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
JSS011 Oral Presentation 2095
Upper mantle seismic anisotropy structure beneath central and
southwestern Japan subduction zone
Dr. Mohamed Salah
Tetsuzo Seno, Takashi Iidaka
In central and northeast Japan, the Pacific plate is subducting from the east beneath the Okhotsk plate
along the Japan Trench in the WNW direction, while in southwest Japan, the Philippine Sea plate is
subducting along the Nankai Trough and the Ryuku Trench in the NW direction. Analysis of seismic
anisotropy in the crust and mantle wedge above subduction zones opens up a new source of
information about rising magma in the region, the induced mantle flow and the stress state in the
forearc and back-arc regions, because the physical property estimated from shear wave anisotropy is
related to the dynamic processes inside the Earth. For this reason, we detect shear wave polarization
anisotropy in the crust and upper mantle above the subducting Pacific plate from local shallow,
intermediate, and deep earthquakes beneath central-southwestern Japan. We analyze local S phases
from 198 earthquakes that occurred in the subducting Pacific plate and are recorded at 42 Japanese F-
net broadband seismic stations. This data set yields a total of 980 splitting parameter pairs for the
central-southwestern Japan subduction zone. The measured splitting pattern is generally complicated
especially beneath Kanto-Tokai. Dominant fast polarization directions of shear waves obtained at most
stations in the Kanto-Izu-Tokai area are oriented nearly WNW-ESE, which are sub-parallel to the Sagami
Trough or parallel to the subduction direction of the Pacific plate. At some stations, however, fast
polarization directions are also oriented in NE-SW directions especially in the north of Izu Peninsula and
Tokai district. Fast directions obtained at stations located in Kii Peninsula are generally oriented ENE-
WSW, almost perpendicular to the Pacific plate subduction, although some directions have NW-SE
trends. Delay times vary considerably and range from 0.1 1.25 s. These lateral variations suggest that
the nature of anisotropy is quite different between the regions beneath the studied areas. Beneath
Kanto-Tokai, the slab morphology is relatively complicated as the Philippine Sea slab is overriding the
Pacific slab. This is also a region where the Izu-Bonin forearc is subducting in the east and the collision
of the Bonin ridge occurs in the west. This complex tectonic setting may induce lateral heterogeneity in
the flow and stress state in the forearc mantle wedge, on very short length scales. Fast directions
beneath Kii Peninsula and its eastern extension, on the other hand are trench-parallel, which is
sometimes seen in the back-arc regions.
Keywords: shear wave splitting, central southwestern japan, subduction zone