5:15 PM - 6:45 PM
[SSS08-P12] The Shear Strain Energy Changes of The 2016 Kumamoto Earthquake Sequence
Keywords:Shear strain energy change, the 2016 Kumamoto earthquake sequence
We investigated the 2016 Kumamoto earthquake sequence in viewpoints of shear strain energy change in the crust. The foreshock of MW6.1 occurred on April 14th followed by earthquakes. We referred to these earthquakes before the mainshock occurrence as the aftershocks of the foreshock. The mainshock of MW7.1 then occurred on April 16th. We estimated the shear strain energy change brought by these seismic activities: the foreshock, the aftershocks of the foreshock, the mainshock, and the aftershocks.
Shear strain energy change depends not only on the stress change by the earthquake faulting but also the background stress field. In this study, we assume that the background stress field is uniform in the crust with the differential stress of 100 MPa. We also assume that uniform slip distributions of all the events including the mainshock. We analyze the aftershocks of the foreshock (24 events) and the main shock (240 events) by setting fault geometry according to the F-net moment tensor solutions.
The changes of the shear strain energy were ~-2.62×1015 J for the foreshock, ~-3.03×1015 J for the total aftershocks of the foreshock, ~-26.4×1015 J for the main shock, and ~-2.43×1015 J for the total aftershocks of the mainshock. The results suggest that the shear strain energy was released from the crust, in other words, the shear strain energy drove these seismic activities. We found that there was a significant difference about the aftershock activities between the foreshock and the mainshock. The ratios of the shear strain energy release by the aftershocks were ~110 % and ~10 % for the foreshock and the mainshock, respectively. For comparison, we characterized these events by seismic moment. The seismic moments were estimated to be ~2.48×1018 Nm for the foreshock, ~1.29×1018 Nm for the aftershocks of the foreshock in total, ~45.4×1018 Nm for the mainshock, and ~1.69×1018 Nm for the aftershock in total. The ratios of the total seismic moments of each set aftershock activity to the foreshock and to the main shock were ~52 % and ~4 %, respectively. The ratios were different from the estimations using the shear strain energy. This suggests that we would make a wrong estimation in energy budget if we use the seismic moment as proxy of the energy release associated with the seismic activity. We also confirmed that more aftershocks occurred in areas where shear strain energy increased by the mainshock as the previous studies reported (e.g., Noda et al. 2020). This result suggests that the shear strain energy increased by the mainshock is decreased by the aftershocks. However, our results show that in many parts of the analyzed areas, the aftershocks did not decrease the shear strain energy increased by the mainshock, but the aftershock also increased the shear strain energy. These results suggest that the number of earthquakes or the seismic moment do not correctly reflect the amount of energy release by the earthquake activities. In order to discuss the energy budget, we need to estimate the energy change, but the seismic moment do not always work as proxy of the energy release.
Shear strain energy change depends not only on the stress change by the earthquake faulting but also the background stress field. In this study, we assume that the background stress field is uniform in the crust with the differential stress of 100 MPa. We also assume that uniform slip distributions of all the events including the mainshock. We analyze the aftershocks of the foreshock (24 events) and the main shock (240 events) by setting fault geometry according to the F-net moment tensor solutions.
The changes of the shear strain energy were ~-2.62×1015 J for the foreshock, ~-3.03×1015 J for the total aftershocks of the foreshock, ~-26.4×1015 J for the main shock, and ~-2.43×1015 J for the total aftershocks of the mainshock. The results suggest that the shear strain energy was released from the crust, in other words, the shear strain energy drove these seismic activities. We found that there was a significant difference about the aftershock activities between the foreshock and the mainshock. The ratios of the shear strain energy release by the aftershocks were ~110 % and ~10 % for the foreshock and the mainshock, respectively. For comparison, we characterized these events by seismic moment. The seismic moments were estimated to be ~2.48×1018 Nm for the foreshock, ~1.29×1018 Nm for the aftershocks of the foreshock in total, ~45.4×1018 Nm for the mainshock, and ~1.69×1018 Nm for the aftershock in total. The ratios of the total seismic moments of each set aftershock activity to the foreshock and to the main shock were ~52 % and ~4 %, respectively. The ratios were different from the estimations using the shear strain energy. This suggests that we would make a wrong estimation in energy budget if we use the seismic moment as proxy of the energy release associated with the seismic activity. We also confirmed that more aftershocks occurred in areas where shear strain energy increased by the mainshock as the previous studies reported (e.g., Noda et al. 2020). This result suggests that the shear strain energy increased by the mainshock is decreased by the aftershocks. However, our results show that in many parts of the analyzed areas, the aftershocks did not decrease the shear strain energy increased by the mainshock, but the aftershock also increased the shear strain energy. These results suggest that the number of earthquakes or the seismic moment do not correctly reflect the amount of energy release by the earthquake activities. In order to discuss the energy budget, we need to estimate the energy change, but the seismic moment do not always work as proxy of the energy release.