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[SGD01-P10] Influence of pore pressure changes on the crustal movements just after the arrival of seismic motions
Keywords:strain step, pore pressure, fracture zone
Strain changes due to the source processes of the large earthquakes such as the 2011 off the Pacific coast of Tohoku earthquake and the 2018 northern Osaka earthquake have been observed at Rokko-Takao station in Kyoto (Mukai et al., 2020). The observed strain changes had some discrepancies from the calculated ones. These discrepancies might be caused by the local pore pressure variations just after the arrival of the seismic motions.
Rokko-Takao station passes through the fracture zone of Manpukuji fault. In this station, the continuous observation of crustal movements have been performed by using the Ishii-type three-component strainmeter, the pore pressure meters, the discharge meter and so on. Some large earthquakes have made the extreme increase of discharge which caused the strain changes in the order of 1 micro-strain (Mukai and Otsuka, 2012). These observed strain changes were 10 times or greater than the calculated ones based on the source processes, and were caused by the pore water migration. Such pore water migration is considered to be caused by the flowing out of mud from cracks or the connecting of cracks in the surrounding crust due to seismic motions, and is one of major disturbances for the crustal movement observations. However, pore water migration cannot start just after the arrival of seismic motions. Therefore, we might be able to observe the strain changes due to source processes during a short interval just after the arrival of seismic motions. Mukai et al. (2020) reported that they could detect the strain changes due to the time development of co-seismic fault slips during a few seconds just after the arrival of the seismic motions in the 2011 off the Pacific coast of Tohoku earthquake and the 2018 northern Osaka earthquake.
However, there are some discrepancies between the observed strain changes and the calculated ones based on the source processes. For instance, the discrepancies after removing the calculated strain changes from the observed ones in the 2016 central Tottori earthquake showed the extension in the N14°W-S14°E direction. In the 2018 northern Osaka earthquake, the discrepancies showed the extension in the N70°E-S70°W direction. Such directions of the extensions are almost along the observation tunnel or its orthogonal direction. This suggests that the discrepancies might be caused by some local influences in the fracture zone.
In the Rokko-Takao station, pore pressure changes has been observed at 10 Hz sampling rate at 3 points with different distances from the fault. The observed pore pressure showed the step-like increase just after the arrival of the seismic motions before the beginning of significant pore water migration. The spatial extent of the step-like increase in the pore pressure depends on the magnitude of the seismic motions. For small seismic motions, the pore pressure could increase in a narrow area, whereas the step-like increase in pore pressure could occur over a slightly wider area for large seismic motions.
In this study, I investigated the cause of the discrepancies between the observed strain changes and the calculated ones based on the source processes, and verified that the discrepancies might be caused by the crustal movements due to such local step-like increase in pore pressure with various spatial extent.
Rokko-Takao station passes through the fracture zone of Manpukuji fault. In this station, the continuous observation of crustal movements have been performed by using the Ishii-type three-component strainmeter, the pore pressure meters, the discharge meter and so on. Some large earthquakes have made the extreme increase of discharge which caused the strain changes in the order of 1 micro-strain (Mukai and Otsuka, 2012). These observed strain changes were 10 times or greater than the calculated ones based on the source processes, and were caused by the pore water migration. Such pore water migration is considered to be caused by the flowing out of mud from cracks or the connecting of cracks in the surrounding crust due to seismic motions, and is one of major disturbances for the crustal movement observations. However, pore water migration cannot start just after the arrival of seismic motions. Therefore, we might be able to observe the strain changes due to source processes during a short interval just after the arrival of seismic motions. Mukai et al. (2020) reported that they could detect the strain changes due to the time development of co-seismic fault slips during a few seconds just after the arrival of the seismic motions in the 2011 off the Pacific coast of Tohoku earthquake and the 2018 northern Osaka earthquake.
However, there are some discrepancies between the observed strain changes and the calculated ones based on the source processes. For instance, the discrepancies after removing the calculated strain changes from the observed ones in the 2016 central Tottori earthquake showed the extension in the N14°W-S14°E direction. In the 2018 northern Osaka earthquake, the discrepancies showed the extension in the N70°E-S70°W direction. Such directions of the extensions are almost along the observation tunnel or its orthogonal direction. This suggests that the discrepancies might be caused by some local influences in the fracture zone.
In the Rokko-Takao station, pore pressure changes has been observed at 10 Hz sampling rate at 3 points with different distances from the fault. The observed pore pressure showed the step-like increase just after the arrival of the seismic motions before the beginning of significant pore water migration. The spatial extent of the step-like increase in the pore pressure depends on the magnitude of the seismic motions. For small seismic motions, the pore pressure could increase in a narrow area, whereas the step-like increase in pore pressure could occur over a slightly wider area for large seismic motions.
In this study, I investigated the cause of the discrepancies between the observed strain changes and the calculated ones based on the source processes, and verified that the discrepancies might be caused by the crustal movements due to such local step-like increase in pore pressure with various spatial extent.