ABSTRACT

Earthquakes occur to release the compressional forces (stress) that accumulate in the Earth due to plate tectonics. Such accumulation and release of stress is oftentimes studied using models that treat rocks as spring-like elastic materials. However, real rocks - especially those surrounding fault zones - contain many defects that cause non-elastic (permanent) deformation. Therefore, movement between tectonic plates may not only be accommodated by slip along faults, but also by deformation distributed in the surrounding rocks. Ignoring such effects may lead to overestimating stress accumulation in the Earth. Accounting for rock deformation around faults is thus crucial to accurately assess seismic hazards. Here, the team aims to characterize such rock behavior through laboratory experiments. They use rock specimens with existing defects; specimens are either collected around natural faults or prepared in the laboratory. They quantify the rock behavior and assess its impact on earthquake mechanics through numerical modeling. The project provides support for an early-career scientist and graduate and undergraduate students at University of Wisconsin - Madison. It also fosters knowledge transfer and community building among experimentalists experts in rock mechanics.

The goal of the project is to evaluate the impact of fault damage-zone rheology on fault mechanics. Field and laboratory evidence shows that fault damage-zone rocks exhibit time-dependent deformational behavior and that shear stress relaxation within the damage zone is a ubiquitous phenomenon. Viscous properties of fault damage-zone rocks are not acknowledged in current fault mechanical models. This may lead to an over-estimation of fault shear stress accumulation. Here, the researchers characterize the damage distribution around selected fault zone outcrops. The emphasis is on quantifying the heterogeneities across and along the faults. Laboratory experiments are conducted on both natural and synthetic damage-zone rocks to constrain the viscous constitutive properties of damaged rocks in the framework of viscoplasticity. A synthesis modeling incorporates findings from the field and lab. The goal is to evaluate how slow distributed deformation in rough damage zones influence the interseismic loading of fault shear stress. The team also examines how the distribution of stress and fault slip evolve during the interseismic period; this provides insights on how earthquake statistics and kinematics evolve during interseismic loading. Another goal is to evaluate the validity of upscaling friction laws to field scale problems. Finally, geophysical observations on transient changes in seismic velocity and strain from the San Jacinto Fault Zone will be examined to explore consistency with the viscous properties constrained in the laboratory.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

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