Naoko Kitada

Integrated study of high resolution geophysical and geological information of Osaka Bay, Southwest Japan

Abstract The stratigraphic framework of Quaternary sediments in the Osaka Basin, Southwest Japan was revealed by high resolution geophysical and geological surveys acquired after the 1995 Kobe Earthquake. Osaka Bay is located in the central part of the Osaka Basin and is underlaid with Pre-Neogene basement rocks covered by an unconsolidated sequence of Plio-Pleistocene marine, fluvial and lacustrine sediments. Fifteen laterally continuous marine clays (from Ma-1 to Ma13, in ascending order) have been identified throughout the Osaka Basin that have been correlated with the marine eustatic record. Deep borehole and high resolution seismic data were acquired in Kobe (northern part of the basin) and Kansai International Airport (southern part of the basin). Sequence stratigraphy defined by seismic reflectors was used to reveal the stratigraphic differences between the two areas. By identifying reflectors as marine clay layers throughout the basin, we were able to divide the study area into three parts (northern, middle and southern parts) and to estimate the sedimentation rate in each location. The sedimentation rate increases from the northern and southern margins of the basin towards central Osaka Bay in the middle of the basin. In the southern parts, the sharp decline of sedimentation rate between Ma2 and Ma4 and thinning or complete lack of the reflectors corresponding to Ma5 and Ma6 layers result from tilting in this region.

Geological structure of the Kego fault zone from using the boring database

In this study to assess the stratal architecture of the surface layer around the Kego fault zone, we collected and studied boring data. Approximately 17,000 boring data was collected, most of which is publicly available. The Kego fault is known for being primarily left-lateral, with one portion in the south-western direction being dip-slip. As such, this study focused on changes in the altitude at which the boring data’s bedrock appeared and investigated the locations at which the fault structure passes along. As a result, changes in the altitude at which the bedrock appeared was notable at Hakata Bay, and this gradually became hilly towards the south. Due to prominent gravel layers, the bedrock appearance altitude was difficult to ascertain, however we were able to confirm the locations at which the fault structure passed through from Hakata Bay to the southern part of Kasuga City.