1995年日本新泻北部M6.0地震及其由地震烈度和地下水地热异常分布所揭示的隐伏地震断层

1995年日本新泻北部M6.0地震发生在新泻地震空区的东部边缘.由于此次地震震源较浅(10km),造成了55栋房屋倒塌、165栋房屋半倒.通过计算倾倒墓碑的地震矩,对本区地震烈度进行了分析,发现烈度6度区(据日本JMA度)为一覆盖面积6.1×1km2,呈NNE-SSW向分布的条带,表明震中区的冲积平原下存在一条隐伏断层.震中区地下水的温度、电导率和Cl-浓度等异常区与呈线性分布的6度烈度区大致吻合,也有力地证明了震中区下面存在一条隐伏断层.这次地震可能是由高压型热水系沿隐伏活动断层喷溢引起.带着高温的高压热水降低了岩石的断裂强度,从而触发了地震.

1995 Northern Niigata Earthquake of M6.0 and its buried earthquake fault imaged by the distributions of seismic intensity and thermal anomaly of groundwater, Japan

The 1995 Northern Niigata earthquake of M6.0 occurred at the eastern margin of the Niigata seismic gap and might have been a precursor of a large destructive earthquake. The epicenter was so shallow at 10 kilometers depth that 55 houses were complete collapsed and 165 houses were half collapsed. The distribution of seismic intensity was analyzed by the moment calculated for overturned gravestones. The seismic intensity 6 on the scale of the Japan Meteorological Agency (JMA) was a belt covering an area of 6.1 × 1 km2 trending NNE - SSW, which may indicate the existence of a buried earthquake fault beneath the plain of the epicentral area, was found. The anomaly areas in temperature, electrical conductivity and C1- concentration of groundwater were approximately coincident with the area of the seismic intensity 6 with linear distribution and convincingly demonstrated the presence of a buried fault beneath the epicentral area. This earthquake might have been triggered by the expulsion of geopressured hydrothermal system along the buried active fault. The accumulation of geopressured hydrothermal water in combination with high rock temperature might reduce fracture strength of the rock, and trigger earthquake occurrence.