Three-Dimensional Seismic Attenuation Structure in the Ryukyu Arc, Japan

Abstract:

Tomographic studies have been conducted to retrieve 3D seismic attenuation structure around Japan Arc since 1980s. However, in the Ryukyu Arc, 3D attenuation structures has never been estimated. It is important to estimate the 3D attenuation structure in this region, since there are highly active volcanos and seismicity between the Okinawa Trough and the Ryukyu Trench. In this study, we estimate 3D seismic attenuation structure in the Ryukyu Arc. We use seismic waveform data recorded by seismic observation networks of NIED, JMA and Kagoshima University, from 2004/06 to 2014/05. We select seismic events of more than 4,500. Since the Ryukyu Arc region are so wide, we separate it into three subregions: Sakishima Islands, Okinawa Islands and Amami Islands subregions. Before calculating the attenuation quantity t^*, the corner frequency of the source spectrum for each event is estimated by using an omega square model. The t^* is estimated from the amplitude decay rate from the source-corrected spectra. We then invert the t^* data to the attenuation structure by a 3D tomographic technique using the non-negative least squares method. Our estimated attenuation structure has the remarkable features: in Sakishima Islands subregion, high attenuation zone exists beneath northern Ishigaki Island. This region corresponds to the Okinawa Trough. High attenuation zone also exists beneath Hateruma Island in upper crust. It corresponds to the accretionary prism formed by subducting Philippine Sea Plate. In Amami Islands subregion, high attenuation zone is located along volcanic front. Low attenuation zone spreads over subducting Philippine Sea slab in all subregions.

Acknowledgements: We used JMA Unified Hypocenter Catalogs and seismic waveform data recorded by NIED, JMA and Kagoshima University. We also used a computer program by Zhao et al. (1992, JGR) for the tomographic analysis.