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Keywords:

  • deep earthquakes;
  • rupture velocity;
  • seismology;
  • back-projection

[1] Explaining deep seismicity is a long-standing challenge in earth science. Between 300 and 700 km depth, earthquakes are scarce except between ∼530 and ∼600 km, where the majority of events occur. By imaging the seismic rupture process for a set of recent deep earthquakes using the back projection of teleseismic P-waves, we found that the rupture velocities are less than 60% of the shear wave velocity except in the depth range of 530 to 610 km. We propose that large fracture surface energy (Gc) values for deep earthquakes generally prevent the acceleration of dynamic rupture propagation and generation of earthquakes between 300 and 700 km depth, whereas small Gc value in the exceptional depth range promote dynamic rupture propagation and explain the seismicity peak near 600 km.