Evidence from high frequency seismic waves for the basalt-eclogite transition in the Pacific slab under northeastern Japan

Seismic multi-pathing effects, attributed to a contrast in seismic attenuation between the back-arc mantle wedge and subducted crust, are detected in central Honshu, northeastern Japan. We observe an initial broadened P-wave which is followed by a delayed higher frequency P-wave signal. Their discrepant frequencies are best explained by attenuation effects: delayed P-wave signals travel in the low-attenuation oceanic crust and therefore contain more high frequency components. The time separation between the initial broadened P-waves and the delayed P-wave signals are affected by the seismic velocity in the subducted oceanic crust. We observe systematic variation in the delay times of the later waves indicating an increase in seismic velocity in the oceanic crust (relative to the mantle wedge) at ∼130-150 km depth. High-frequency seismic simulations incorporating mineral-physics derived models show that a 4% Vp increase due to the blueschist decomposition and a 9% Vp increase associated with the (lawsonite, talc)-eclogite transition replicate the observed delay time variation. The blueschist breakdown may occur at a depth of ∼100 km and the (lawsonite, talc)-eclogite transition might be linked with the reduced seismicity level at depths greater than 150 km. Distinct from traditional guided waves, the multi-pathing effects in this study are mainly controlled by attenuation contrast and therefore may not require the oceanic crust to have low velocity and any special decoupling mechanism. The multi-pathing effects offer us another important tool to image subducted oceanic crust below back-arc mantle wedges, especially where guided waves are not observable. In this study, we demonstrate the value of observing and simulating high frequency seismic waves (>20 Hz) in advancing our understanding of subduction zones.