3:45 PM - 4:00 PM
[S08-03] Shear Strain Energy Changes in the Coseismic and Postseismic Periods of the 2016 Kumamoto Sequence: Constraints on Background Stress Field
The shear strain energy density allows us to investigate whether stress is accumulated or released in the lithosphere. The energy is released during the occurrence of a large earthquake and the subsequent postseismic period, in the form of afterslip. Using the shear strain energy density, Noda et al. (2020 GRL) investigated the energy changes due to the mainshock of the 2016 Kumamoto earthquake, a large intraplate earthquake sequence that considerably affected the crustal deformation in the area. Their research showed that the shear strain energy patterns strongly depended on the background stress, and the correlation of aftershocks with the areas of shear strain energy increase. However, a quantitative relation between the large earthquake and the following afterslip was not investigated at this stage.
In this research, we extend the work of Noda et al (2020) to investigate the energy release, not only in the coseismic period but also in the postseismic period by considering energy absorption due to the viscoelastic response. For the analysis, we employ GPS observations from 93 GEONET sites in the research area. We performed a time series analysis to extract the deformation associated with the coseismic period and afterslip one year after the earthquake. These results are used to estimate detailed kinematic models of the deformation. To estimate the shear strain energy change, we introduce a depth-dependent background stress field within the seismogenic zone following Noda et al. (2020). We evaluate the dependence of the coseismic and postseismic change in shear strain energy as a function of the effective frictional coefficient μeff. We consider four cases of μeff: 0.0, 0.1, 0.2, and 0.4.
The total energy estimated during the coseismic period indicates energy release for each investigated case, in agreement with shear faulting to release the stored shear strain energy from the lithosphere and the results of Noda et al (2020). On the other hand, the total energy change estimated 1 year after the earthquake indicated that the energy was released if we assume μeff>= 0.2, but the energy was increased if we assume μeff=0.1. Since the energy increase by the afterslip is not physically reasonable, we can reject the background stress model with μeff=0.1. This study highlights the importance of examining mechanical parameters by integrating different stages of the earthquake cycle to avoid nonphysical models and accurately establish the mechanics of earthquake sequences.
In this research, we extend the work of Noda et al (2020) to investigate the energy release, not only in the coseismic period but also in the postseismic period by considering energy absorption due to the viscoelastic response. For the analysis, we employ GPS observations from 93 GEONET sites in the research area. We performed a time series analysis to extract the deformation associated with the coseismic period and afterslip one year after the earthquake. These results are used to estimate detailed kinematic models of the deformation. To estimate the shear strain energy change, we introduce a depth-dependent background stress field within the seismogenic zone following Noda et al. (2020). We evaluate the dependence of the coseismic and postseismic change in shear strain energy as a function of the effective frictional coefficient μeff. We consider four cases of μeff: 0.0, 0.1, 0.2, and 0.4.
The total energy estimated during the coseismic period indicates energy release for each investigated case, in agreement with shear faulting to release the stored shear strain energy from the lithosphere and the results of Noda et al (2020). On the other hand, the total energy change estimated 1 year after the earthquake indicated that the energy was released if we assume μeff>= 0.2, but the energy was increased if we assume μeff=0.1. Since the energy increase by the afterslip is not physically reasonable, we can reject the background stress model with μeff=0.1. This study highlights the importance of examining mechanical parameters by integrating different stages of the earthquake cycle to avoid nonphysical models and accurately establish the mechanics of earthquake sequences.