Basement and Active Structures Revealed by the Seismic Reflection Survey in Osaka Bay

Abstract

Multi-channel seismic reflection surveys at seven tracks were conducted in Osaka bay. Osaka bay fault whose length is more than 37 kilometers, was recognized from the south of Kobe to the east of Sumoto, Awaji island. Very thick sediments, maximum thickness being more than 2, 700 meters, were recognized to the eastside of Osaka bay fault. The surface of the basement of Osaka bay becomes shallower gently toward the southeast. Besides, the basement morphology to the northwest of the fault is steeper. Distribution and thickness of each sedimentary unit suggest that the basement of Osaka bay was subsiding in ductile in late Pliocene to early Pleistocene. Then, the basement was ruptured and Osaka bay fault was formed in early Pleistocene. Since then, subsiding of the basement was concentrated along the fault. The fault is developed at constant rate since early Pleistocene. Another burried fault was also found at the southeast of Tsuna. Activity of the fault is mcreasing smce middle Pleistocene.