Earthquake Nucleation Size: Evidence of Loading Rate Dependence in Laboratory Faults

Abstract

Recent GPS observations of major earthquakes such as the 2014 Chile megathrust show a slow pre‐slip phase releasing a significant portion of the total moment (Ruiz et al., 2014). Despite advances from theoretical stability analysis (Rubin & Ampuero, 2005; Ruina, 1983), and modeling (Kaneko, Carpenter, & Nielsen, 2017), it is not fully understood what controls the prevalence and the amount of slip in the nucleation process. Here we present laboratory observations of slow slip preceding dynamic rupture, where we observe a dependence of nucleation size and position on the loading rate (laboratory‐equivalent of tectonic loading rate). The setup is composed of two polycarbonate plates under direct shear with a 30 centimeters long slip interface. The results of our laboratory experiments are in agreement with the pre‐slip model outlined by Ellsworth and Beroza (1995) and observed in laboratory experiments (Latour, Schubnel, Nielsen, Madariaga, & Vinciguerra, 2013; Nielsen, Taddeucci, & Vinciguerra, 2010; Ohnaka & Kuwahara, 1990), which show a slow slip followed by an acceleration up to dynamic rupture velocity. However, further complexity arises from the effect of (1) rate of shear loading and (2) inhomogeneities on the fault surface. In particular, we show that when the loading rate is increased from 10−2 MPa.s−1 to 6 MPa.s−1, the nucleation length can shrink by a factor of three and the rupture nucleates consistently on higher shear stress areas. The nucleation lengths measured fall within the range of the theoretical limits Lb and L∞ derived by Rubin and Ampuero (2005) for rate‐and‐state friction laws.