Three‐dimensional velocity structures of Mount Fuji and the South Fossa Magna, central Japan

Abstract

[1] We present the three-dimensional structures of the P wave velocity (V P ), S wave velocity (V S ) as well as the P wave to S wave velocity ratio (V P /V S ) beneath Mount Fuji and the South Fossa Magna, Japan, using arrival time data collected from 2002 to 2005 by a dense seismograph array. The high-resolution data set and improved methodology reveal not only several velocity features that are consistent with previous studies but also important new details that clarify the velocity structures associated with volcanic processes beneath Mount Fuji and the collision tectonics of the South Fossa Magna. One such particular feature is a low-V P , low-V S and low-V P /V S anomaly at depths of 7-17 km beneath Mount Fuji that corresponds with the locations of deep low-frequency (DLF) earthquakes. The coincidence of the velocity anomaly and the DLF locations suggests that supercritical volatile fluid, such as H 2 O and CO 2 , may be abundant in the low-V P /V S region and may play an important role in generating DLF earthquakes. This anomaly overlies a deeper low-V P , low-V S and high-V P /V S anomaly at depths of 15-25 km that may represent a zone of basaltic partial melt. A low-V P , low-V S and low-V P /V S anomaly is seen at depths of 6-14 km beneath Mount Hakone. Isovelocity surfaces (V P = 6.0 km/s and V S = 3.5 km/s) corresponding to the upper limit of hypocenter distribution below Mount Fuji may define the upper surface of the Philippine Sea plate whose existence in a seismic gap beneath Mount Fuji has been controversial.

The 2011 off the Pacific coast of Tohoku Earthquake related to a strong velocity gradient with the Pacific plate

“…We propose that fluid with high aspect ratio pores exists there, as proposed by Matsubara et al (2004Matsubara et al ( , 2009) based on the result of Takei (2002). Nakamichi et al (2007) and Kato et al (2010) also considered the high aspect ratio crack with low-V p /V s zone. If a large seamount has already subducted beneath the plate, a low-V zone would be imaged (Cummins et al, 2002).…”

Seismotectonics in the Tanzawa Mountains area in the Izu-Honshu collision zone of central Japan, as revealed by precisely determined hypocenters and focal mechanisms

“…The seismic velocity tomography revealed that a lowvelocity wedge exists between the Tanzawa Mountains and Mount Hakone (Nakamichi et al, 2007;Nagai and Tanada, 2010). These studies interpreted the low-velocity wedge as trough-filled deposits, which accreted between the Tanzawa and Izu block.…”

Influence of the March 11, 2011 M w 9.0 Tohoku-oki earthquake on regional volcanic activities

“…According to our numerical modeling, the dilatation induced by the Hokodu-oki earthquake around Mount Fuji is about 220 nano-strains. Because of the existence of basaltic magma at depths of 15-25 km beneath Mount Fuji [3], exsolution of water is less likely. Therefore, volumetric expansion may lead to magma ascension and increase the risk of volcanic eruption.…”

“…Mount Fuji is located southwest of Tokyo, on the border between Shizuoka and Yamanashi prefectures in central Honshu. The 3-D P-and S-wave velocity structures suggest existence of supercritical volatile fluid, such as H 2 O and CO 2 , at depths of 7-17 km beneath Mount Fuji, and there might be partial melting of basaltic rocks at depths of 15-25 km [3]. Changbaishan volcano is located at the border between NE China and North Korea and has been active in the past.…”