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Speaker: Camilla Cattania, Assistant Professor, Massachusetts Institute of Technology

Host: Demian Saffer

Title: Seismicity and rupture behavior in geometrically complex fault zones

Abstract: Faults are geometrically complex at all scales: from the fractal topography of individual surfaces, to kilometer-scale features such as bends and step-overs, to widespread damage zones surrounding major faults. Although this complexity has long been recognized as an essential aspect controlling earthquake rupture and seismicity patterns, a quantitative understanding of the link between fault geometry and its seismic behavior is still limited.

In this talk I will present recent progress in this area, focusing on a few studies across a range of spatial scales and tectonic settings: 1. Slip complexity mediated by fault roughness during slow slip events (SSEs); 2. The role of subducting seamounts in earthquake nucleation and arrest; 3. Seismicity patterns in the damage zone.

Back-propagating rupture fronts are commonly observed during slow slip events, offering an opportunity to probe the frictional properties of heterogeneous faults. We find that this behavior spontaneously emerges in numerical simulations of SSEs on rough, rate-state faults with a rate-strengthening transition at high slip rates. Back propagating fronts are triggered as a consequence of a “delayed stress drop” near the crack tip, caused by large local fluctuations slip rate as the rupture propagates through the heterogeneous stress field, coupled with rate-strengthening friction. While small scale heterogeneity favors this process, we demonstrate that back-propagating ruptures can also be included by large scale stress heterogeneity induced by far-field loading, offering a possible explanation for “boomerang earthquakes” along transform faults.

Subducted seamounts are an example of large-scale structural heterogeneity, and they have often been associated with earthquake nucleation and arrest. Here we use quasi-dynamic, elastic simulations to investigate how seamounts affect the seismic cycle. Seamount subduction modifies the state of stress on the subduction interface, creating regions of enhanced and reduced normal stress downdip and updip of the seamount. We find that this pattern can facilitate creep and earthquake nucleation, although the enhanced compression can cause ruptures to arrest. We identify different regimes, controlled by the amplitude of stress heterogeneity, and find that they are well explained by fracture mechanics criteria that account for spatial variations in stress drop and fracture energy modulated by normal stress. These results extend previous criteria for the occurrence of partial ruptures and super-cycles on heterogeneous faults, and qualitatively explain the variability in earthquake behavior observed along subduction megathrusts.