1993年能登半島沖地震

Abstract

An earthquake of magnitude 6.6 (JMA) occurred about 10 km off the northeastern tip of Noto peninsula on February 7, 1993. P and S wave arrival times of the main shock and aftershocks recorded at the network stations of the Kamitakara Observatory, Kyoto university and the Shin'etsu Earthquake Observatory, University of Tokyo together with those at a temporary station set at the northeasern tip of Noto peninsula were used for the precise determination of hypocenters and focal mechanisms. Epicenters of aftershocks are distributed in a circular region with a radius of about 25 km, but southeastern half of them are activated a few days after the main shock occurrence and they can be calld "induced aftershocks". In the early stage of the earthquake sequence, epicenters of aftershoks are well concentrated on an area of about 15 km long and 8 km wide striking northeasterly. No large aftershocks occurred in the area, but foreshocks in December, 1993 were located very close to the main shock. Focal depths, which are poorly constrained compared to epicenters, are 5-12 km, when they are determined adding the temporary station, of which data are very much effective on the accuracy of focal depth. Focal mechanisim of the main shock determined from P-wave first motions and that determind from body and surface wave centroid moment tensor (CMT) inversion are of reverse fault type with P-axis in NW-SE direction. Most of the focal mechanisms of major aftershocks heve reverse fault type with P-axes of WNW-ESE to NW-SE direction, while those of the "induced aftershocks" have strike-slip or normal fault type. Moreover, the trend of the aftershock distribution in the early stage of the earthquake sequence is concordant with submarine active fault, which defines the northwestern margin of a rise. On the contrary, "induced aftershocks" were located on the northeast and southeastern margion of the rise, southeast side of which is also delineated by another active fault.