Exploring fault preparation and earthquake nucleation from the laboratory

The initiation of unstable fault slip leading to earthquakes involves intricate physical processes and interactions. Understanding these mechanisms is crucial for advancing our knowledge in earthquake seismology. Investigations at both field and laboratory scales have highlighted the existence of spatio-temporal variations in seismic or aseismic observations near the epicenter of a major seismic event, such as a rise in frequency of precursory earthquakes (Kato and Ben-Zion, 2021)  or even strong fluctuations in seismic velocities (e.g., Campillo & Paul, 2003). These variations are often associated with the preparatory phase of major earthquakes believed to involve processes resulting from progressive localization of deformation around the eventual rupture zone that eventually accelerates leading up to failure. However, the time and spatial scales of this behavior are not well understood due to our lack of understanding into the physical mechanisms within the preparatory zones. In this study, we combined innovative laboratory techniques and numerical modelling to investigate (a)seismic preparatory deformation during a triaxial failure test in the laboratory. Employing distributed strain sensing (DSS) with optical fibers, we closely monitored strain rates on the sample surface. This was supplemented by active ultrasonic surveys and passive acoustic emission (AE) monitoring to investigate changes in P-wave velocity and locate regions prone to AEs within the sample. Using a physics-based computational model, we investigated strain localization within the sample by monitoring rock regions exhibiting high dissipation of mechanical energy. Highly dissipative regions spatio-temporally correlated with the observed AE locations and with sample regions experiencing P-wave velocity reduction. By further tracking the dissipation field within the sample, we recognized a system of conjugate bands that first emerged and quickly merged into a single band growing from the center towards the sample surface. The latter was interpreted to be related to the preparation of a weak plane. Shortly prior to failure, the model showed an acceleration of deformation that was also observed during the laboratory test with the DSS measurements and correlated with an increase of the seismicity rate in a similar volume of the sample. The combination of increased deformation and seismic rates mimics observations of precursory seismicity in nature. By methodically segregating the laboratory experiments from the numerical modeling, this study provides a comprehensive analysis of the physical processes underlying earthquake nucleation. The integration of cutting-edge laboratory techniques with advanced numerical modeling offers a novel perspective on the (a)seismic preparatory deformation that sets the stage for major seismic events.   References: Campillo M., Paul A. (2003) Science 299, 547-549. Kato, A., Ben-Zion, Y. (2021) Nat Rev Earth Environ 2, 26–39.