Seismological evidence for a discontinuity In subduction zones

Abstract

The mean local magnitude M is independent of the rate of earthquake ocurrence, and can be used to monitor the changes of stress in a region. Three estimators of earthquake size are discussed: the mean local magnitude de M, the b-factor (related to the reciprocal of M) and the mean local moment M0 . Only M0 is an unbiased estimator of earthquake size: An analysis of the variation of b in some subduction zones discloses systematic changes with depth. The evidence suggests that downgoing slabs in subduction zones are not continuous down to depths of 600 km, excepting the Tonga-Kermadec subduction zone which exhibits no significant variation of b-values with depth.