Studies of Matsushiro earthquakes: 4. Observation of microearthquakes around Mt. Minakami



In 1966, local cracks formed on the ground surface in the northeast area of Mt. Minakami (see Figure 16). These cracks run almost parallel, and the general trend can be traced to a distance of 1.5 km in the NW-SE direction. The leveling survey made by the Earthquake Research Institute (ERI) indicates an extraordinary uplift of the ground in the area relative to surrounding regions. The triangular survey by the Geographical Survey Institute covering a wider area also shows horizontal ground movements toward the NW and SE directions being bounded by the area. This situation suggests the possibility that the ground cracks might be related to a latent fault movement caused by tectonic forces working in the whole region. For this reason, we made a temporary observation of microearthquakes around the area in question. The purpose of the observation was focused to examine a possible correlation between the spatial distribution of microearthquakes and the local cracks or latent faults.