The relationship of intermediate- and deep-focus seismicity to the hydration and dehydration of subducting slabs

Previous studies suggest that intermediate- and deep-focus earthquakes in subducting slabs may result from dehydration reactions. We investigate the importance of dehydration in facilitating intermediate- and deep-focus earthquakes by comparing the seismicity rate with the calculated slab dehydration flux (van Keken et al.. 2011) as a function of depth in 56 subduction zones worldwide. For each region, the seismicity rate per km of trench length per year as a function of depth between 1990 and 2009 was determined from the Preliminary Determination of Epicenters (PDE) catalog. The number of earthquakes between 75 and 240 km depth was compared to the calculated slab dehydration flux in 25 km increments. Deeper than 240 km, dehydration rates were not calculated, so seismicity is instead compared to the calculated flux of water that remains mineralogically bound in the slab at that depth. No strong correlation between the dehydration flux and the seismicity rate was found at arty depth. This suggests that some factor besides the presence of water, such as the stress state of the slab, controls intermediate-focus seismicity. For depths greater than 240 km, there is a correlation between the amount of mineralogically-bound H2O and the seismicity rate. In particular, although deep slabs with small amounts of bound water show a wide range of seismicity rates (and many have no seismicity whatsoever), all deep slabs with appreciable water transported below 240 km show significant seismicity. We conclude that the presence of water may be a necessary condition for deep seismicity in conjunction with other factors. We also note that slab temperature and dehydration are linked. Traditionally the limitations on deep earthquake occurrence have been linked to temperature, but this study suggests that similar arguments can be made linking deep earthquakes with the presence of water. (C) 2012 Elsevier B.V. All rights reserved.