Foreshocks during the nucleation of stick‐slip instability

Abstract

[1] We report on laboratory experiments which investigate interactions between aseismic slip, stress changes, and seismicity on a critically stressed fault during the nucleation of stick-slip instability. We monitor quasi-static and dynamic changes in local shear stress and fault slip with arrays of gages deployed along a simulated strike-slip fault (2 m long and 0.4 m deep) in a saw cut sample of Sierra White granite. With 14 piezoelectric sensors, we simultaneously monitor seismic signals produced during the nucleation phase and subsequent dynamic rupture. We observe localized aseismic fault slip in an approximately meter-sized zone in the center of the fault, while the ends of the fault remain locked. Clusters of high-frequency foreshocks (M w~À 6.5 to À5.0) can occur in this slowly slipping zone 5-50 ms prior to the initiation of dynamic rupture; their occurrence appears to be dependent on the rate at which local shear stress is applied to the fault. The meter-sized nucleation zone is generally consistent with theoretical estimates, but source radii of the foreshocks (2 to 70 mm) are 1 to 2 orders of magnitude smaller than the theoretical minimum length scale over which earthquake nucleation can occur. We propose that frictional stability and the transition between seismic and aseismic slip are modulated by local stressing rate and that fault sections, which would typically slip aseismically, may radiate seismic waves if they are rapidly stressed. Fault behavior of this type may provide physical insight into the mechanics of foreshocks, tremor, repeating earthquake sequences, and a minimum earthquake source dimension.

Variations in precursory slip behavior resulting from frictional heterogeneity

“…Foreshocks were also reported in recent megathrust earthquakes in subduction zones, such as the 2011 Tohoku-Oki earthquake in Japan and the 2014 Iquique earthquake in Chile, in association with slow slip around the eventual hypocenter of the mainshock (Ando and Imanishi 2011;Kato et al 2012;Ito et al 2013;Ruiz et al 2014). Similar observations have been reported in rock fracture experiments, where local seismic sources are located within the aseismically slipping (nucleation) zone before slip occurs on the rock surface (McLaskey and Kilgore 2013). Foreshocks are considered to be driven by the nucleation process of the mainshock and therefore are expected to act as precursors to large earthquakes (Dodge et al 1996;McGuire et al 2005).…”

Preslip and cascade processes initiating laboratory stick slip

“…Previous work on meter-sized laboratory samples at 5 MPa normal stress indicated that the critical nucleation size R C~1 m [Okubo and Dieterich, 1984;Beeler et al, 2012;McLaskey and Kilgore, 2013]. The current experiments were conducted at about 20 times higher stress levels (~100 MPa) so we estimate that R C is about 20 times smaller.…”

Analogue earthquakes and seismic cycles: experimental modelling across timescales

“…It was studied experimentally using fault block models using precut rock (e.g. Dieterich, 1978a;Okubo and Dieterich, 1984;Ohnaka and Shen, 1999;McLaskey and Kilgore, 2013;McLaskey and Glaser, 2011;McLaskey et al, 2012) as well as rock analogues, e.g. polycarbonate (e.g.…”