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A deep earthquake goes supershear

Seismic analysis of an earthquake off Russia's Kamchatka Peninsula last year offers evidence that deep earthquakes are more complicated than geoscientists realized.

On 24 May 2013 the tectonic plate that subducts under Russia ruptured 607 km below the Sea of Okhotsk and produced the strongest deep earthquake ever recorded, with a moment magnitude of 8.3. The same day, a 6.7 magnitude aftershock struck even deeper, at 642 km. A new seismic analysis of the aftershock by Zhongwen Zhan at the Scripps Institution of Oceanography and his colleagues there and at Caltech reveals a surprise: The aftershock's fault ruptured at an astonishing 8 km/s, nearly 50% faster than the shear-wave velocity at that depth. The analysis puts the aftershock in rare company as one of only seven so-called supershear earthquakes ever identified, and the only deep one. That rock can rupture so quickly—and thus radiate energy efficiently—in a part of the mantle thought to deform plastically suggests that deep earthquakes are strange indeed. Resolving rupture velocity is usually straightforward, because shear and compression waves contain information about what parts of the fault slipped at any given instant of time. The researchers had access to a global network of seismic stations, pictured here, but the nature of the aftershock—with the rupture unzipping steeply downward into the mantle—meant that the distant seismometers alone were insufficient to yield details of rupture position along the fault. To resolve the roughly 12-km length of the rupture and thus its 8-km/s speed, they had to untangle the much more complicated waves recorded at the one regional station in the network. The waves at that station (marked PET), located directly above the aftershock (red star), had been severely diffracted while passing through the cold subducting plate and hot mantle. (Z. Zhan et al., Science 345, 204, 2014.)

A deep earthquake goes supershear

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