11:30 AM - 11:45 AM
[SCG44-21] Explaining Slow Slip Events and Tremors with a Frictional-Viscous Faulting Model
Keywords:Slow slip event, Slow earthquake, Theoretical model, Frictional-viscous model, Tremor
We consider a frictional-viscous fault zone model to explain the puzzling slow earthquake phenomena, with a particular focus on slow slip events (SSEs). The frictional-viscous model is inspired by the recent geological observations that imply the occurrence of SSEs in fault zones with a finite thickness of ~hundreds of meters. The bulk matrix of the fault zone deforms viscously, while pervasive frictional surfaces are distributed in the viscous matrix. To simultaneously consider both the 10s-kilometer-scale rupture propagation and the 100s-meter-scale fault zone features in the same model, We treat a fault zone as a zero-thickness "surface" embedded in an elastic medium. The "frictional-viscous" characteristics are parameterized into a constitutive relation where fault strength is partitioned into a frictional and a linear viscous component, while the slip for both components is the same (e.g., Ando et al., 2010; Nakata et al. 2011; Ando et al., 2012).
We explored the above model setup by analyzing the corresponding boundary integral equations both analytically and numerically. Two key parameters in the frictional-viscous model are the viscous coefficient $\eta_v$ and the event stress drop. We found that the frictional-viscous model can simultaneously explain various kinematic source parameters of SSEs to first order when the viscous coefficient $\eta_v$ is about $10^{4} - 10^{5}~\mu/(2\beta)$, and the average stress drop in a slip transient is about 10 kPa. $\mu$ is the shear modulus and $\beta$ is the shear wave speed. The characteristic kinematic source parameters of SSEs that are explained by the above model parameters include the slip rate (~10^{-8} m/s), rupture propagation speed (~10^{-2} m/s), rise time and source duration (~days), diffusive migration diffusivity (~10^{-3} m^2/s), and radiation energy to moment ratio (10^{-9}). Qualitatively, this frictional-viscous model can also explain the shorter inter-event interval and lower average stress drop observed in subduction zone SSEs, compared to what is observed in the fast earthquakes at the seismogenic depth. These agreements imply that the frictional-viscous model is a promising representation of the actual SSE source processes.
The present model provides many hypotheses, which can be further tested with future geophysical, geological, and experimental data. For example, our results imply that the frictional-viscous model has a moment-duration scaling relation of M0-T^3. Also, to explain the observed kinematic source parameters of SSE, the effective viscosity of the shear zone in the model need to be ~10^12-10^14 Pa*s (assuming shear zone width = 100m), which is significantly lower than the typical ambient rock viscosity (usually above 10^18 Pa*s).
We explored the above model setup by analyzing the corresponding boundary integral equations both analytically and numerically. Two key parameters in the frictional-viscous model are the viscous coefficient $\eta_v$ and the event stress drop. We found that the frictional-viscous model can simultaneously explain various kinematic source parameters of SSEs to first order when the viscous coefficient $\eta_v$ is about $10^{4} - 10^{5}~\mu/(2\beta)$, and the average stress drop in a slip transient is about 10 kPa. $\mu$ is the shear modulus and $\beta$ is the shear wave speed. The characteristic kinematic source parameters of SSEs that are explained by the above model parameters include the slip rate (~10^{-8} m/s), rupture propagation speed (~10^{-2} m/s), rise time and source duration (~days), diffusive migration diffusivity (~10^{-3} m^2/s), and radiation energy to moment ratio (10^{-9}). Qualitatively, this frictional-viscous model can also explain the shorter inter-event interval and lower average stress drop observed in subduction zone SSEs, compared to what is observed in the fast earthquakes at the seismogenic depth. These agreements imply that the frictional-viscous model is a promising representation of the actual SSE source processes.
The present model provides many hypotheses, which can be further tested with future geophysical, geological, and experimental data. For example, our results imply that the frictional-viscous model has a moment-duration scaling relation of M0-T^3. Also, to explain the observed kinematic source parameters of SSE, the effective viscosity of the shear zone in the model need to be ~10^12-10^14 Pa*s (assuming shear zone width = 100m), which is significantly lower than the typical ambient rock viscosity (usually above 10^18 Pa*s).