1. Introduction

The Uemachi Fault in Osaka is an important active fault that controls tectonics and sedimentation in the Osaka Basin. It across the center of Osaka and lies in N–S direction mainly and is more than 40 km in length. The sediments that cover this basin are over 1000 m thick and, start to subsidence of the basin and aggradation from 3.5 Ma (Kitada et al., 2011 [1]). Cyclic climate change has occurred since 1.3 Ma, with at least 15 layers of marine clay having been deposited in central Osaka during warm intervals. The stratigraphy of this sedimentary sequence, known as the Osaka Group, has been summarized for upland areas on the basis of correlation of marine clay beds and volcanic ash layers.

2. Characteristics of Uemachi Fault and Borehole Database

In Osaka, Uemachi Fault is important active fault control the tectonic movement of sedimentation of Osaka basin. Pliocene to Quaternary sediment “Osaka Group” and terrace sediment are found to be deposited in the Osaka Plain and Holocene marine clay layers (Ma13) are covered these plains in order to sea level change. Fifteen layers of marine clay beds are deposited due to the cyclic sea level changes during glacial retreats. These sediment are very thick layers over 1000m therefore, fault structure are appeared as flexure zone (only vending the strata) and hidden the fault displacement around the surface (Fig.1). The up side on the fault (east side) is modified by erosion and urban development however, many seismic reflection surveys information the fault trace line on a piecemeal basis. One of the subway construction project across the fault, are carried out the many borehole drilling survey around the fault. It is the good case to understand the subsurface structure around fault. The active fault survey is almost done by the seismic reflection survey. However the surface structure is very complicated and difficult to estimate the folding structure. The result of this study along the subway, these marine clay layers are correlated using tephra and microfossil analysis. Fault displacement is resolved by the 600m flexture zone. These flexture zone need attention of amplified the seismic wave not only the fault displacement. From 2010, Project of focus investigations for Uemachi fault start and in this program, to make clear the distribution of this flexture zone, we use borehole database of KG-Net.

Figure 1. Geological and tectonic map of area around Osaka Area (Active fault map in urban area, 1996 [2])

In the Osaka basin, about 60,000 borehole data was collected and in the Uemachi fault zone, about 2000 borehole data can used to study about subsurface structure (Fig 2). Geological subsurface structure in the Osaka basin is considered about delta deposit area (lowland) and sometimes in the marine environment due to the sea level change. Figure 3 shows the cross sections of the Osaka basin. Blue and green strata show the marine clay layers. Uemachi uphill were formed by the structural movement of Uemachi fault. The seismic profile shows the displacement of the top of the basement rock over several hundred meters (Fig.4).

Figure 2. Distribution of borehole database in the Osaka Basin Square shows the study area

Figure 3. Geological cross section across the Osaka plain [3]

Figure 4. Sceismic profile inth Osaka basin [4]

3. Subsurface Structure around Uemachi Fault and Result

We first examined the borehole data along the seismic reflection line and then considered the surrounding area. An example of this analysis is shown in Figure 5, which highlights the fact that the buckling is expressed particularly well in the marine clay layers. An Upper Pleistocene marine clay (Ma12) is a good indicator of the flexure zone. We constructed many cross sections in and around the fault zone and classified the deformation form into three categories around the flexure zone. First, Ma12 marine clay layers were folded and were recognized on both the up and down sides of the flexure. Second, Ma12 marine clay layers appeared to be folded, but did not continue toward the hanging wall. Third, old marine clay, which was folded or displaced, was observed. An example of the correlate between seismic profile and borehole data shows in Fig 5. The results of this study allowed us to map the distribution of folding in a zone in the west of the Osaka area. Folding can be classified into three types: (1) Ma12 folding, (2) Ma12 folding that does not continue toward the hanging wall, and (3) folding or displacement of old marine clay. Ma12 marine clay is folded near the flexure, but the hanging-wall side appears to be denuded. Type (3) is distributed around Uemachi in upland areas, but the Ma13 and Ma12 layers are not found in these areas; therefore, it is difficult to estimate later activity along the Uemachi Fault [5] (Fig 6).

Figure 5. An example of compare seismic profile and borehole data

Figure 6. Distribution of flexure zones (Kitada et al, 2012) [5]

In this time we correlate more borehole data before and show the detail the folding zone in the Osaka basin. Figure 7 show the folding zone map in Osaka basin. Folding zone include Ma12 (pink zone) is the positively resent activity because the Ma12 (about 12,500 years before) sediment is folding. These zone are NE-SW direction not N-S. It might be changed the stress direction in the Osaka basin.

Figure 7. Distribution of the folding zone of Uemachi Fault

ACKNOWLEDGEMENTS

This research is funded by the integrated research project for the Uemachi active fault system in FY2011 by MEXT.