[S13-P] Poster

[S13-P-13] Radiation Efficiency of Intraslab Earthquakes beneath Kyushu

^○Yumenari Adachi¹, Junichi Nakajima¹, Toru Matsuzawa² (1.Tokyo Institute of Technology, Tokyo, Japan, 2.Tohoku University, Miyagi, Japan)

Although earthquakes in some subduction zones occur down to a depth of ~690 km, earthquakes in Kyushu are limited to a depth of ~200 km. As the Philippine Sea (PHS) slab is aseismically subducting to a depth of >400 km (Huang et al. 2013), physical properties in the PHS slab that control the genesis of intraslab earthquakes may change around a depth of ~200 km. This study estimates radiated energy and radiation efficiency for earthquakes that occurred at depths of 60-200 km beneath Kyushu, and discusses the depth variation in radiation efficiency.
To obtain reliable stress drop and radiated energy, we need to precisely estimate corner frequency of earthquakes from amplitude spectra. However, there is a strong trade-off between the corner frequency and quality factor, and so it is difficult to determine the two parameters accurately. Therefore, the corner frequency is first estimated by the coda wave spectral ratio method, and then the static stress drop and the quality factor are obtained from the shape of amplitude spectra for S waves using the pre-estimated corner frequency.
The obtained results show that the radiation efficiency does not show a depth dependency and the average value of the radiation efficiency is ~0.1, which is relatively small compared to those observed in the Pacific slab beneath Tohoku and Hokkaido. We infer that the small value of the radiation efficiency is due to high slab temperature in Kyushu.