FUJII, Naoyuki

INUZUKA, Yoshihiro

IT0, Takamori

KAWABE, Iwao

MIYAKAWA, Kazuya

MORl, Yasunori

NAGAMINE, Koichiro

OOIDA, Tooru

Groundwater gas anomalies were observed at the 1,500 m deep Nagashima spa well near Nagoya for the M=5.7 earthquake (Oct. 31, 2000) in southern Mie Prefecture, Japan. In spite of the very long focal distance of 98 km, H2/Ar, He/Ar, N2/Ar, and CH4/Ar ratios of groundwater gas bubbles significantly changed not only at the time of the shock but also 3 days before it. Water temperature indicated a sharp coseismic increase. The earthquake was a dip-slip reverse event occurred approximately beneath the plate boundary in the eastern part of Kii Peninsula at 44 km depth. The gas anomalies are strikingly different from the previous ones at the same well, which were restricted to those in H2/Ar ratio for nearby events with M=1.6-5.4 within 25 km. The apparently "long-distance" nature of the groundwater gas anomalies may suggest a coherent subsurface deformation over a roughly 100 km x 100 km area. In view of the size and occurrence timing, we speculate that such a subsurface episode may possibly be related to the onset of unsteady crustal displacements of the 2001 Tokai silent earthquake revealed by the nationwide GPS observation system

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Preseismic groundwater gas anomalies at Nagashima spa near Nagoya for the M=5.7 earthquake (Oct. 31, 2000) in southern Mie Prefecture, Japan: An episodic subsurface process related to the 2001 Tokai silent earthquake

Preseismic groundwater gas anomalies at Nagashima spa near Nagoya for the M=5.7 earthquake (Oct. 31, 2000) in southern Mie Prefecture, Japan: An episodic subsurface process related to the 2001 Tokai silent earthquake

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