5:15 PM - 7:15 PM
[SSS11-P25] Friction law for shallower region than the seismogenic layer: Application to depth-dependent stress field
Keywords:shallower region, friction law, depth-dependent stress field, characteristic source model
In conventional earthquake ground motion predictions (Irikura and Miyake, 2011), slip in a near-surface layer shallower than a seismogenic layer has not been modeled because its contribution to strong-ground motion was considered small. Recently, however, necessity of near-surface long-period motion generation area has been pointed out to synthesize the long-period seismic waveforms and near-fault permanent displacements observed in the main shock of the 2016 Kumamoto earthquake (e.g., Matsumoto et al., 2016; Irikura et al., 2019). Kaneko and Goto (2022) implied that the physical origin of the long-period motion generation area may reflect a dynamic interaction of propagating rupture and the Earth's surface enhanced by reflected waves from the boundaries of low-velocity layers, and noted that a gentle rise in the seismic waveform near the fault would be inconsistent with a slip-weakening friction with small Dc or velocity-weakening friction without off-fault plasticity.
Rock experiments, on the other hand, have shown that frictional coefficient increases with increasing slip rate in the intermediate (about 0.05-0.2 m/s) velocity range (e.g., Tsutsumi and Shimamoto, 1997; Reches et al., 2010). The histories of slip rate and shear stress in rock experiments (Ohnaka and Kuwahara, 1990) and reconstruction of seismic waveform inversion results (Ide and Takeo, 1997) also showed velocity-strengthening friction at the beginning of slip and followed velocity-weakening friction. Kase et al. (2018, 2019) proposed a velocity-strengthening friction law below a certain velocity in addition to a slip-weakening law and then showed that the velocity-strengthening friction causes large slip and small slip rate time histories on a fault in the near-surface layer. In this study, we apply our friction law to a characterized source model in a depth-dependent stress field.
An 18 km wide vertical left-lateral strike-slip fault is located in a medium consisting of a five-layered, 2 km thick shallow region and a seismogenic layer. One or more asperities are distributed depending on the length of the fault. The area of the asperities is 15.6 % of the fault, and the depth of the top of the asperities is 2 km. Principal stresses are proportional to depth. Stress drops in the shallow and background regions are 0 MPa, and stress drops in the asperities increase with depth. The friction constitutive laws are a slip-weakening in the seismogenic layer (the asperities and the background region) and a slip-weakening and velocity-strengthening in the shallow region. We investigate the effect of the velocity-strengthening friction in slip rate functions in the shallow region and velocity waveforms at near fault sites, simulating spontaneous ruptures initiating at an initial crack in the asperity.
The slip-weakening and velocity-strengthening friction law applied to the shallow region enhances results in a longer period of the slip rate function in the shallow region and of the ground velocity waveforms near the fault (within about 1 km) compared to the no velocity-strengthening case. The larger frictional coefficient increment resulting from velocity-strengthening (δ) and the smaller threshold value of slip-rate resulting in velocity-strengthening (Vc) cause the more suppression the slip-rate. In the frictional constitutive relationship of the 1995 Kobe earthquake reconstructed from the waveform inversion result (Ide and Takeo, 1997; Guatteri et al., 2001), δ and Vc are estimated to be about 0.02 and 0.1-0.2 m/s, respectively. In the range of these values, the influence of the value Vc is small. The slip-weakening and velocity-strengthening friction law suppresses the maximum slip at the ground surface to about 90-95% of that in the no velocity-strengthening case, while the seismic moment is about 97-98%. Therefore, the scaling law can be satisfied almost as well as that in the slip-weakening case.
Rock experiments, on the other hand, have shown that frictional coefficient increases with increasing slip rate in the intermediate (about 0.05-0.2 m/s) velocity range (e.g., Tsutsumi and Shimamoto, 1997; Reches et al., 2010). The histories of slip rate and shear stress in rock experiments (Ohnaka and Kuwahara, 1990) and reconstruction of seismic waveform inversion results (Ide and Takeo, 1997) also showed velocity-strengthening friction at the beginning of slip and followed velocity-weakening friction. Kase et al. (2018, 2019) proposed a velocity-strengthening friction law below a certain velocity in addition to a slip-weakening law and then showed that the velocity-strengthening friction causes large slip and small slip rate time histories on a fault in the near-surface layer. In this study, we apply our friction law to a characterized source model in a depth-dependent stress field.
An 18 km wide vertical left-lateral strike-slip fault is located in a medium consisting of a five-layered, 2 km thick shallow region and a seismogenic layer. One or more asperities are distributed depending on the length of the fault. The area of the asperities is 15.6 % of the fault, and the depth of the top of the asperities is 2 km. Principal stresses are proportional to depth. Stress drops in the shallow and background regions are 0 MPa, and stress drops in the asperities increase with depth. The friction constitutive laws are a slip-weakening in the seismogenic layer (the asperities and the background region) and a slip-weakening and velocity-strengthening in the shallow region. We investigate the effect of the velocity-strengthening friction in slip rate functions in the shallow region and velocity waveforms at near fault sites, simulating spontaneous ruptures initiating at an initial crack in the asperity.
The slip-weakening and velocity-strengthening friction law applied to the shallow region enhances results in a longer period of the slip rate function in the shallow region and of the ground velocity waveforms near the fault (within about 1 km) compared to the no velocity-strengthening case. The larger frictional coefficient increment resulting from velocity-strengthening (δ) and the smaller threshold value of slip-rate resulting in velocity-strengthening (Vc) cause the more suppression the slip-rate. In the frictional constitutive relationship of the 1995 Kobe earthquake reconstructed from the waveform inversion result (Ide and Takeo, 1997; Guatteri et al., 2001), δ and Vc are estimated to be about 0.02 and 0.1-0.2 m/s, respectively. In the range of these values, the influence of the value Vc is small. The slip-weakening and velocity-strengthening friction law suppresses the maximum slip at the ground surface to about 90-95% of that in the no velocity-strengthening case, while the seismic moment is about 97-98%. Therefore, the scaling law can be satisfied almost as well as that in the slip-weakening case.