Imaging crust and upper mantle beneath Mount Fuji, Japan, by receiver functions

Abstract

Mount Fuji has ejected a huge amount of basaltic products during the last 100,000 years. Even though the region around Mount Fuji is tectonically active, the seismicity below Mount Fuji is low, resulting in little knowledge about the seismic structure there. To gain more insight into the magma-plumbing system, we obtain the seismic structure beneath Mount Fuji by the receiver function (RF) technique. RFs at seismic stations around Mount Fuji show positive phases at ~3 and ~6 s, representing the conversion of P to S waves at a positive velocity boundary in the Philippine Sea plate. Cross sections of RF amplitudes reveal two distinct velocity boundaries around Mount Fuji, at depths of 40-50 km and 20-30 km, which we interpret to be the boundary between the crust-mantle transition layer and the uppermost mantle of the Izu-Bonin arc and the velocity discontinuity just below the region where low-frequency earthquakes (LFEs) of Mount Fuji have occurred, respectively. The velocity boundary at about 50 km depth shows a clear gap just beneath Mount Fuji. We suggest that this gap represents a weaker velocity contrast zone through which the magma of Mount Fuji ascends from the Pacific plate. A thorough grid search reveals that a low-velocity zone at depths of ~13-26 km explains all the characteristics of RFs around Mount Fuji, leading us to interpret the high-velocity boundary just below the LFE region as the lower boundary of Mount Fuji's magma chamber