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A Large intraplate earthquake triggered by latent magmatism

マグマ活動が引き起こしたプレート内地震

梅田 浩司; 浅森 浩一; 根木 健之; 草野 友宏

Umeda, Koji; Asamori, Koichi; Negi, Tateyuki; Kusano, Tomohiro

2000年10月に発生した鳥取県西部地震は$$M$$$$_{j}$$=7.3の巨大な内陸地震であったにもかかわらず地表には明瞭な地震断層を生じなかった。しかしながら、その後の解析によって、地震を引き起こしたのは未成熟な断層であり、震源断層からの主破壊は地下1-2kmまで達したものの、地表では幾つかに分散するわずかな横ずれ変位しか生じなかったことが明らかにされた。本研究では、未成熟な震源断層を検出する手法として、地磁気・地電流観測に基づく比抵抗構造解析と地下水中のヘリウム同位体の観測をそれぞれ適用した。その結果、鳥取県西部地震を引き起こした震源断層の南西側の中部-下部地殻には溶融体が存在することが明らかになった。また、溶融体の周辺には低周波地震も発生していることから、溶融体から放出された流体が震源断層の活動と密接に関連することが示唆される。

A magnitude ($$M$$$$_{j}$$) 7.3 intraplate earthquake occurred in the western Tottori area, southwest Japan, on 6 October 2000, where there was no apparent prefaulting subsurface indication of the source fault of the 2000 earthquake. Magnetotelluric soundings were taken in and around the aftershocks occurred in order to image 3D electrical resistivity structure at depths of up to 40 km, which could indicate an anomalous conductor in the crust to the upper mantle on the southwest side of the source fault. Free gas and dissolved gases collected from groundwater wells around the seismic source region are characterized by $$^{3}$$He/$$^{4}$$He ratios several times higher than the atmospheric value. Although alkali basalts of early Pleistocene age are sparsely distributed in the western Tottori area, the observed $$^{3}$$He/$$^{4}$$He ratios are higher than the calculated $$^{3}$$He/$$^{4}$$He ratio derived from the ancient magmatism, considering post-extrusive radiogenic ingrowth of $$^{4}$$He by decay of U and Th included in the magma. Therefore, it is concluded that the geophysical anomaly imaged to the southwest of the source fault is attributed to latent magmatism in the present-day subduction system. Aqueous fluids separated from the cooling crustal magma could cause deep low-frequency earthquakes around the Moho discontinuity and migrate into the brittle upper crust. In addition, the presence of aqueous fluid is expected to weaken the crustal materials. Locally anelastic deformation, implying notable compressive deformation in the E-W direction, was observed in the region where aftershocks were distributed. Under overpressure conditions, the existing fault could serve as a pathway for aqueous fluids expelled from magma with high $$^{3}$$He/$$^{4}$$He ratios, so that the upwelling of overpressurized fluids toward the Earth's surface results in the emanation of groundwaters with high $$^{3}$$He/$$^{4}$$He ratios along the trace of the source fault segments.

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パーセンタイル:42.25

分野:Geochemistry & Geophysics

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