel to the rupture. Their data show that the earthquake, which began in the west, accelerated to above the shear wave velocity after ripping 175 km eastward, at a bend in the fault. Rupture speeds nearly reached the compressional (p) wave velocity before decelerating at another bend. Much of the high-frequency seismic energy from the quake was radiated during these transitions. — BH

J. Geophys. Res. 113, B07305 (2008).

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