On 24 May 2013, the tectonic plate that subducts under Russia and Eurasia ruptured 607 km beneath the Sea of Okhotsk, west of the Kamchatka Peninsula, and produced the strongest deep earthquake ever recorded, with a moment magnitude of 8.3. Nine hours later a magnitude 6.7 aftershock struck even deeper, at 642 km. A new seismic analysis 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.1 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 cracks can propagate so quickly in a part of the mantle thought to plastically deform rather than fracture only adds to most geophysicists’ perception that deep earthquakes are strange. According...
