Propagation of High-Frequency Seismic Waves Inside the Subducted Lithosphere From Intermediate-Depth Earthquakes Recorded in the Vanuatu Arc

Three seismic phases associated with the slab of subducted lithosphere are identified on seismograms of intermediate-depth earthquakes (depths >70 km) recorded in the distance range up to 350 km by the Vanuatu (formerly New Hebrides) seismograph network. They are interpreted as (1) a P wave refracting into the inner portion of the slab, (2) a P wave traveling mainly through the outer(upper) portion of the slab, and (3) a P to S converted wave in which the conversion occurs across the upper surface of the slab. The existence of the two P arrivals from intermediate-depth earthquakes suggests that these earthquakes are not located in the strong and cold inner core of the slab but within an outer zone where the velocity is lower than that within the inner portion of the slab. The low-frequency character of the shear wave converted from the P wave across the outer “boundary” of the slab indicates that either the boundary is a transition layer with a finite thickness or the material above the slab has high attenuation for high-frequency shear waves. Large spatial variations in the transmission of high-frequency shear waves also indicate that there is a localized zone of anomalous shear wave attenuation in the mantle wedge lying just above the subducted slab and beneath the active volcanos.