Nature of the low velocity zone in Cascadia from receiver function waveform inversion

A dipping low-velocity zone (LVZ) is a ubiquitous structural element of subduction zones worldwide. In this study we map seismic attributes characterizing the LVZ beneath the Cascadia subduction zone from northern Vancouver Island to northern California using receiver function waveform inversion. Throughout this region, the LVZ is characterized by high V P / V S ratios (mean=2.77), strong S-velocity contrasts ( ∼ 50%) and thicknesses averaging 3.37 km. The LVZ is immediately underlain by a second, weaker layer exhibiting moderate V P / V S ratios (mean=1.85) with mean thickness of 4.62 km. We interpret the combined structure in terms of subducting oceanic crust, based on classical structural/petrological descriptions and constraints from previous studies of ophiolites and ocean drill cores. The LVZ is identified with pervasively hydrated, high porosity pillow basalts and sheeted dikes of Layer 2 with possible contributions from sediments (Layer 1). Fluids released from metamorphic dehydration reactions are maintained near lithosphere fluid pressures through an impermeable plate boundary above, and a low porosity, gabbroic/mafic-cumulate dominated Layer 3 below.