メジアンフィルターを用いた2016年熊本地震の断層近傍における加速度記録の基線補正と変位波形の推定

The 2016 Kumamoto earthquake sequence occurred on April 14 (MJMA 6.5) and April 16 (MJMA 7.3). Seismic intensity of 7 on the Japan Meteorological Agency (JMA) scale was observed in Mashiki Town, Kumamoto Prefecture for the both events and in Nishihara Village, Kumamoto Prefecture for the April-16 event. We estimate the displacement waveforms from these acceleration records. Since the acceleration seismograms include the long-period noise due to tilting of the ground and instrumental effects, the baseline corrections are required to derive the accurate velocity and displacement waveforms. We apply a median filter to the velocity waveforms to identify the linear trends on them due to the steplike noise on the acceleration records, and determine the time at which baseline shifts take place and the step value of each shift for the baseline correction through trial and error. Our baseline correction can successfully reconstruct the velocity and displacement waveforms from the acceleration records. The displacement waveforms show the static components consistent with the geodetic data.