Nucleation processes and foreshocks of slow and fast laboratory earthquakes illuminated with waveform similarity measurements

Earthquake nucleation is a key problem in earthquake science with rich implications for earthquake early warning systems, earthquake forecasting, and understanding foreshock properties. Foreshocks are often viewed as a byproduct of the nucleation phase, and thus, their properties could lend insights into the impending mainshock. Hence, understanding properties of foreshocks and the factors that control their spatiotemporal behavior is key for determining how earthquakes get started. We report on a suite of well-controlled laboratory friction experiments instrumented with an array of acoustic emission (AE) transducers. We systematically modulated the stiffness of the loading apparatus in tandem with the fault zone normal stress to produce a spectrum of slow and fast slip behaviors. AE data were measured in parallel with fault zone properties, allowing for a more robust understanding of the casual processes driving foreshock activity. We measured waveform similarity using AE templates and tracked their spatiotemporal evolution throughout the seismic cycle using differential travel-times. We demonstrate that AE templates with high-similarity (cross-correlation coefficients >= 0.70) are co-located to within <11 mm. We interpret these events as repeaters that have similar source mechanisms and rupture a single fault patch. Fast laboratory earthquakes are preceded by a late and rapid increase in fault slip rate, which in turn, broadcasts repeating AEs that coalesce in space and time. In contrast, slow laboratory earthquakes lack an abrupt nucleation phase because the fault continues to creep throughout the entire seismic cycle. As a result, slow instabilities show a modest increase in waveform similarity without any robust signs of foreshock coalescence prior to failure. Our work suggests that laboratory foreshocks with high-similarity (i.e. repeaters) are fingerprints of pre-seismic fault slip and could serve as a useful tool for tracking precursory processes along tectonic fault zones.