Velocity anomalies and spatial distributions of physical properties in horizontally lying slabs beneath the Northwestern Pacific region

Abstract

We calculated velocity anomalies by constructing thermal structural models of horizontally lying slabs beneath Kuriles, Central Japan, Northern Izu–Bonin and Central Izu–Bonin arcs. The values of V p and V s anomalies become maximum in the depth range of 100 to 200 km within the slabs, reaching 5 to 6.4% for V p anomalies and 7 to 8.5% for V s anomalies. The calculated V s anomalies are larger than the V p anomalies, and the ratio of the former to the latter is 1.2 to 1.5 within the slab. This suggests that V s anomalies are more sensitive to temperature variation than V p anomalies, indicating that the existence of the horizontally lying slabs is detectable in more detail by using S-wave. We also calculated spatial distributions of physical properties in the horizontally lying slabs beneath the four regions. In the calculation, we assumed that the material of the slabs consists of olivine (Mg 0.89Fe 0.11) 2SiO 4. The estimated values of physical properties within the slabs are larger than those in the surrounding upper mantle, except for Poisson's ratio, while the values within the metastable wedge of olivine become smaller. This is most remarkable within the slab beneath the Central Izu–Bonin arc due to the lowest temperature among the four regions. The horizontally lying slab above the 660 km seismic discontinuity beneath the Central Izu–Bonin arc is denser up to 60 kg/m 3 and the slab within the metastable wedge of olivine is lighter up to −120 kg/m 3. The values of the physical properties, except for Poisson's ratio, decrease with increasing the horizontal distances of the horizontally lying portions of the slabs. This is due to thermal assimilation, which is largely dependent on moving velocity of the slabs.