草津白根火山白根火砕丘の地震反射断面 : 擬似反射記録法による反射断面の推定

概要

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Shallow subsurface structure of Shirane pyrocrastic cone, Kusatsu-Shirane Volcano is discussed with Pseudo Reflection Profiling. Pseudo Reflection Profiles are obtained from a controlled source seismic experiment in September-October 2003 as a part of the Fourth Joint Observation of Kusatsu-Shirane Volcano. A significant horizon is defined through correlation of the profile with borehole logging data. The horizon is compatible with a top face of Tertiary volcanic rocks as a substratum of Kusatsu-Shirane Volcano and is estimated at 1700 m a. s. l. in the craters. The horizon strikes NNE-SSW direction and dips eastward. Its dipping is not uniform and there is a bench zone beneath the craters which strikes NNE-SSW direction. Seismicity beneath Mizugama crater is concentrated and the active magnetization region locates beneath the horizon. Other clear reflection events within the substratum are also observed in the profiles but their geological correlation is not defined. One of deeper horizon at 1000m a. s. l. may correlate a sort of density contrast in Tertiary volcanics layer. Seismicity along the traverse line tends to concentrate under the deeper horizon beneath Yugama crater. The density contrast may constrain seismicity beneath the crater.
特定非営利活動法人日本火山学会の論文
2008-02-28