Hydrous minerals in the mantle wedge and the maximum depth of subduction thrust earthquakes

In many subduction zones the downdip limit of thrust earthquakes approximat ely coincides with the intersection of the subduction thrust with the forea rc mantle. This limit may be explained by aseismic hydrous minerals present in the mantle wedge. During subduction, fluids released from the subductin g slab infiltrate the overlying forearc mantle forming serpentine + brucite , especially in cool subduction zones. At the slab interface itself, talc-r ich rocks form in the mantle by the addition of silica transported by risin g fluids and by mechanical mixing of mantle and siliceous rocks. In the lab oratory, serpentine generally exhibits stable-sliding aseismic behavior. Th e behavior of talc, a layered hydrous silicate, and brucite, a layered hydr oxide, has not been investigated, but their structures also suggest weak st able-sliding behavior. We suggest all three layered hydrous minerals promot e aseismic behavior and that their presence controls the downdip limit of t hrust earthquakes in many subduction zones.