Creep on seismogenic faults: Insights from analogue earthquake experiments

Tectonic faults display a range of slip behaviours including continuous and episodic slip covering rates of more than 10 orders of magnitude. To gain insight into the slip behaviour of brittle faults, we performed laboratory stick-slip experiments at low pressure using dry “sticky” rice as rock analogue material. Rice has been shown to be a valuable material obeying the rate and state friction laws qualitatively and quantitatively and mimicking the full spectrum of seismogenic fault behaviour (“ricemic cycles”) depending on boundary conditions. The deformation mechanism is granular flow and as such transient hardening and weakening phenomena such as strain localization or stick-slip are accompanied by dilation and compaction, respectively. Such a rheology might be similar in rocks at various scales (grain scale to regional tectonic scale).

We here report on ring shear test experiments on a range of rice varieties, including full-grain and crushed sorts. We imposed boundary conditions (i.e., normal load, shear velocity) scaled down from nature under which our fault analogue shows a variety of slip behaviours ranging from slow and quasi-continuous creep to episodic slow slip to dynamic rupture. The experiments demonstrate that significant interseismic creep (up to far-field loading rate) and earthquakes may not be mutually exclusive phenomena for a given location along a fault. Moreover, creep signals vary systematically with the fault’s seismic potential. Accordingly, the transience of interseismic creep scales with fault strength and seismic coupling as well as with the maturity of the seismic cycle. Loading rate independence of creep signals suggests that the long-term stationary mechanical properties of faults (e.g. seismic coupling) can be inferred from short-term observations (e.g. aftershock sequences). Moreover, we observe the number and size of small episodic slip events to systematically increase towards the end of the seismic cycle providing an observable proxy of the relative shear stress state on seismogenic faults. 

Importantly, very weak faults (with low effective normal loads) in a late stage of their seismic cycle might creep at rates very close to far-field loading for extended periods of the interseismic stage (~decades before failure). Given that we typically observe only a fraction of seismic cycles with high resolution (with geodetic methods) in nature, this might lead to the false belief of the fault being aseismic and not hosting large earthquakes. We thus demonstrate that seismic and aseismic behaviour might not necessarily be mutually exclusive.