[S-SS06] Fault Rheology and Earthquake Physics

[SSS06-08] Vibration-induced dynamic weakening in sheared gouge layer

*Ryunosuke Sakamoto¹, Takahiro Hatano¹ (1.Osaka University)

Keywords:dynamic triggering earthquake, granular material, discrete element method

Seismic waves radiated from a epicenter can trigger other earthquakes. Observational studies show that dynamic triggered earthquakes are caused by stress changes (O(100~0.1)kPa) due to the passing seismic waves that are much smaller than the earthquake stress drop (O(10MPa)) (e.g., Hill 2015). One of the reasons that such small stress changes can trigger earthquakes is believed to be the strength weakening of fault gouge by the vibrations. This is because several experiments show that the vibration weakens the fault gouge strength (e.g., Johnson et al., 2005). However, most of these experiments were conducted under non-sheared condition. In numerical simulation, there are a few studies in which vibrations are applied to sheared gouge layers (Ferdowsi et al., 2015;Giacco et al., 2015). However, these models may not be valid because the inter-particle contact force does not depend on the relative velocity between particles, or the longitudinal and transverse wave velocity are same.Therefore, our understanding of the strength weakening of sheared gouge layers due to vibration is not yet sufficient.

In this study, we focused on clarifying the mechanism of strength weakening of sheared gouge layer caused by vibration using the discrete element method.Our model is that the fault gouge is confined by two substrates, one of which is pulled at a constant velocity V through a spring with spring constant k.The response of the sheared gouge layer to vibration was studied by applying transient vibrations to the model.

Numerical result showed that the strength weakening of sheared gouge layer due to vibration is caused by increase in slipping contact ratio(ratio of the number of slipping contacts to the number of inter-particle contacts). This result implies that slipping contact causes reduction in contact stiffness, resulting in modulus softening (strength weakening) of the fault gouge (Jia et al., 2011;Tong et al., 2020). Furthermore, we found that this strength weakening is more pronounced when the thickness of the fault gouge and the applied vibration wavelength are on the same scale.