Imaging quasi-static slip during the nucleation of laboratory earthquakes

Pierre Dublanchet¹, F.X. Passelègue², H. Chauris¹, A. Gesret¹, C. Twardzik², C. Noël²

1: Mines Paris, PSL University, Centre de Geosciences, Fontainebleau
2: Université Côte d’Azur, CNRS, Observatoire de la Côte d’Azur, IRD, Géoazur, Valbonne

Abstract:

Earthquakes manifest as the sudden reactivation of shear slip on critical faults in the upper earth crust. Geophysical monitoring reveals that the initiation (or nucleation) of earthquakes on faults, involves a transient acceleration of quasi static to dynamic slip, associated with a significant increase in the production of small to moderate earthquakes (foreshocks). This preparatory phase, observed during hours to months before the main shock has motivated many observational, experimental and theroretical studies aiming at better characterizing the spatio temporal evolution of precursory fault slip, in the perspective of improving seismic hazard assessment. Many aspects of the physical control on earthquake nucleation nevertheless remain poorly understood.

Here we present an attempt to image quasi-static fault slip during the nucleation of stick slip events within centimetric scale saw-cut granite samples loaded in a triaxial cell. Experiments were performed at ESEILA (Experimental SEIsmology LAboratory, Géoazur, Nice). This controlled environment allows to reproduce a range of stress conditions prevailing on seismogenic faults. Our approach relies on a Bayesian inversion scheme allowing to retrieve the spatio-temporal evolution of slip from the measurements of strain gauges distributed around the experimental fault, as well as the related uncertainties. For that we use Green’s functions computed with finite elements in order to account for the real geometry and loading conditions of the sample. We show that nucleation of stick slip events involves the growth of quasi-static slip patches expanding at a speed of the order of 200 m/day, and accumulating a few microns of slip before the onset of dynamic phase. We explore the effect of confining stress, static friction, and loading rate on the dynamics of these slip events, providing new insights into the physical control on earthquake nucleation. Overall, our results are constant with observations on natural faults, and extend at higher confining stresses the conclusions of previous experiments involving PMMA under direct shear conditions.

Short bio:

Pr. Pierre Dublanchet completed his bachelor and master studies at École Normale Supérieure Paris (ENS Paris) and the Institut de Physique du Globe de Paris (IPGP) in 2009. He earned a PhD in Seismology from the Seismology Laboratory at IPGP in 2013. Following this, he conducted postdoctoral research at ETH Zurich, working with the Swiss Seismological Service from 2013 to 2015. Since 2015, he has been a research scientist at the Geosciences Center of Mines Paris.