変動地形に基づく2004年中越地震の断層モデル

抄録

The fault model for the 2004 Mid-Niigata Prefecture Earthquake is examined based on the nature of surface fault, the results of leveling survey and the aftershock activity. The surface faults of the Mid Niigata Prefecture Earthquake in 2004 were found along the pre-existing active fault traces; the Obiro fault and the northern part of the western marginal fault of the Muikamachi basin (WFM). The amount of vertical displacement was 10 to 30cm. The elevation change of benchmarks indicates that the southwestward extension of the Obiro Fault (Suwa-toge Flexure) corresponds to the source fault. The slip on the fault was ca. 1.8m. The hypocenter distribution shows that the Obiro Fault is dipping toward WNW with about 50 degrees, and that the WFM extends to the north with the same dip as the Obiro Fault. In the central part of epicentral area, there is a double planar structure composed of these faults. Both the slip on the Obiro Fault and the small displacement of the surface earthquake fault (WFM) have proved indispensable in matching the calculated deformation with the observed one. There are two fault models for the earthquake; 1) low-angle thrust (WFM) branches off from the tip of the high-angle Obiro Fault at a depth of 2.8km (upward-flattening structure), 2) WFM is a high-angle reverse fault parallel to the Obiro Fault (double planar structure). Geomorphologically, the double planar structure is the most probable one. The Obiro Fault is responsible for the main shock. The WFM can be correlated with the source fault of the largest aftershock with a small amount of slip.