Two-Dimensional Analysis of Gravity Anomaly across the Rokko Fault System.

Abstract

We carried out dense gravity measurements with the Fast Static GPS positioning along five survey lines across the Rokko fault system. We calculated the Bouguer anomaly to estimate the underground structure relevant to the 1995 Hyogo-ken Nanbu earthquake. We applied the two-dimensional Talwani's method to the modeling, assuming that the density structure is similar for all the five survey lines. All the Bouguer anomaly profiles are characterized by flat areas on the Rokko Mountains and Kobe Plain, with steep gradient between them. The gradient is most likely due to the fault scarps between the Rokko granite and the sedimentary layer of the Osaka and Kobe groups. It also indicates that the faults are vertical or reverse ones, at least just below their surface traces. This finding is consistent with the tectonic background of the Rokko Mountains that have upheaved under the east-west compression in the Quaternary. Moreover, we found the southwestward extension of the Koyo Fault underneath the sedimentary layer. The extension runs on the edge of the earthquake disaster zone. We inferred that the thickness of the sedimentary layer is 1-2 km near the sea, decreasing gradually toward the mountain side. The wedge-like structure and the hidden fault under the Kobe Plain may have served as a focusing lens of seismic rays during the earthquake.