統計数理研究所

講演題目: Coseismic change and recovery of scattering environment in the crust after a large earthquake

作者: Katsunori Sugaya, Yoshihiro Hiramatsu, Muneyoshi Furumoto, and Hiroshi Katao

概要: The coda waves mainly consist of scattered S waves in the crust. The attenuation property of coda waves, coda Q^-1 or Qc^-1, reflects the scattering and absorption environments in the crust and is supposed to be a good tool to investigate medium properties in the crust. Furthermore, Qc^-1 is a good indicator of the stress condition in the crust.
We observe a unique temporal variation in crustal heterogeneity from the analysis of Qc^-1 for 14 years in the Tamba region, northeast to the rupture zone of the 1995 Hyogo-ken Nanbu earthquake (Mjma 7.3) in southwest Japan. The values of Qc^-1 at lower frequencies (1.5-4.0 Hz) that increased coseismically due to the static stress change decreased back to the pre-event values in about two years. No such variations are found at higher frequencies (5.0-24.0 Hz). We confirm that no tectonic events that cause a significant stress change occurred during the recovery period. The time required for the recovery of the scattering environment, such as the number density of cracks and cracks opened by the stress change, observed here is consistent with those of previous studies focused on the brittle shallower crust. This suggests a possibility that a similar mechanism of the recovery operates both in the brittle and the ductile parts of the crust.