2:45 PM - 3:00 PM
[SSS06-15] Estimation of friction parameters and slip distribution from shear strain array data beneath the fault plane (Part 2: Model with free surfaces)
Recently, induced earthquakes have caused some problems due to underground water injection associated with shale oil gas mining and enhanced geothermal power generation. A contribution to reduce the risk of induced earthquakes by investigating the mechanisms of fault rupture propagation. It is important to estimate the spatiotemporal distribution of slip on the fault plane and the frictional constitutive law of rocks to estimate the generation process of induced earthquakes. Even in laboratory experiments, it is difficult to directly measure the slip distribution on a two-dimensional fault plane, although it is possible to measure the strain distribution near the fault plane. In this study, we use the shear strain array data close to the fault plane obtained by Fukuyama et al. (2018, Tectonophys.) from a large-scale rock friction experiment to estimate friction parameters and spatiotemporal distribution of slip on the fault plane. Noda and Fukuytama (2022, SSJ) used dummy elements around the fault plane to simulate the edge of the fault. In the present study, we introduced free surface boundary conditions by using free surface elements.
Noda and Fukuyama (2022, JpGU) used the slip-weakening friction (Ida, 1972, JGR; Andrews, 1976, JGR), which is used to describe unstable slip of a fault plane, and the boundary integral equation method to calculate dynamic rupture propagation for plane rupture (Hok and Fukuyama, 2011, GJI) to estimate the friction constitutive law parameters and slip distribution from the shear stress distribution in numerical experiments. Using that method, Noda and Fukuyama (2022, SSJ) estimated a slip distribution as well as the distribution of slip weakening friction parameters from spatiotemporal distributions of shear strain near the fault plane .
In the calculations, the fault plane is divided into triangular elements, and the nucleation zone, where the initial slip is thought to have started, is defined. Within the nucleation zone, the static friction is set to be slightly smaller than the initial shear stress, and the slip weakening distance Dc is set to a constant value. Outside the nucleation zone, the difference between the static friction and the initial shear stress was assumed to vary in proportion to the distance from the center of the nucleation zone. In the previous study, dummy elements were added around the fault plane to reduce the influence of the edge of the fault plane, but in the present study, vertical free surface elements were added around the fault plane to achieve free surface boundary condition there.
The final model was selected to minimize the difference in stress drop and rupture propagation velocity between the measured data and the calculated results. From that model the spatio-temporal distribution of slip and the friction constitutive law were obtained. We confirmed that more accurate reproduction of the measured data by using free-surface elements than using dummy elements.
References
Andrews, J. D. (1974) https://doi.org/ 10.1029/JB081i032p05679
Fukuyama, E. et al. (2018) https://doi.org/10.1016/j.tecto.2017.12.023
Hok, S. and E. Fukuyama (2010) https://doi.org/10.1111/j.1365-246X.2010.04835.x
Ida, Y, (1972) https://doi.org/10.1029/JB077i020p03796
Noda and Fukuyama (2022, JpGU) used the slip-weakening friction (Ida, 1972, JGR; Andrews, 1976, JGR), which is used to describe unstable slip of a fault plane, and the boundary integral equation method to calculate dynamic rupture propagation for plane rupture (Hok and Fukuyama, 2011, GJI) to estimate the friction constitutive law parameters and slip distribution from the shear stress distribution in numerical experiments. Using that method, Noda and Fukuyama (2022, SSJ) estimated a slip distribution as well as the distribution of slip weakening friction parameters from spatiotemporal distributions of shear strain near the fault plane .
In the calculations, the fault plane is divided into triangular elements, and the nucleation zone, where the initial slip is thought to have started, is defined. Within the nucleation zone, the static friction is set to be slightly smaller than the initial shear stress, and the slip weakening distance Dc is set to a constant value. Outside the nucleation zone, the difference between the static friction and the initial shear stress was assumed to vary in proportion to the distance from the center of the nucleation zone. In the previous study, dummy elements were added around the fault plane to reduce the influence of the edge of the fault plane, but in the present study, vertical free surface elements were added around the fault plane to achieve free surface boundary condition there.
The final model was selected to minimize the difference in stress drop and rupture propagation velocity between the measured data and the calculated results. From that model the spatio-temporal distribution of slip and the friction constitutive law were obtained. We confirmed that more accurate reproduction of the measured data by using free-surface elements than using dummy elements.
References
Andrews, J. D. (1974) https://doi.org/ 10.1029/JB081i032p05679
Fukuyama, E. et al. (2018) https://doi.org/10.1016/j.tecto.2017.12.023
Hok, S. and E. Fukuyama (2010) https://doi.org/10.1111/j.1365-246X.2010.04835.x
Ida, Y, (1972) https://doi.org/10.1029/JB077i020p03796