The earthquake arrest zone

Abstract

Summary Earthquake ruptures are generally considered to be cracks that propagate as fracture or frictional slip on preexisting faults. Crack models have been used to describe the spatial distribution of fault offset and the associated static stress changes along a fault, and have implications for friction evolution and the underlying physics of rupture processes. However, field measurements that could help refine idealized crack models are rare. Here we describe large-scale laboratory earthquake experiments, where all rupture processes were contained within a 3-m long saw-cut granite fault, and we propose an analytical crack model that fits our measurements. Similar to natural earthquakes, laboratory measurements show coseismic slip that gradually tapers near the rupture tips. Measured stress changes show roughly constant stress drop in the center of the ruptured region, a maximum stress increase near the rupture tips, and a smooth transition in between, in a region we describe as the earthquake arrest zone. The proposed model generalizes the widely used elliptical crack model by adding gradually tapered slip at the ends of the rupture. Different from the cohesive zone described by fracture mechanics, we propose that the transition in stress changes and the corresponding linear taper observed in the earthquake arrest zone are the result of rupture termination conditions primarily controlled by the initial stress distribution. It is the heterogeneous initial stress distribution that controls the arrest of laboratory earthquakes, and the features of static stress changes. We also performed dynamic rupture simulations that confirm how arrest conditions can affect slip taper and static stress changes. If applicable to larger natural earthquakes, this distinction between an earthquake arrest zone (that depends on stress conditions) and a cohesive zone (that depends primarily on strength evolution) has important implications for how seismic observations of earthquake fracture energy should be interpreted.

Earthquake breakdown energy scaling despite constant fracture energy

“…2e), where scaling case A and B bound closely a scaleinvariant behavior, consistent with seismologically inferred measurements [28][29][30][31] . With the self-similar initial stress distribution of scaling case A, the arrest zone width 17 w az = R − a is a nearly constant percentage of the average rupture radius R (Fig. 2d).…”

“…A running earthquake arrests either because the rupture front enters a region of the fault that is stronger or has a more stable rheology compared to the nucleation region, i.e., a barrier 9,10,15,16 , or the front enters a region with low initial stress and hence subcritical driving force 17 . While the first scenario implies a larger fault fracture energy for a larger earthquake rupture, the second scenario does not.…”

Subduction earthquakes controlled by incoming plate geometry: The 2020 M>7.5 Shumagin, Alaska, earthquake doublet

“…The preferred shape of the crack model, an ellipse, is supported by mechanical considerations (Sendeckyj, 1970). Ke et al (2020) proposed an analytical model of the slip profile from the centre of the crack to the rupture tip, and we expand this one-dimensional model into a two-dimensional model with an elliptical shape, by assuming one of the focal points of the ellipse to be the crack centre and the elliptical perimeter to be the crack tip.…”

“…Traditional kinematic fault slip inversion method used static observations to solve for the slip displacements but neglected to consider the driving forces or stresses that cause these motions. Recently, a laboratory-derived crack model was introduced to describe the relationship between stress and slip on the fault (Ke et al, 2020).…”

Earthquake Breakdown Energy Scaling Despite Constant Fracture Energy

“…A running earthquake arrests either because the rupture front enters a region of the fault that is stronger or has a more stable rheology compared to the nucleation region, i.e., a barrier (e.g., ref. 5,6,8,9 ), or the front enters a region with low initial stress and hence subcritical driving force 10 .…”