IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS014 Poster presentation 2275
Three-Dimensional velocity structure of the South Fossa Magna, Central
Japan
Dr. Haruhisa Nakamichi
Graduate School of Environmental Studies Nagoya University IAVCEI
Hidefumi Watanabe, Takao Ohminato
We present the three-dimensional structures of the P wave velocity (VP), S wave velocity (VS) as well as
the P wave to S wave velocity ratio (VP/VS) 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-VP, low-VS and low-VP/VS 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 H2O and CO2,
may be abundant in the low-VP/VS region and may play an important role in generating DLF
earthquakes. This anomaly overlies a deeper low-VP, low-VS and high-VP/VS anomaly at depths of 15-
25 km that may represent a zone of basaltic partial melt. Iso-velocity surfaces (VP =6.0 km/s and VS
=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.
Keywords: fossa magna, mt fuji, tomography