Unconventional Damage Patterns in Mashiki town during the 2016 Kumamoto earthquakes
Abstract
In engineering seismology, it is well known that thick sedimentary deposits tend to amplify ground motions which can cause severe damage to structures. However, the 2016 Kumamoto earthquake is an unusual example where the opposite was observed: there were fewer collapsed houses and overall less damage on the soft sites of a nearby river floodplain. The 2016 Kumamoto earthquakes caused serious building damage in the near-source regions, especially Mashiki town. The first earthquake (Mj6.5) occurred at 21:26, April 14 and the second event (Mj7.3) occurred at 1:25, April 16. 30% of the totally collapsed buildings were concentrated in Mashiki town. There was a very heterogeneous distribution of the affected buildings that varied within a few kilometers from severe damage to almost all structures, to very light damage. In order to understand the relationship between damage distribution and local site response, we performed a detailed damage survey of wooden structures and carried out microtremor surveys. Our results show that the area where buildings were relatively less severely affected has thick sediment deposits with a low shear wave velocity of 70 m/s. From a nonlinear analysis of the site response, the lower frequency content caused by the shallow layer was dominant during the Mj7.3 event, and had relatively smaller amplitudes at 1-2 Hz. We suggest that this nonlinear response was an important factor contributing to the damage distribution.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFM.S21D..02Y
- Keywords:
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- 7212 Earthquake ground motions and engineering seismology;
- SEISMOLOGY;
- 7255 Surface waves and free oscillations;
- SEISMOLOGY;
- 7290 Computational seismology;
- SEISMOLOGY