Gravity Surveys over Asama and Kusatsu-shirane Volcanos

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  • 浅間・草津白根山周辺の重力測定
  • アサマ クサツ シラネサン シュウヘン ノ ジュウリョク ソクテイ

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Abstract

Gravity surveys have been carried out over Asama and Kusatsu-shirane volcanos, the central part of Japan, for the purposes of obtaining a distribution map of Bouguer anomaly and establishing a precise gravity net for detecting gravity changes associated with volcanic activities. The Bouguer anomaly map over an area of about 6,300km2 centring Volcano Asama is obtained by compiling the data of the present survey together with the gravity survey in the past. The distribution of the Bouguer anomalies reflects well the geological feature in the surveyed area. The high and low anomaly areas correspond to plutonic or volcanic rocks and sedimentary areas respectively. The gravity high anomaly corresponds to the central belt of uplift running south-westwards from the northerb part of Volcano Kusatsu-shirane. Bisecting the low anomaly zone, another high anomaly zone, a branch of the central belt of uplift, runs along the mountain chain of Mt. Eboshidake, Mt. Takamine and Volcano Asama. Mt. Arafune is also occupied by a high anomaly. 0n the other hand, there is no remarkable anomaly around Volcano Kusatsu-shirane. In order to calculate the subterranean structure around Volcano Asama from the obtained gravity distribution, the well-known sinx/x method was used for analyzing the gravity data. With reference to the explosion seismic data, a density contrast of two layers that have a compressional wave velocity of 4.4km/sec and 6.Okm/sec respectively is chosen as 0.3g/cm3, and the depth of the boundary between both the layers was taken as 3km beneath ground surface at the eastern part of Ueda. In the underground structure computed in such a way, we see the low anomalies in the northern part of Volcano Asama and the southern part of Komoro. They may indicate thick sediments about 5km deep. The latter shows a caldera-like structure.

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