Aftershock Distribution of the 1993 M6.6 Earthquake Off Noto Peninsula and its Relation to Tectonic Features

An M6.6 earthquake occurred on February 7,1993, around a sea rise extending southwest-northeast direction off the northeastern tip of Noto Peninsula. The hypocenters of the mainshock and aftershocks were located using telemetered data from university stations. The aftershocks during the first two days are concentrated in the narrow active fault zone along the northwest side of the rise. Other concentrations occurred along active faults on the southeast side of the rise. Most of the focal depths are 10-15km, consistent with the interpretation of T phases and pP phases recorded at some stations. Northwestward dipping 3-dimensional distributions for large aftershocks suggest two possible fault planes, which coincide well with the two fault planes of the CMT solution. The gross nature of the seismic fault is of a thrust type, which contradicts the strike slip solution estimated from the initial motions. The Noto region is part of the tectonic zone along the eastern margin of the Japan Sea, where zonal shortening due to compression is predominant as in the Japan Sea coast region in northern Honshu, Japan. The alignment of aftershocks along the topographic lineaments and submarine active faults may reflect this tectonism.1993年2月7日の能登半島沖地震は,禄剛崎沖の南西から北東へ伸びる海底の高まり付近で発生した.この余震の分布をできるだけ精密に求めるため京大防災研上宝観測所と東大地震研信越地震観測所の観測網のデータを統合して震源計算した.この地域の構造はきわめて不均質であり,その影響を極力さけるためもっとも震源域に近い観測点を用いて地震決定を行なった