Intraslab Seismicity and its Generation Mechanisms

History of studies of intraslab seismicity and its generation mechanisms, since the work by Wadati, is reviewed. Through the initial stage of studies of morphology of the intraslab seismicity, there followed the stage of discussion of focal mechanisms and terminal depths by mechanics and temperature of the slab. In 1970s, double seismic zones were discovered and their generation mechanisms were discussed in terms of bending or thermal stresses. In recent years, to overcome a difficulty that very high pressure prevents intraslab seismicity, dehydration embrittlement and phase transformation have been invoked for the mechanisms of intermediate and deep earthquakes, respectively. If the intermediate seismicity represents dehydration, it may give us a key to understand the distribution of fluids to the upper plate and to the seismogenic interplate thrusts, and finally to understand tectonics and volcanism in subduction zones.