Rupture Complexities of Slow Slip Events Controlled by Fault Friction Mechanics

Slow slip events (SSEs) are usually observed in elongated transitional zones between the seismogenic and creeping regions of the subduction zones, with the potentials to trigger large subduction earthquakes. Geodetic observations of SSEs in the Cascadia subduction zone (Michel et al., 2019) reveal contrasting complexities of rupture segmentations in the northern and southern segments separated by 44°N, with the northern segment preferring longer ruptures and the southern part preferring shorter ruptures. However, it remains unclear what mechanisms control the observed contrasting rupture segmentations of SSEs. Additionally, understanding the mechanisms behind the rupture complexities of SSEs can provide physical insights into the processes governing characteristic or complex earthquakes. Here, we conduct numerical simulations of SSE cycles along an elongated fault with a finite width W , which is governed by the rate-and-state friction with velocity-strengthening. We find that the rupture complexities of SSEs on a fault – classified as characteristic ruptures, complex ruptures, or creeping – depend on two non-dimensional ratios  L_nuc/W and L_c/W, where L_nuc is the critical nucleation length and L_c is the critical cohesive zone length. When L_nuc/W  is larger than 0.5, the fault keeps creeping and cannot produce any SSEs, which is consistent with previous theoretical predictions of 0.5 to 1. In addition, we find that runaway characteristic ruptures are enabled if the fault satisfies the energy balance condition between the energy release rate G₀ and the fracture energy G_c,, G₀ = G_c, derived from the three-dimensional theory of dynamic fracture mechanics that accounts for finite rupture width (Weng and Ampuero, 2022). If G₀ < G_c, ruptures prefer to arrest in a short distance and form complex events. This work proposes that a wide spectrum from creeping to characteristic ruptures is controlled by two length ratios in the framework of fracture mechanics, providing new physical insights into the mechanisms of SSEs.

References:

Michel, S., Gualandi, A., & Avouac, J.-P. (2019). Similar scaling laws for earthquakes and Cascadia slow-slip events. Nature, 574(7779), 522–526. https://doi.org/10.1038/s41586-019-1673-6

Weng, H., & Ampuero, J.-P. (2022). Integrated rupture mechanics for slow slip events and earthquakes. Nature Communications, 13(1). https://doi.org/10.1038/s41467-022-34927-w