Characteristics of Strong Ground Motions and Fragility Curves of Buildings during the 2011 off the Pacific coast of Tohoku Earthquake

Abstract

The 2011 off the Pacific coast of Tohoku Earthquake was an Mw9.0 mega thrust interplate earthquake. It was the largest earthquake in Japan and the fifth largest one in the world since seismology records began in 1900. The coastal area, especially in the Iwate and Miyagi Prefecture, was subjected to great damage by huge tsunami. Many villages and towns including the buildings and people were entirely flushed away. Except the damage caused by tsunami, the ground shaking during this earthquake also brought about the damage to buildings over the east Tohoku and Kanto regions. It is essential to investigate the relationships between building damage ratios and ground motion characteristics, i.e., fragility curves, during this earthquake, which will be useful for disaster mitigation and damage assessment in future disastrous earthquakes. First, the relationships between building damage ratios in administrative districts, such as city, town and village (shi, cho, mura in Japanese), and observed ground motions during the 2011 Tohoku Earthquake are investigated. Three different damage ratios, i.e., total collapse ratio (TCR), collapse ratio (CR) and damage ratio (DR), are related to four ground motions indices, i.e., peak ground acceleration (PGA), peak ground velocity (PGV), JMA instrumental seismic intensity (IJMA) and spectral ratio (SI), respectively. However, the ground motions are sensitive to geological conditions in administrative districts, it suggests that the ground motions should be estimated within a smaller administrative unit, e.g., subdistrict (subdivision of an administrative unit), to be related to the damage ratios. Therefore, in this study, the method of estimating ground motions during the mainshock is proposed and verified. This method indicates that the ground motions at the sites where no strong motion stations were installed can be estimated with the underground velocity structures identified from the microtremor H/V spectral ratios and the ground motions at the bedrock during the mainshock calculated by use of the short-period source model. The underground velocity structures at damaged sites are identified from microtremor H/V spectral ratios. By applying the proposed method, the ground motions in subdistricts are estimated with the identified underground velocity structures and the bedrock motions during the mainshock. Fragility curves based on the damage ratios and estimated ground motions in the subdistricts of Osaki and Kurihara cities, and Wakuya town, Miyagi prefecture during the mainshock are then investigated.