24. Spatial Variation of Earthquake-Generating Stress in and around the Kanto District, Central Japan

A number of fault-plane solutions of earthquakes (n=454) in and around the Kanto District were determined by a numerical method, and the spatial variation of earthquake-generating stress is derived by using individual and average focal mechanisms. Averaging of direction cosines is made for both axes of maximum pressure and tension, and other axes are located by computation. The structure of earthquake-generating stress is derived from the spatial variation of fault types which are systematically classified into six types based on dip angles and azimuths of pressure and tension axes and null vector. The :spatial variations of focal mechanisms are derived by summarizing pressure and tension axes and comparing them with the distribution of earthquake hypocenters previously determined by correcting the Pn station biases. The northern focal hemispheres of fault-plane solutions are used for comparison with the vertical distributions of the relocated hypocenters. A reverse fault with the horizontal E-W compression is predominant for crustal and subcrustal earthquakes on the Pacific side, and it is directly related to the westward convergence of the Pacific Plate underneath the Eurasian Plate. Axes of maximum pressure show some variations for individual earthquakes and hypocenter locations. An eastward increase of dip angle of T axis is observed along the E-W section. The E-W compression appears also in the inland region of the northern Kanto District. In the southwestern part of the Kanto District a variety of focal mechanisms are observed. For very shallow earthquakes near Izu-Oshima and the Izu Peninsula a strike-slip with an N-S compression is predominant. Another type is the normal faulting for earthquakes located south off the Kanto District. These two types of faulting commonly have the horizontal E-W extension. Besides the down-dip compression observed for mantle earthquakes, the down-dip extension along the inclined seismic zone is observed for intermediate-depth earthquakes located on the Pacific side. But the tensional stresses along the lower plane of the double seismic zone show a variety of azimuths and dip angles. A significant contrast is observed in fault types between the IzuBonin region and the Pacific coast of the Kanto District and Northeast Japan. The thrust faulting is predominant for earthquakes with depths down to 60 km along the plate boundary below the Pacific coast, but in the Izu-Bonin region various types of faulting are observed without showing an obvious inclined seismic zone. Some source regions with similar focal mechanisms are observed; the reverse fault with the NW-SE compression in the sequence of the 1964 Niigata and 1972 Hachijojima Earthquakes, and the strike-slips in the Matsushiro Swarm and in the Izu Peninsula and Izu Islands region.数値解法によって関東地方と周辺に起った454個の地震(1963～1979年,M≧5.0)の地震メカニズムを解析し起震歪力の分布を調べた.個々のメカニズム解・平均メカニズム・メカニズムタイプの分布が,Pn観測点補正を行なって求められた高精度震源の分布と比較された.関東地方・東北日本の太平洋側と伊豆・小笠原諸島の間には起震歪力の分布に著しいコントラストがみられる.関東・東北日本の太平洋側では太平洋プレートの上面境界に沿った逆断層が卓越しているが,伊豆・小笠原諸島では西に傾き下がる地震帯も明瞭でなく種々のタイプのメカニズムがあらわれている.こうした相違は両地域における太平洋プレートの,ユーラシア・フィリピン海プレートとの結合の違いによると考えられる.関東・東北日本の地殻内・地殻直下の地震では水平東西の主圧力による逆断層が卓越しているが,個々の地震や地域によって主圧力軸の方位に変化がみられる.又主張力軸の傾きが東側で大きくなる地域変化もみられる.東西水平の主圧力は内陸部にもみられ,新潟地震では縦ずれ,松代地震では横ずれのメカニズムであらわれている.関東地方の南西部ではいくつかのタイプのメカニズムがみられる,伊豆半島・伊豆大島附近の極浅発地震では南北あるいは北西-南東主圧力の横ずれ断層が卓越し,関東南方沖では正断層があらわれる.これらのメカニズムでは水平東西の張力が共通している.マントル地震では主圧力軸が深発地震帯に沿って傾き下がるメカニズムの他,特に稍深発地震では地震帯に沿って傾き下がる主張力がみられる.これら2種類のメカニズムは二重深発地震面の上・下面にあたるが,特に下面に沿った主張力軸の方位や傾きは必ずしも地震帯に沿うものばかりではない