Hydration-induced faulting as a trigger of intermediate-depth earthquakes

Subduction zones experience numerous earthquakes at intermediate depths (~50–300 km), despite high pressure at great depths inhibiting brittle behavior in rocks. Several mechanisms for intermediate-depth earthquakes, including dehydration-related processes, have been proposed; however, the exact processes by which brittle failure evolves into large faults and eventually develops into seismic slip remain unclear. Here, we propose the hydration-induced faulting of feldspars as a new mechanism of intermediate-depth earthquakes. We conducted rock-deformation experiments of natural rock containing alkali feldspar and plagioclase in dry and wet environments under pressure and temperature conditions similar to those occurring at subduction zones. We identified faulting associated with the hydration of alkali feldspar and plagioclase, accompanied by ultrafine-grained, weak hydration products, such as illite and smectite. Under high pressure, dynamic weakening resulting from the increased rheological contrast between mechanically weak hydration products and surrounding minerals lead to the localization of strain and development of faults. The increased number of intermediate-depth earthquakes at depths of ~100 km at subduction zones can be attributed to hydration-induced faulting of feldspars under high pressure. This mechanism may also contribute to the seismicity of subducting slabs composed of blueschist and oceanic sediments enriched in alkali feldspar and plagioclase.