Seismological Stress Drops for Confined Ruptures Are Invariant to Normal Stress

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.

Automated summary

Seismic moment and rupture length can be used together to estimate stress drop, an important parameter for evaluating earthquakes. Surprisingly, stress drops in natural earthquakes are largely independent of depth, despite the expected correlation between frictional stress and normal stresses. By studying a transparent experimental fault, we found that observed stress drops are independent of normal stress magnitude and heterogeneity, similar to what is seen in natural earthquakes. However, we observed larger, normal stress-dependent stress drops when the fault area is reduced, allowing slip events to reach the edge of the fault interface more frequently. This suggests that the depth-independence of stress drops in tectonic earthquakes may be a result of their confined nature.