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

Abstract

Recent Global Positioning System observations of major earthquakes such as the 2014 Chile megathrust show a slow preslip phase releasing a significant portion of the total moment (Ruiz et al., 2014, https://doi.org/10.1126/science.1256074). Despite advances from theoretical stability analysis (Rubin & Ampuero, 2005, https://doi.org/10.1029/2005JB003686; Ruina, 1983, https://doi.org/10.1029/jb088ib12p10359) and modeling (Kaneko et al., 2017, https://doi.org/10.1002/2016GL071569), 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‐cm long slip interface. The results of our laboratory experiments are in agreement with the preslip model outlined by Ellsworth and Beroza (1995, https://doi.org/10.1126/science.268.5212.851) and observed in laboratory experiments (Latour et al., 2013, https://doi.org/10.1002/grl.50974; Nielsen et al., 2010, https://doi.org/10.1111/j.1365-246x.2009.04444.x; Ohnaka & Kuwahara, 1990, https://doi.org/10.1016/0040-1951(90)90138-X), 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 to 6 MPa/s, the nucleation length can shrink by a factor of 3, 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, https://doi.org/10.1029/2005JB003686) for rate‐and‐state friction laws.

Contained Laboratory Earthquakes Ranging From Slow to Fast

“…From equation (4), we would expect h * to remain relatively constant in a given sequence as long as the normal stress, rigidity of the fault rocks, and friction properties remain constant, but recent experiments have shown that h * can vary based on loading rate, healing time, and other factors (Guérin‐Marthe et al, ; McLaskey & Yamashita, ). In particular, h * has been shown to shrink in response to “kicks” such as after sudden increases in loading rate or upon resumption of loading after holding periods where the fault is held in essentially stationary contact.…”

Earthquake Initiation From Laboratory Observations and Implications for Foreshocks

“…The USGS 2-m block is likely somewhat rougher than others, while Onaka and Kuwahara, (1990) is smoother. Variability due to loading rate effects is shown inKato et al (1992) andGuérin-Marthe et al (2019).…”

Numerical and experimental simulation of fault reactivation and earthquake rupture applied to induced seismicity in the Groningen gas field

“…Similarly, normal stress concentrations in the direct shear experiments form at one or both ends, as observed from strain gauge measurements (Bayart et al, 2016;Ben-David et al, 2010), pronounced fault wear (Mclaskey & Yamashita, 2017), pressure sensitive films (Yamashita et al, 2018), and modeling (Kammer et al, 2015). The distribution of the shear and normal stresses along the experimental faults is furthermore affected by other geometrical factors and boundary conditions, such as the sizes of the PMMA blocks with respect to each other, whether shear loading is applied uniformly or at a point (Bayart et al, 2016), the location at which the shear load is applied (Ben-David et al, 2010), deformation of the frame containing the experimental setup (Ke et al, 2018) and whether motion is allowed between hydraulic presses and the forcing blocks (Guérin-Marthe et al, 2019;Langer et al, 2013). Depending on the relative magnitude of the shear and normal stresses, maxima in τ/σn will form at some distance from the fault end(s) (e.g.…”

“…When the nucleation length is close to the laboratory fault length, the mode of fault sliding (seismic vs. slow or aseismic slip) can be controlled by changing the loading conditions such as the normal stress (Mclaskey & Yamashita, 2017). Both experimental (Guérin-Marthe et al, 2019;Kato et al, 1992;Xu et al, 2018) and numerical (Kaneko et al, 2016, Kaneko & Lapusta, 2008 studies further show that the nucleation length decreases with increasing loading rate, whereas it increases with fault roughness (Ohnaka & Shen, 1999;Yamashita et al, 2018).…”

“…Ohnaka & Kuwahara, 1990;Okubo & Dieterich, 1981) or granodiorite (Ma et al, 2002), or on polymers (e.g. Guérin-Marthe et al, 2019;Latour et al, 2013). In nature, fault zones are usually filled with fault gouge or cataclastic rocks produced by wear of the formations flanking the fault.…”