NW Pacific slab rheology, the seismicity cutoff, and the olivine to spinelphase change

Along the Kamchatka-Kuril-Japan-Izu-Bonin-Mariana subduction zones, the old age of the subducting Pacific Plate and the rapid subduction rate together suggest that earthquakes should occur to the bottom of the transition zone . However, the seismicity cutoff varies in depth between 358 km and 650 km. Along these subduction zones, the largest deep-focus earthquakes invariabl y occur near the depth of the local seismicity cutoff regardless of its dep th. The events near the seismicity cutoffs also have systematically differe nt focal mechanisms than shallower events. Furthermore, data from S660P arr ivals, residual sphere analysis, and tomographic studies all show that the slab dip consistently steepens to a near-vertical orientation at the seismi city cutoff. This change in slab dip indicates a strength loss in the slab. We hypothesize the following causal connection among all these observation s: The cold temperatures in the slab kinetically hinder the olivine to spin el phase change and allow the olivine to persist metastably to depths well below its equilibrium pressure. When the phase transition occurs, it nuclea tes very fine-grained spinel which acts as a lubricant, allowing the initia tion of earthquake faulting at high confining pressures which further nucle ates additional fine-grained spinel. The cold anomaly of the slab severely inhibits the growth of the nucleated spinel crystals. The presence of the f ine-grained spinel crystals reduces the strength of the coldest part of the slab by several orders of magnitude, allowing high slab deformation rates. Additionally, the phase change, by increasing the density, provides a nega tive buoyancy force. Combined, these processes reduce the slab membrane str ength and allow the slab to descend at a steeper dip.