Seismological Stress Drops for Confined Ruptures Are Invariant to Normal Stress

Abstract

Seismic moment and rupture length can be combined to infer stress drop, a key parameter for assessing earthquakes. In natural earthquakes, stress drops are largely depth‐independent, which is surprising given the expected dependence of frictional stress on normal stresses and hence overburden. We have developed a transparent experimental fault that allows direct observation of thousands of slip events, with ruptures that are fully contained within the fault. Surprisingly, the observed stress drops are largely independent of both the magnitude of normal stress and its heterogeneity, capturing the independence seen in nature. However, we observe larger, normal stress‐dependent stress drops when the fault area is reduced, which allows slip events to frequently reach the edge of the interface. We conclude that confined ruptures have normal stress independent stress drops, and thus the depth‐independent stress drops of tectonic earthquakes may be a consequence of their confined nature.

Laboratory Earthquake Rupture Interactions With a High Normal Stress Bump

“…Steinhardt et al. (2023) found that seismological stress drop was independent of normal stress for fully confined slip events, and Schaal and Lapusta (2019) similarly found that high normal stress bumps did not produce abnormally high seismological stress drops when they ruptured. This is likely because the locally higher stress drop on a bump will be offset by further rupture propagation into unfavorably stressed regions, resulting in a rupture‐averaged stress drop that is more modest.…”

“…Steinhardt et al. (2023) investigated the effect of a normal stress asperity on the scale independence of stress drop. Buijze et al.…”

“…showed that direct fluid injection onto the fault could also promote contained rupture events. Steinhardt et al (2023) investigated the effect of a normal stress asperity on the scale independence of stress drop. Buijze et al (2020Buijze et al ( , 2021 observed normal stress heterogeneities from differential gouge compaction and highlighted nucleation complexity caused by normal stress variations and the importance of fault stress state.…”

Frictional instabilities in clay illuminate the origin of slow earthquakes

“…To explore the dynamics of frictional instabilities, we designed a method to directly observe sample deformation and fault slip, similarly to experiments performed in analog materials ( 33 – 39 ). Our experiment was performed using the same protocol and boundary conditions of experiments reported in Fig.…”

“…2B and movies S1 to S18). We use the local evolution of the gray level as a proxy to monitor changes in the state of stress associated with rupture propagation, in a way similar to previous works that documented rupture propagation using strain gauges, digital image correlation, or local fault slip measurements ( 37 – 39 ). Each slip event corresponds to a systematic variation of the overall surface reflectivity ( Fig.…”