Koji Hada

Development of Inference Methods for Rotational Motions on Ground Surface and Application to Microtremors Acquired with a Small-Size Dense Array

In this study, we develop two methods for the inference of rotation vector on ground surface, two rocking rotations and a single torsional rotation. The first, termed nth-order elastic method, is based on the elasticity of the ground surface. The rotation vector is constructed from the first derivative with respect to the space of the ground surface motions. The first derivative is calculated from simultaneous equations by n-th order Taylor expansion obtained by difference motion between multiple observation points. Meanwhile, the second, termed rigid method, is based on the rigidity of ground surface and the rotation.

Estimation of Strong Motion During the 2011 Northern Nagano Earthquake and an Associated Building Damage Survey

We conducted a damage survey of the wooden structures and collected very dense ambient noise measurements in the near-source region of the 2011 Northern Nagano earthquake in central Japan. The percentage of totally collapsed buildings exceeded 30 % in the Aokura and Yokokura colonies of Sakae village, Nagano prefecture. The percentage in the Mori colony, where a strong motion was recorded during the mainshock, was <10 %. We estimated the strong motion in the Aokura and Mori colonies from the ambient noise measurements and strong motion records. The estimated strong motion distribution reflects the soil conditions and varies within that small area. The correlation of the estimated strong motion and damage ratio of the wooden structures is reasonably high, which indicates that the estimated ground motions are realistic. The damage curve obtained in this research shows that the collapse ratio exceeds 50 % at around 150 cm/s of input ground motion.