Exploring patterns and mechanisms of seismicity in the absence of tectonic loading

Within stable continental interiors such as the Australian, South African or North American cratons, seismicity occurs in the absence of measurable tectonic loading. This seismic activity has surprising characteristics. Relative to plate-boundary seismicity, it is more sensitive to seasonal load variations and it seems to develop aftershock sequences sustained for a much longer duration. Both observations are unexpected evidence that the crust in regions with no active tectonics has still found a way to reach a critical stress state, allowing it to be modulated by small variations of stress and to sustain long, efficient cascades of seismicity. Different mechanisms may be considered to explain how the crust reaches failure in the (supposed) absence of loading, that is either by reducing strength or by increasing stress by other means than tectonics. Among others, we propose (i) a progressive weakening of the crust through a brittle-creep-like mechanism, slowly driving cracks to near-critical conditions, (ii) the slow development of a deviatoric load due to erosive exhumation. Understanding which mechanism may dominate the activity, the activity timescales associated and which observables can be used to constrain them is key to make an assessment of the seismic risk in stable continental interiors.

In this work, we explore patterns of activity in high-resolution catalogs of seismicity in Eastern Australia,  the Northeastern USA and Northwestern France, as well as in acoustic emissions catalogs from brittle-creep of natural rocks in laboratory experiments. Using aftershock and triggering patterns in time and space, we attempt to constrain elements of the stress-to-failure distribution in the crust and how it evolves in time. These observations are then compared to the order-of-magnitude predictions from both (i) brittle-creep and (ii) erosive theories on how the crust fails in the absence of tectonic loading.