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.

Joint inversion of refraction and gravity data for the three-dimensional topography of a sediment-basement interface

“…The velocity derived in the basement is 5.347 km s −1 , which implies ρ b is equal to 2.386 g cm −3 with a Δρ of −0.349 g cm −3 . These values are rather lower than those for fresh granites, probably because the rocks in the basement around this area have been heavily crushed and deeply weathered by Quaternary tectonic movements (Kobayashi et al 1996). The rhyolites and metamorphic rocks in the north may also reduce the average velocity and density.…”

“… Distribution of the gravity stations by Inoue et al (1998) and Kobayashi et al (1996) over contours of the initial sediment–basement interface from Kagawa et al (1993). …”

“…The joint inversion procedure described in the preceding sections is examined here by using synthetic data for the simple basin model in Osaka Bay Kobe Figure 9. Distribution of the gravity stations by Inoue et al (1998) and Kobayashi et al (1996) over contours of the initial sediment-basement interface from Kagawa et al (1993). Figure 10.…”

“…8). The dense gravity data from Inoue et al (1998) and Kobayashi et al (1996) are also introduced into this study (Fig. 9).…”

“…They succeeded in recovering general features of the basin, but the fine structure around the north‐western edge of the basin could not be recovered owing to the poor coverage of the refraction data. Kobe, which suffered a devastating earthquake in 1995, is located there, and the fine structure was identified from gravity data after the earthquake (Kobayashi et al 1996; Inoue et al 1998). Since the structure greatly affected the distribution of strong ground motion from the earthquake (Kawase 1996; Furumura & Koketsu 1998; Pitarka et al 1998), it is very important to construct a detailed model using all available data.…”