Three-dimensional P- and S-wave attenuation structures around the source region of the 2016 Kumamoto earthquakes 4. Seismology

We investigate the three-dimensional P- and S-wave attenuation (Q P - 1 and Q S - 1) structures of the crust around the source region of the 2016 Kumamoto earthquakes, Japan. To estimate the attenuation structures, the path-averaged attenuation factor t ∗ is estimated from the amplitude decay rate of the P- and S-wave spectra corrected for the source spectrum. The (Q P - 1 and Q S 1) structures are estimated by tomography using t ∗ for the P- and S-waves, respectively. Several features are found in the attenuation structures as follows: In the source region, two high- Q P and high- Q S zones exist along the Futagawa and the Hinagu fault segments in the upper crust. The high- Q P and high- Q S zone along the Futagawa fault segment is found to include the large-slip area of the mainshock obtained from a source inversion study. In the lower crust, the low Q P is distributed beneath the entire source region. A low- Q P and low- Q S zone also exists beneath the Kuju and Aso volcanoes, which is consistent with the shallow limited depth extent of the seismogenic zone due to high temperature. The western edge of this zone adjoins the eastern edge of the high- Q P and high- Q S area, including the large-slip area.