2-DIMENSIONAL ANALYSIS OF GRAVITY-ANOMALY ACROSS THE ROKKO FAULT SYSTEM

We carried out dense gravity measurements with the Fast Static GPS pos itioning along five survey lines across the Rokko fault system. We cal culated the Bouguer anomaly to estimate the underground structure rele vant to the 1995 Hyogo-ken Nanbu earthquake. We applied the two-dimens ional Talwani's method to the modeling, assuming that the density stru cture is similar for all the five survey lines. All the Bouguer anomal y profiles are characterized by flat areas on the Rokko Mountains and Kobe Plain, with steep gradient between them. The gradient is most lik ely due to the fault scarps between the Rokko granite and the sediment ary layer of the Osaka and Kobe groups. It also indicates that the fau lts are vertical or reverse ones, at least just below their surface tr aces. This finding is consistent with the tectonic background of the R okko Mountains that have upheaved under the east-west compression in t he Quaternary. Moreover, we found the southwestward extension of the K oyo Fault underneath the sedimentary layer. The extension runs on the edge of the earthquake disaster zone. We inferred that the thickness o f the sedimentary layer is 1-2 km near the sea, decreasing gradually t oward the mountain side. The wedge-like structure and the hidden fault under the Kobe Plain may have served as a focusing lens of seismic ra ys during the earthquake.