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. Plain Language Summary Despite being a common feature of global seismicity, intermediate‐depth earthquakes (70–350 km in depth approximately) and their physical mechanism are not well understood. These earthquakes occur at pressures and temperatures incompatible with our current models of brittle failure. At those depths, most subducting slabs feature two separate layers of seismicity, with little activity in‐between. It is commonly believed that the release of high‐pressure fluids enables the brittle‐like behavior; however, it is not yet clear whether this mechanism can operate on both layers. We have applied a recently developed earthquake location technique to construct a new global catalog of intermediate‐depth seismicity. We use this data set to study the geometrical structure of the two layers and their statistical characteristics. Our results point to a relatively dry lithospheric mantle—lower layer—regardless of plate age, convergence velocity, or composition and suggest that the physical mechanism enabling rupture in the lower layer is fundamentally different from the one in the upper. Key Points Double seismic zone geometrical structure is controlled by plate age and the slab's thermal parameter We find consistently larger b values for the upper seismicity layer, correlating with slab age, and roughly constant values for the lower Our results are consistent with dehydration embrittlement operating in the upper layer but point to a relatively dry lithospheric mantle