Hydrogeologic Monitoring of High-Risk Volcanoes in the Cascade Range, Northwestern United States

Heat flow, multiphase fluid flow, seismicity, and deformation are tightly coupled in volcanically active areas.  Changes in hydrothermal systems can signal intrusive events and otherwise reveal some of the physical processes surrounding volcanic unrest.  Thus the U.S. Geological Survey has recently implemented a hydrogeochemical monitoring network for the Cascade Range, focusing on the 12 highest-risk Cascade volcanoes.  Mass flux, energy flux, and/or temperature are measured continuously at ~30 sites from Mount Lassen in northern California to Mount Baker in northern Washington.  The objectives of this effort are to (1) develop meaningful hydrothermal-monitoring methods and (2) generate a synchronous, multiyear baseline data set for the most obvious hydrogeochemical monitoring targets in the U.S. portion of the Cascade Range.  The approach is necessarily experimental, because continuous long-term, high-resolution time series focused on hydrothermal areas are rare.  Conclusive interpretation of most preexisting data is limited by data gaps and inadequate sampling frequency.  Preliminary analysis of selected records from our Cascade Range study suggests sensitivity to Earth tides and to both large remote earthquakes (e.g. 2011 Tohoku earthquake) and local seismicity (e.g. a September 2011 <M1.4 swarm near Devils Kitchen, Lassen Volcanic National Park).  The hydrothermal signals associated with small, local earthquake swarms can be ephemeral (lasting perhaps a few hours), such that they may only be captured by relatively high-resolution sampling.