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Time-lapse monitoring of seismic velocity at volcanic areas can provide unique insight into the property of hydrothermal and magmatic fluids and their temporal variability. We established a quasi real-time velocity monitoring system by using seismic interferometry with ambient noise to explore the temporal evolution of velocity in the Lassen Volcanic Center, Northern California. Our monitoring system finds temporal variability of seismic velocity in response to stress changes imparted by an earthquake and by seasonal environmental changes. Dynamic stress changes from a magnitude 5.7 local earthquake induced a 0.1 % velocity reduction at a depth of about 1 km. The seismic velocity susceptibility defined as ratio of seismic velocity change to dynamic stress change is estimated to be about 0.006 MPa−1, which suggests the Lassen hydrothermal system is marked by high-pressurized hydrothermal fluid. By combining geodetic measurements, our observation shows that the long-term seismic velocity fluctuation closely tracks snow-induced vertical deformation without time delay, which is most consistent with an hydrological load model (either elastic or poroelastic response) in which surface loading drives hydrothermal fluid diffusion that leads to an increase of opening of cracks and subsequently reductions of seismic velocity. We infer that heated-hydrothermal fluid in a vapor-dominated zone at a depth of 2–4 km range is responsible for the long-term variation in seismic velocityGraphical Abstract(Top panel) Time history of relative velocity change dv/v for stack of 5 days in a frequency range of 0.5–0.9 Hz with two sigma standard deviations in the time intervals January 2012 through June 2016. Dashed red line is the occurrence time of the 24 May 2013 magnitude (M) 5.7 Greenville earthquake. Also shown are the time series of dv/v for stack of 30 days (red lines). (Bottom panel) A long-term velocity change and vertical deformation at the Lassen Volcanic Center. Red line is dv/v for stack of 30 days in a frequency range of 0.3-0.7 Hz. Grey circles are up-down displacements at global positioning system (GPS) site P668 with the one-sigma error bars. Also shown is a 30-day median GPS displacement (green line). Note that the mean and the linear trend were removed from GPS and dv/v data. Dashed red line is the occurrence time of the 24 May 2013 M 5.7 Greenville earthquake.

Response of hydrothermal system to stress transients at Lassen Volcanic Center, California, inferred from seismic interferometry with ambient noise