LAUSR

Significance Earthquakes at subduction zones extend to great depths where slip on faults cannot occur by conventional frictional processes. Earthquakes at depths greater than 15 km are commonly attributed to high pore pressures, low effective pressures, and dehydration embrittlement of serpentine and other hydrous silicates. However, recent evaluations of temperatures and pressures of intermediate-depth earthquakes do not always correspond to breakdown reactions of antigorite serpentine, amphiboles, or talc. Altered mantle rocks of the upper lithosphere and mantle wedges also contain magnesian carbonates, which are stable to great depths, pressures, and temperatures. Carbonates of downgoing slabs are weaker than anhydrous mantle silicates. Our model suggests that intermediate-depth earthquakes may occur by strain localization and thermal shear instabilities within carbonate horizons.