Earthquake nucleation and slip behavior altered by stochastic normal stress heterogeneity

Abstract

Recent laboratory experiments exhibit varying nucleation locations with changing nucleation lengths. This could be caused by spatial variations in effective normal stress, due to its controlling influence on fault strength and fracture energy. We use an earthquake sequence model with stochastically variable, spatially heterogeneous normal stress, analogous to a 2-m scale laboratory experiment, to quantitatively understand its impact on earthquake nucleation and the following earthquake sequences. We identify five regimes of earthquake nucleation and slip behaviors, controlled by the ratio of the heterogeneity wavelength (λ) over the nucleation length (Lc). When λ is much smaller than Lc, full ruptures are observed. Magnitudes and recurrence intervals are similar to those on homogeneous faults with comparable averaged normal stress. When λ is much larger than Lc, slow slip events and partial ruptures occur frequently and the nucleation length of each earthquake depends on the local stress level. We find that the nucleation location becomes migratory when λ and Lc are of the same magnitude. When λ is larger than Lc, earthquakes nucleate in low normal stress regions (LSRs), and high normal stress regions (HSRs) function as barriers. However, HSRs and LSRs exchange their roles when λ is smaller than Lc. Interestingly, we observe that nucleation locations migrate from an LSR to an HSR in one earthquake, when λ is between the minimum and maximum nucleation length on a heterogeneous fault. This means that nucleation can change its preferred location from weak to strong regions. In this case, we observe a large amount of aseismic slip and associated stress drop in the initial LSR, which might be linked to the migration of foreshocks as documented in natural and laboratory observations. We summarize this in a conceptual model, where these five regimes are linked to the wavelength of the heterogeneity. This improved understanding of earthquake nucleation is important in estimating the seismic potential of natural faults, if the local stress field is well understood, but becomes especially relevant in induced seismicity settings where the effective normal stress is affected by human activities.