5:15 PM - 6:30 PM
[MIS34-P21] Temporal changes of seismic velocity structure after the 2016 Kumamoto earthquake
Keywords:The 2016 Kumamoto earthquake, temporal changes of seismic velocity structure, Auto Correlation function, Hi-net
We use seismic records of Hi-net, which were provided by National research Institute for Earth science and Disaster resilience (NIED). We use Auto-Correlation-Functions (ACFs) of the ambient seismic noise record to calculate velocity fluctuation of the subsurface structure. To obtain an ACF, we applied a 1 - 3 Hz bandpass filter to 1 hour waveform data, and applied one-bit amplitude normalization to the filtered data to reduce effects of large seismic signals. To avoid daily pattern of ambient seismic noise, we calculate one-week average of the ACFs. These stacked ACFs were calculated every day, and they were compared to the referenced ACF through the stretching method. The referenced ACF was reproduced from average of the ACFs obtained in 2013. Temporal velocity change was calculated if the velocity change was larger than twice the standard deviation of the velocity fluctuation obtained one month before the 2016 Kumamoto earthquake.
Our preliminary result demonstrated the co-seismic velocity changes at the source region of the Kumamoto earthquake, Mt. Aso, and a part of the northern Oita prefecture. For the southwest part of the source area, we obtained 0.5 – 3.0 % velocity decreases at three stations: N.MSIH, N.MSMH, and N.TYNH. For the northeast part of the source area near Mt. Aso, significant velocity decreases of 1.5 – 6.0 % were obtained at stations N.ASVH, N.HKSH, and N.NMNH. In a part of the northern Oita prefecture, velocity decreases of 0.5 – 1.0 % were obtained at stations N.KKEH and N.SNIH, which are located at a seismic gap between the swarms of Mt. Aso and northern Oita prefecture.