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.

Three‐Dimensional Seismic Attenuation Structure of Central Japan and Deep Sources of Arc Magmatism

“…The location of the uppermost part of this high-attenuation zone is in good agreement with the locations of a low-velocity anomaly (e.g., Kinoshita et al, 2015;Nakajima et al, 2009;Nakamichi et al, 2007) and a low-resistivity conductor (Aizawa et al, 2004), both of which have been interpreted as a deep basaltic magma chamber (Fujii, 2007). The location of the uppermost part of this high-attenuation zone is in good agreement with the locations of a low-velocity anomaly (e.g., Kinoshita et al, 2015;Nakajima et al, 2009;Nakamichi et al, 2007) and a low-resistivity conductor (Aizawa et al, 2004), both of which have been interpreted as a deep basaltic magma chamber (Fujii, 2007).…”

“…The high-attenuation body, which originates from~100-km depth, extends upward to 20-to 30-km depth beneath Mount Fuji. The location of the uppermost part of this high-attenuation zone is in good agreement with the locations of a low-velocity anomaly (e.g., Kinoshita et al, 2015;Nakajima et al, 2009;Nakamichi et al, 2007) and a low-resistivity conductor (Aizawa et al, 2004), both of which have been interpreted as a deep basaltic magma chamber (Fujii, 2007). Furthermore, since felsic compositions (2.8-2.9 g/cm 3 ; Tatsumi et al, 2008) are dominant in the middle crust of the PHS plate, which lies immediately below Mount Fuji (e.g., Kodaira et al, 2007), basaltic melt (~2.8 g/cm 3 ; Kushiro, 1983) could be stagnated in the lower crust (Fujii, 2007).…”

“…For example, the long-term eruption rate of Mount Fuji is an order of magnitude higher than that of Mount Hakone (Tsukui et al, 1986). Although the crustal magma plumbing systems beneath these two volcanoes have been analyzed in detail via seismic velocity tomography (e.g., Lees & Ukawa, 1992;Nakamichi et al, 2007;Yukutake et al, 2015), receiver function analysis (Kinoshita et al, 2015), magnetotelluric surveys (e.g., Aizawa et al, 2004;Yoshimura et al, 2018), and petrological studies (e.g., Fujii, 2007;Kaneko et al, 2010;Yoshimoto et al, 2004), the details of their deeper magmatic systems are poorly understood (for details, see Aoki et al, 2019). These differences in volcanic activity between Mount Fuji and Mount Hakone probably result from key differences in their melt transport pathways.…”

Elastostatic effects around a magma reservoir and pathway due to historic earthquakes: a case study of Mt. Fuji, Japan

“…Fuji remains unclear, a seismic tomography study (Nakamichi et al 2007) revealed a high-Vp/Vs anomaly around 15-25 km depth that is interpreted as a zone of basaltic partial melting. Another study (Kinoshita et al 2015) used receiver function analysis to show a seismic velocity contrast at 20-30 km depth, which can be related to the bottom of the magma chamber. In addition, over 90 % of (magmatic) low-frequency earthquakes in Fig.…”

Seismic Detection of a Magma Reservoir beneath Turtle Island of Taiwan by S-Wave Shadows and Reflections

“…To improve the understanding of the spatial distribution of molten magmas within a reservoir, seismic images often play one of the most important roles. The general geometries of magma reservoirs have been successfully delineated either from seismic low-velocity 3 – 5 or anisotropy zones 6 , but those seismic images only reflect the general characteristics of the partial melting rocks in the magma reservoir. Until recently, there was no direct seismic evidence to detect liquid magma within the reservoir.…”