LAUSR

Double seismic zones are ubiquitous features of subduction zones, where seismicity is distributed along two layers separated by a region with significantly less seismic activity. Dehydration embrittlement is thought to be responsible for earthquakes in the subducting crust (upper layer), but the case for it in the lithospheric mantle (lower layer) is less clear. We apply a recently developed relative relocation technique to characterize seismicity in 32 slab segments. The high‐precision hypocentral depths allow us to assign events to either the upper or lower layer and to separately estimate frequency size distributions for each plane. We find consistently larger b values, correlating with slab age, for the upper layer and roughly constant values for the lower. We also show that thermal parameter and plate age are the key controls on double seismic zone geometry. Our results point to a relatively dry lower layer and suggest a fundamentally different mechanism for lithospheric mantle earthquakes.