Segmentation and Rupture Propagation of the Nojima Earthquake Fault

The Nojima Earthquake Fault 18km long appeared along the northwestern coastal line in Awaji Island in the 1995 Southern Hyogo Prefecture Earthquake. This fault can be divided into four segments based on the morphological characteristics, geological structures, gravity anomaly, and distributions of aftershocks. These segments are named here from north to south, Nojima segment (Awajicho to Nojima river), Ogura segment (Nojima river to Toshima river), Ikuha segment (Toshima river to Murotsu), and Kareki segment (Murotsu to Ozaki). These fault segments form a dextral strike-slip fault system showing an en echelon form and generally trend toward N30°-60°E. The Nojima and Ogura segments coinside with the Nojima geological fault. The Ikuha and Kareki segments appeared as new fault ruptures, the former following almostly along the axis of the Mitsukoshi flexure, and the latter parallel to and partly along the Shizuki geological fault.The boundaries between two adjacent fault segments along the Nojima Earthquake Fault are geological and geomorphological boudaries between the basement of Pre-Neogene granitic rocks and the Kobe Group and Quaternary deposits, and between the basin and hill. These boundries show the characters of dilational jogs. Furthermore, it is clear that the aftershock distributions are concentrated in the areas around these dilational jogs. It seems that the main earthquake rupture propagated southwestward from the main epicenter along the Nojima geological fault and then jumped to the next en echelon segment at the dilational jog to the southwestern end area.As stated above, it is clear that the morphological distributions of the four fault segments of the Nojima Earthquake Fault are controlled by the deep geological structures. The topography and morphology of the Nojima Earthquake Fault provides both useful information on the mechanics of faulting and predictions regarding the heterogeneity of earthquake.