Physical properties of subducted slab and surrounding mantle in the Izu-Bonin subduction zone based on Broadband Ocean Bottom Seismometer data

摘要

Seismic attenuation and traveltime anomalies in the vicinity of the Izu-Bonin subduction zone are inferred from waveform data recorded by Broadband Ocean Bottom Seismometers (BBOBSs) deployed as part of the Stagnant Slab Project. We measure the path-averaged attenuation of P waves and traveltime anomalies of both P and S waves using waveform data from 10 regional earthquakes within the Izu-Bonin slab. The observed relation between the attenuation and traveltime anomalies is quantitatively evaluated on the basis of mineral physics. For the data that sample the slab and the Pacific mantle beneath the slab, the relation between the attenuation and traveltime anomalies is consistent with predictions based on the thermal effect. The data that sample the mantle wedge show pronounced positive traveltime anomalies unaccompanied by comparable high attenuation. The traveltime anomalies are too intense to be explained solely in terms of the temperature effect. The temperature anomalies estimated from attenuation suggest that the temperature in the mantle wedge reaches the dry solidus of peridotite. The relative intensity of S to P wave traveltime anomalies of mantle wedge data, after correction for the thermal effect, is relatively small (Rs /p = 0.5-1.2). Similarly low-velocity anomalies without comparable high attenuation and with relatively small Rs g, (<1.5) have been reported in previous studies of subduction zones at Tonga, in the southwest Pacific, and Tohoku, Japan. We discuss to what extent the present seismological observations can be explained by partial melt on the basis of recent theoretical and experimental studies on partially molten rocks.