Spatio-Temporal Organization of Earthquakes: Insights from Aseismic Transients and Seismic Triggering in Rock Fracture

Recent observations of many large earthquakes suggest a pronounced interaction of seismic sequences and aseismic slip (Kato & Ben-Zion, 2020). Among them, the spatio-temporally clustered seismic sequences may be related to internal stress transfer through event-event triggering processes (Davidsen et al., 2021). These evolving stress correlations at different length scales may be a key component to earthquake nucleation. However, the coupling between aseismic deformation and seismic triggering remains poorly understood due to observational limitations. In this study, we performed a triaxial experiment at 50 MPa confining pressure on a Rotondo granite sample. The sample was pre-notched to induce localized stress concentrations. The deployment of distributed strain sensing (DSS), based on fiber-optic technology, captures the transient behavior of aseismic deformation leading to system-size failure. We show that, at peak stress (σ_p = 420 MPa), the strain first builds up and accelerates near the edges of notches. During a subsequent small stress drop (Δσ_d = 1.5 MPa), a sharp contrast between positive and negative strain rates appears. This asymmetric deformation reflects the local stress redistribution associated with the development of shear cracks emanating from the notches. Following this, a transient recovery is observed, as the concentrated strain rate transfers to the surrounding regions. This indicates that the shear crack is growing away from the notches towards the middle of sample. Acoustic emissions (AEs) recorded throughout the failure sequence were analyzed in conjunction with local strain rates, strain gradients, and the speed at which the strain concentrations propagate. This study provides novel insights into the spatio-temporal evolution of shear cracks, revealing the complex interplay between strain localization, aseismic transients, foreshock activity and stress redistribution during the preparatory phase preceding dynamic failure.

 

References:

Davidsen, J., Goebel, T., Kwiatek, G., Stanchits, S., Baró, J., & Dresen, G. (2021). What Controls the Presence and Characteristics of Aftershocks in Rock Fracture in the Lab? Journal of Geophysical Research: Solid Earth, 126(10), e2021JB022539. https://doi.org/10.1029/2021JB022539

Kato, A., & Ben-Zion, Y. (2020). The generation of large earthquakes. Nature Reviews Earth & Environment, 2(1), 26–39. https://doi.org/10.1038/s43017-020-00108-w