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Dynamic Tensile Cracking During Earthquake Ruptures

PhD student Ashley Griffith, working with Professor David Pollard and colleagues, uses optical experiments and high-speed photography to interpret the origins of tensile, off-fault fractures which are often observed to form in damage zones of exhumed geological faults. This study provides a new quantitative link between the geometry of such fractures and earthquake dynamics thereby proposing diagnostic criteria for interpreting velocity, directivity, and static pre-stress state associated with past earthquake rupture events. Subsonic shear ruptures in an analogue material Homalite-100 produce off-fault damage consisting of an array of tensile cracks, providing a glimpse into near-source mechanics of natural earthquakes. These microcracks are different than commonly observed wing cracks concentrated at the ends of shear discontinuities and classical micro-branches which form ahead of dynamically growing tensile cracks. They are observed to nucleate and grow behind the tips of shear ruptures propagating dynamically along interfaces (faults) with frictional and cohesive strength, simulating a “strong” geological fault. The tensile micro-ruptures are produced only along one side of the interface where they are associated with the highly transient, fault-parallel, tensile stress perturbation associated with the cohesive end zone behind the growing shear rupture tip in an otherwise compressive far-field environment. Results of this study represent an important potential bridge between geological observations of structures preserved along exhumed faults and theoretical models of earthquake propagation.