THE 2011 OFF THE PACIFIC COAST OF TOHOKU EARTHQUAKE TSUNAMI HYDRAULIC DATA OBSERVED AT CHOSHI OFFSHORE

Abstract

 The 2011 off the Pacific coast of Tohoku Earthquake, Mw9.0, occurred on the 11^th March 2011. High tsunamis were observed on the Pacific coast of Japan and caused devastating damages. The temporal variation of water depth, hydraulic pressure and horizontal components of current velocity was observed by using the Ultrasonic-type wave gauge placed on th e sea bottom 11m deep and 3km offshore of Choshi city, Chiba prefecture, Japan. The following findings were made. 1) Tsunamis and the largest aftershock may occur simultaneously. 2) The amount of water depth decrease due to drawback is larger than that of water depth increase due to runup. 3) The tsunami coming after the drawback can be breaking wave.