INVESTIGATION OF BUILDING DAMAGE IN MASHIKI-TOWN FOR THE 2016 KUMAMOTO EARTHQUAKE—EFFECT OF LOCAL SOIL CONDITIONS—

Abstract

 We performed a detailed survey of buildings in central Mashiki town, which was heavily damaged by the 2016 Kumamoto earthquakes. The percentage of severely damaged buildings (damage ratio) varies greatly within this small area. In order to analyze the cause of the heterogeneous distribution, we performed a microtremor survey to study the effect of the local site response on ground motions during the Kumamoto earthquakes. Our results show that the area where buildings were less severely affected has a thick sediment deposit with a low shear wave velocity of 70m/s. From a nonlinear earthquake response analysis, the lower frequency content caused by the shallow layer was dominant during the mainshock, and there were relatively smaller amplitudes at 1-2 Hz. We suggest that this nonlinear response was an important factor contributing to the damage distribution.