| 論文要旨 |
Phase velocity distributions of fundamental-mode Rayleigh waves
in the Japanese islands are reconstructed by using a newly developed
multi-station method with a regional broad-band seismic network, F-net,
which has been deployed by the NIED (National Research Institute for
Earth Sciences and Disaster Prevention, Japan).
We estimate local phase velocities and arrival angles of
the fundamental-mode Rayleigh waves from a group of stations using the
multi-station method in a period range between 40 and 128 seconds.
To avoid some unwanted effects caused by higher modes as well as ambient
noise, we restrict seismic events at depth shallower than 50 km
and with surface-wave magnitude Ms >= 6.0.
In our multi-station analysis, we consider at first regular grids with a
1-degree intervalin both longitude and latitude.
Each grid point is supposed to be the center of a circle with radius of
150 km.
F-net stations located in each circle are used as a group to measure
a local phase velocity dispersion, and one of the stations in each group
is set to be the reference station.
We then measure phase differences for the rest of stations in the group
with respect to the reference station.
Perturbations of phase speeds and arrival angles are expanded in a set
of B-spline functions, whose coefficients are determined by a
least-squares inversion.
The local phase velocities estimated by the multi-station analysis are
then used to invert for phase velocity maps in order to investigate
the upper mantle structure beneath the Japanese islands.
The results show that the phase velocities
in a period range between 40 and 50 seconds in the Chubu region
are slower 6-15 % than those estimated from a model based on PREM
with crustal correction using an average crust model in Japan,
suggesting a thicker crust beneath this region.
In addition, the phase velocities in the same period range are 3-10 %
higher than the reference values in the Chugoku and the eastern
Kyusyu regions, indicating the effect of the
subducting Philipine Sea plate beneath these regions.
In a period range longer than 68 seconds, the phase velocities
in the eastern Japan (from Hokkaido to the northern Kanto)
are 2-6 % faster than the reference, suggesting
the effect of the subducting Pacific plate with higher shear
wave velocities beneath these regions.
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