Vp and Vs Structures in the Crust of Tokai Region, Central Japan, Estimated by Seismic ACROSS Signals

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  • 弾性波アクロスによる東海地域地殻のP波およびS波速度構造の推定
  • ダンセイハ アクロス ニ ヨル トウカイ チイキ チカク ノ Pハ オヨビ Sハ ソクド コウゾウ ノ スイテイ

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Abstract

We estimated seismic velocity structures for both Vp and Vs in the crust of the Tokai region, central Japan, with observations of seismic ACROSS signals. “ACROSS” is the abbreviation of the terms “Accurately Controlled, Routinely Operated, Signal System”. The seismic ACROSS aims at highly-stable monitoring of the Earth’s crust using continuous elastic waves. This study is the first investigation of deep crustal structure with the use of the seismic ACROSS. In the Tokai region, three seismic ACROSS vibrators are constantly transmitting seismic signals for monitoring the focal region of impending Tokai earthquake. We deployed a seismic linear array consisting of 81 seismometers with 120 km aperture from April to August in 2008 in order to receive ACROSS signals from two transmitting stations named TOKI (in Gifu Pref.) and MORI (in Shizuoka Pref.). The ACROSS signals were stacked for four months to obtain the 6-components Green’s functions in time domain. We observed clear arrivals of seismic phases of both P and S waves along the linear array. Some phases were interpreted as reflections originated from deep crust and the upper surface of the Philippine Sea Plate. We estimated a seismic velocity model of the crust beneath the array that can explain these major seismic phases using a ray tracing method. Structural features of the estimated model, such as downward bending of the island-arc Moho and uplift of the upper surface of the plate, are consistent with the result of previous structure survey in the region.

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