長野市松代地域における炭酸ガスの湧出量と地殻変動

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タイトル別名
  • Discharge of Carbon Dioxide and Crustal Movement at the Matsushiro Area, Nagano City
  • ナガノシ マツシロ チイキ ニ オケル タンサンガス ノ ユウシュツリョウ ト

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Large amount of underground water with CO2 gas bubbles had flowed out from many spots for the period of the Matsushiro Earthquake Swarm, 1965-1967. Some of the springs are still alive. We explored possibility for detecting crustal activity, such as crustal deformation and seismicity by observing CO2 concentration in soil gas in the Matsushiro area. We measured CO2 concentration in soil gas once a week for 6 months at many places in two areas: (1) a fault zone of the Matsushiro Earthquake Swarm, and (2) floor of the tunnel of Matsushiro Seismological Observatory which is about 2.5km away from the fault zone. The measurement points are located at various geological conditions. Remarkable high concentration of CO2 in soil gas was detected at measurement points in the fault zone. In particular, high concentration of CO2 was observed at a point on a fault. In contrast, on tunnel floor, out of the fault zone, the concentration is relatively low. Temporal variation, records of CO2 concentration of all measurement points, however, resemble one another in variation pattern. This suggests that the CO2 discharge is controlled by some wide regional scale mechanism. The pattern of the concentration variation is compared with those of seismic data, crustal strain data and meteorological data observed at the observatory. Good correlation is found between temporal variation in CO2 concentration and that in crustal strain observed with north-south orientation strainmeter, that is, CO2 concentration increase with expansion of the NS70-100 component of strainmeter. We propose a model of the variation in CO2 discharge in relation to crustal strain and fracture distribution. The strainmeter data shows that the crustal strain is largely influenced by rainfall at the Matsushiro area, for example, NS70-100 component of strainmeter expands after rainfall. Since many fractures having nearly the east-west direction are observed on the surface in this area, expansion in the north-south direction is expected after rainfall. Some pre-existing cracks will open with the expansion of neighboring cracks caused by water absorption. Since large amount of CO2 gas presents under this area, CO2 discharge rate increase with increase of width of passages. We can detect strain change of the fracture zone by CO2 monitoring.

収録刊行物

  • 地震　第２輯

    地震　第２輯 51 (1), 41-50, 1998

    公益社団法人 日本地震学会

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