Abstract
Non-volcanic tremor (NVT) is weak rumbling of ground associated with slow slip on fault plane. It is a recently discovered seismic signal with the potential to answer many puzzling questions about faults, and may have important implications on seismic hazard analysis. However, its driving mechanism, and the details of spatiotemporal distribution remain poorly understood, mainly due to the inherent difficulty in detecting and locating NVT. I developed a novel beam-backprojection (BBP) technique to detect and locate tremor. This technique gives higher detectibility, and unprecedented resolution in relative tremor location, compared to the conventional methods. The BBP method allows tracking NVT minute-by-minute during a slow slip event, and reveals fine-scale features in tremor migration.

I find that spatiotemporal distribution of tremor varies remarkably over many time scales. Over the short time scale of several minutes, tremor shows rapid, continuous, slip-parallel migration with a velocity of ~50 km/hr. Over the time scale of several hours, slip-parallel tremor bands sweep Cascadia along-strike from south to north with a velocity of ~10 km/day. Finally, over the time scale of several days, tremor activity fills distinct moment patches that coincide with geodetic slip patch on the plate interface. I integrate these varied and enigmatic observations over different time scales, strive to explain their relationship, explore possible physical models, and present a unified picture of tremor distribution in space and time. It will help better understand the mechanics of the transition zone where tremor occurs, and the possible role this zone plays to govern subduction dynamics.