Concurrent tremor and slip associated with the thermal boundary of the seismogenic zone

Abstract:

Large-scale, recurrent slow-slip events and tremors occur below the seismogenic zone of major subduction zones. The physics of slow-slip events is relatively well understood, possibly involving dilatant hardening, semi-brittle flow, fluid migration, and quasi-stable thermal instabilities. In contrast, the mechanics of concurrent slow-slip and tremor remains unclear because the conditions leading to slow or fast ruptures are thought to be mutually exclusive. This contradiction is usually resolved by invoking fine-grain heterogeneities, but a quantitative explanation is still missing. Here, we explore faults dynamics in conditions representative of the thermal boundary at the bottom of the seismogenic zone to show that simultaneous slow slip and tremor emissions are the natural behavior of homogenous, weakly velocity-weakening asperities, as long as the velocity dependence approaches velocity neutral with a small characteristic nucleation size. We simulate slow-slip events on an idealized fault plane using a two-dimensional model based on a physics-based rate- and state-dependent friction law. We consider a single velocity-weakening asperity surrounded by a velocity-strengthening region. Although the frictional properties are controlled by the geothermal gradient, we consider isothermal conditions during fault slip. Every simulated slow-slip event produces multiple fast sub-events that concentrate at the slow-slip rupture front. The spectrum of the isolated fast ruptures have slightly higher fall-off rates than those of regular earthquakes, and short bursts of seismicity following slow-slip accelerations lead to a train of arrivals resembling tremor observations in nature. Despite the seismogenic behavior, the ruptures last several weeks, producing a slow transient detectable geodetically. Within the assumptions of the model, the moment-duration scaling of slow and fast ruptures is different. The model simultaneously explains the location and underlying mechanics of collocated tremor and slow-slip events because velocity neutral is necessarily found at the bottom of the seismogenic zone due to the hydrothermal control on frictional stability. The transitional friction properties from unstable to stable slip behavior is essential to generate concurrent slow and fast ruptures.