[S03-P] Poster

[S03-P-07] Elastic Velocity Change associated with the 2016 Kumamoto Earthquakes, Japan

^○Tomotake Ueno, Tatsuhiko Saito, Kaoru Sawazaki, Katsuhiko Shiomi (NIED, Tsukuba, Japan)

A seismic interferometry analysis of the Hi-net seismograms found temporal change of subsurface velocity structure after the 2016 Kumamoto earthquakes. Significant velocity decreases (-6.0 - -0.5 %) were detected near the main shock fault by the analysis of ambient noise auto correlation function (ACF) with the lag time of 4-15 s. The large ground acceleration (> ~100 cm/s/s) and the large volumetric strain (> ~-5x10^-6) were observed at these stations where the velocity decreases were found. We also analyzed the ACFs with the lag time of 1-5 s and found velocity increase and decrease around the fault. A station N.MSIH near the fault shows velocity increase. Fukuyama and Suzuki (2016) estimated large compressional strain at the N.MSIH station by analyzing permanent vertical displacements recorded at two acceleration sensors deployed different depths. The compressional strain change may cause the velocity increase of N.MSIH. On the other hand, when we analyzed the ACF at the lag time of 4-15 s, velocity change was not recognized due to unstable behavior of the ACFs. The dependence of the velocity change on the lag time suggests that the velocity change considerably varied in space. The velocity increase associated with the earthquakes may reflect localized strain change near the main rupture. High-density observation around the focal area enabled us to detect the localized velocity increase after large earthquakes.