Using pulse shapes of intermediate depth seismicity to locate sources relative to subducting oceanic crust showing low seismic velocity

We are proposing a method to locate intermediate depth earthquakes in respect to the position of a relatively low velocity subducted crust by classifying pulse shapes recorded at specific receiver locations. P wave onsets of intermediate depth earthquakes at subduction zones indicate that low velocity layering of less than 10 km thickness is present in the vicinity of nearly all circum pacific slab surfaces. Whether low velocity is caused by hydrous metabasalt phases persisting to depth, released fluids, dehydration within the subducting slab, a serpentinized layer or even partial melts on top of subducted crust is a subject of ongoing discussion. Defining source locations relative to such a low velocity structure will help to rule out part of these interpretations of intermediate depth seismicity and shed more light on the source processes involved. We use 2D finite difference modelling to calculate full synthetic wave propagation in subduction zones. Our simulations show that a low velocity structure of km scale brings forth characteristic p onsets at near coastal receiver locations for sufficiently long propagation distances along the slab surface. Pulse shapes and frequency of these synthetic onsets are analysed regarding source location relative to the slab surface. We test Double Couple (DC) source position and orientation in an up dip slab geometry derived from local seismicity in Northern Chile. The layered slab consists of eclogized upper crust (5 km) and a layer of slow basaltic lower crust (2 km) on top of fast oceanic mantle. The tests account for velocity undulations in subducted crust, roughness of the low velocity structure and slab geometry as well as scale of layering. We find that sources of intermediate depth events at 21° S in Northern Chile are situated within or directly below the lower crust at distances of less than 2 km from the oceanic Moho.