Hydrothermal response to a volcano‐tectonic earthquake swarm, Lassen, California

The increasing capability of seismic, geodetic, and hydrothermal observation networks allows recognition of volcanic unrest that could previously have gone undetected, creating an imperative to diagnose and interpret unrest episodes. A November 2014 earthquake swarm near Lassen Volcanic National Park, California, which included the largest earthquake in the area in more than 60 years, was accompanied by a rarely observed outburst of hydrothermal fluids. Although the earthquake swarm likely reflects upward migration of endogenous H2O‐CO2fluids in the source region, there is no evidence that such fluids emerged at the surface. Instead, shaking from the modest sized (moment magnitude 3.85) but proximal earthquake caused near‐vent permeability increases that triggered increased outflow of hydrothermal fluids already present and equilibrated in a local hydrothermal aquifer. Long‐term, multiparametric monitoring at Lassen and other well‐instrumented volcanoes enhances interpretation of unrest and can provide a basis for detailed physical modeling. Recent volcanic unrest at Lassen included an earthquake swarm and a hydrothermal outburstThe hydrothermal response owes to a local permeability change, rather than endogenous fluidsAnalyzing such events will help us better determine when unrest signals impending eruption