Seismic monitoring systems detect and locate tens of thousands of earthquakes globally every year, adding to an ever growing archive of instrumentally recorded seismic data that goes back almost a century. Most of these earthquakes occur along the boundaries of tectonic plates and provide the fundamental data to study the physical processes that drive plate tectonics. My research centers on the development of new high-resolution earthquake location algorithms and their application to vast digital seismic archives to study the nature of active faults, the spatio-temporal evolution of seismicity, and the physical processes underlying seismic failure. I am also developing operational real-time procedures that enable high-resolution monitoring of changes in seismic signals and fault properties over time periods of seconds to decades. This has considerable societal and economic impacts in the immediate evaluation and mitigation of seismic hazards, the potential for forecasting future events, and for effective monitoring of compliance with the Comprehensive Nuclear-Test-Ban Treaty.