Large dynamic strains carried by seismic waves are known to trigger seismicity far from their source region. It is unknown, however, whether surface waves trigger only small earthquakes, or whether they can also trigger large, societally significant earthquakes. To address this question, we use a mixing model approach in which total seismicity is decomposed into 2 broad subclasses: "triggered" events initiated or advanced by far-field dynamic strains, and "untriggered" spontaneous events consisting of everything else. The b-value of a mixed data set, bMIX, is decomposed into a weighted sum of b-values of its constituent components, bT and bU. For populations of earthquakes subjected to dynamic strain, the fraction of earthquakes that are likely triggered, fT, is estimated via inter-event time ratios and used to invert for bT. The confidence bounds on bT are estimated by multiple inversions of bootstrap resamplings of bMIX and fT. For Californian seismicity, data are consistent with a single-parameter Gutenberg-Richter hypothesis governing the magnitudes of both triggered and untriggered earthquakes. Triggered earthquakes therefore seem just as likely to be societally significant as any other population of earthquakes.

Granular materials are central to a wide range of earth science processes. The majority of granular flow rheology experiments and theory describes flows composed of smooth spheres, but natural shear flows are composed of particles exhibiting a range of irregular shapes, sizes, and material properties. For natural hazards involving granular media, like landslides and earthquakes, it is important to understand how the complexity of natural grains affects the transition between nearly static slow moving shear flow regimes and dilatational fast moving shear flow regimes. The transition between these end member regimes can take many forms and occur over orders of magnitude of velocity and pressure conditions. Integrating an understanding of the effects of different material properties and grain shape into velocity phase regimes will allow for a complete energy balance to be drawn up for granular shear flow. Such an understanding will result in effective predictions of rheological behavior based on certain material characteristics and boundary conditions and will answer an enduring and fundamental physics question of how energy entering into a granular system is dissipated. I develop a new method for measuring fluctuation energy in granular shear flow experiments with acoustic energy, and I use this method to examine how internally induced vibrations affect the mechanics of transitional regime flows and the dependence of these mechanics on the material properties of the solid grains that make up the flow. I find that grain shape and material capacity for plastic damage can change how energy is dissipated in a shear flow, which in turn affects the overall rheology of the system.