9:45 AM - 10:00 AM
[SSS09-04] Changes in attenuation and its anisotropy associated with seismic ground motion
Keywords:coseismic change, changes in anisotropy, changes in attenuation, artificial seismic source, ACROSS
We found anisotropic change in attenuation associated with seismic ground motions in dataset obtained in the experiment using an artificial seismic source, ACROSS. A larger reduction in amplitude was observed in the direction with a larger delay in travel time. The dataset we analyzed was obtained by Ikuta et al. (2002) at Awaji ACROSS test site from 2000 to 2001. At this site, the ACROSS was installed on the surface and the signal was observed at two seismometers installed at the bottom of 800m and 1700m deep borehole near the source.
We calculated changes in amplitude using the ratio of power of the transfer functions to that of the reference transfer function as reported by Tsuji et al. (2020 JpGU). In this calculation, we removed power of noise that of the transfer functions to reduce effect of temporal change in noise level from the estimated change in amplitude.
To analyze the anisotropic changes in amplitude, we reinvestigated the anisotropic change in S wave observed at the 2000 Western Tottori Earthquake (WT) and the 2001 Geiyo Earthquake (GY). We calculated the azimuthal variation of the coseismic delay of the S wave by rotating the sensor orientation with high resolution and found that the azimuthal pattern of coseismic delay cannot be approximated by an ellipse as Ikuta and Yamaoka (2004) did.
We modeled the azimuthal pattern with parameters that describe the anisotropy. By fitting the azimuthal pattern by the model on to the observation, we estimated directions of the principal axes. For WT, the directions of the major axis were N90°E for both the 800 m and 1700 m borehole sensors. For GY, the directions were N85°E and N95°E for the 800 m and 1700 m borehole sensors, respectively. These directions coincide well with the results of Ikuta and Yamaoka (2004), in which the direction was N100°E for WT and GY at the 800 m sensors.
We estimated the amplitude change in the direction of the principal axes. Changes in the amplitude and result of the model fitting in the principal axes are shown in the Figure. Dots and lines indicate amplitude changes and its fitting. The major and minor axes correspond to the axes of larger and smaller velocity change, respectively. For WT, a greater decrease in amplitude is obtained in the major axis of velocity anisotropy for both depths. For GY, no significant difference was observed for either depth. This result shows that the direction of greater attenuation corresponds to that of larger delay. That is consistent with laboratory experiments with cracked and saturated media.
We calculated changes in amplitude using the ratio of power of the transfer functions to that of the reference transfer function as reported by Tsuji et al. (2020 JpGU). In this calculation, we removed power of noise that of the transfer functions to reduce effect of temporal change in noise level from the estimated change in amplitude.
To analyze the anisotropic changes in amplitude, we reinvestigated the anisotropic change in S wave observed at the 2000 Western Tottori Earthquake (WT) and the 2001 Geiyo Earthquake (GY). We calculated the azimuthal variation of the coseismic delay of the S wave by rotating the sensor orientation with high resolution and found that the azimuthal pattern of coseismic delay cannot be approximated by an ellipse as Ikuta and Yamaoka (2004) did.
We modeled the azimuthal pattern with parameters that describe the anisotropy. By fitting the azimuthal pattern by the model on to the observation, we estimated directions of the principal axes. For WT, the directions of the major axis were N90°E for both the 800 m and 1700 m borehole sensors. For GY, the directions were N85°E and N95°E for the 800 m and 1700 m borehole sensors, respectively. These directions coincide well with the results of Ikuta and Yamaoka (2004), in which the direction was N100°E for WT and GY at the 800 m sensors.
We estimated the amplitude change in the direction of the principal axes. Changes in the amplitude and result of the model fitting in the principal axes are shown in the Figure. Dots and lines indicate amplitude changes and its fitting. The major and minor axes correspond to the axes of larger and smaller velocity change, respectively. For WT, a greater decrease in amplitude is obtained in the major axis of velocity anisotropy for both depths. For GY, no significant difference was observed for either depth. This result shows that the direction of greater attenuation corresponds to that of larger delay. That is consistent with laboratory experiments with cracked and saturated media.