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 L b and L∞ derived by Rubin and Ampuero (2005, https://doi.org/10.1029/2005JB003686) for rate‐and‐state friction laws.
Highlights
The precursory phase of earthquakes and, more generally, the different phases of the seismic cycle remain in large part poorly understood
We conducted a total of 27 individual experiments, at imposed loading rates ranging from 0.01 to 6 MPa/s (Figure 3a), with normal stresses maintained around 4.7 ± 0.8 MPa (Figure 3b)
Stress (≈ 5MPa) while the shear stress is increased at an arbitrary rate from a minimum of 0.01 up to 6 MPa/s
Summary
The precursory phase of earthquakes and, more generally, the different phases of the seismic cycle remain in large part poorly understood. Some promising advances have been made in the past decades thanks to fault observations, theoretical and numerical models, and small-scale laboratory experiments. It is well-known that some faults are able to release a significant portion of the strain energy accumulated during the tectonic loading phase by slow, aseismic creep (Kanamori, 1977; Scholz et al, 1969). Tape et al (2018) showed that a M3.7 earthquake in Alaska initiated with the acceleration of a rupture front 22 s before the main shock This last observation concerns a small earthquake rupture and provides insight in the rupture process at an intermediate scale between laboratory experiments and great earthquakes
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