PDNumerical Modelling of the Three-Dimensional
Stress Field in Southwestern Japan
MANABU HASHIMOTO
Disaster Prevention Research Institute, Kyoto University Uji, Kyoto, 611, Japan
Tectonophysics, Vol.84, P.247-266, 1982.
It has been inferred from the focal mechanism
of earthquakes and their hypocenter distribution
(Shiono, 1977) that the stress field in southwestern
Japan indicates
complicated features; a NW-SE compression at shallow depths along the Nankai
trough, an E-W or ESE-WNW compression in the inland crust, an extension parallel
to the leading edge of the Philippine Sea plate at subcrustal depths in the
region from the southern Chubu to northwestern Shikoku, and a down-dip tension
beneath the Kyushu island. In order to investigate possible sources of these
complex features of the stress state, a three-dimensional finite element
method is employed to model the configuration of the subducting Philippine
Sea plate. taking into consideration the following three possible types of
forces:
(1) A negative buoyancy due to the density contrast between the subducting plate
and the surrounding mantle.
(2) A northwestward compressive force generated by the movement of the Philippine
Sea plate.
(3) A westward compressive force due to the movement of the Pacific plate.
For various combinations of different magnitudes of these forces, and of different
elastic moduli, the stresses at a number of selected sites are calculated, and
their directions are compared with those inferred from the focal mechanism and
other geophysical information. It is found that the observed extensional stresses
parallel to the leading edge of the subducting Philippine Sea plate may be caused
mainly by the negative buoyancy. The northwestward compressive force may not
play an important role in generating the complex stress field in southwestern
Japan. The observed E-W compressional stress field prevailing in the inland region
appears to result mainly from the westward-moving Pacific plate. The present
results suggest that if a thin low-velocity transitional layer exists just above
the subducting Philippine Sea plate, it could give appreciable effects on the
mechanism of low-angled thrust faulting off the Kii peninsula and the Shikoku
island. The magnitude of the shear stress in the continental crust and in the
subducting plate is estimated to be of the order of several hundred bar, although
the calculated shear stresses are considerably affected by the configuration
of the subducting plate and also by the applied forces. It is interesting that
the stress distribution appears to have some relations to seismicity of subcrustal
earthquakes, and to the rupture process of large thrust earthquakes along the
plate boundary. _ΆκΦίι
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