(Guest post by Taka’aki Taira from Earth, Planets and Space)
The Lassen Volcanic Center (LVC) is one of the southernmost active volcanoes in the Cascades Volcanic Arc. The U.S. Geological Survey designates the LVC as “very high threat volcano.” The LVC has an extensive hydrothermal system characterized by a shallow vapor-dominated reservoir (~240°C hot waters). This hydrothermal system may pose potential hazards to the Lassen region including emissions of gases and hydrothermal explosions.
Taira and Brenguier (2016) apply seismic interferometry, in which the cross-correlation of ambient noise reconstructs seismic response (i.e., seismic velocity structure), to continuous seismic recordings for investigating the temporal behavior of the Lassen hydrothermal system. They find the temporal variability of seismic velocity in response to stress changes imparted by an earthquake and by seasonal environmental changes (Figure 1).
A sudden velocity drop (about 0.1%) was observed in late May 2013 (Figure 1a). Taira and Brenguier (2016) argue that this velocity change was induced by dynamic stress changes from a magnitude (M) 5.7 local earthquake. They hypothesize that the velocity reduction is associated with opening of cracks in the hydrothermal system of the LVC due to stress changes triggered by the M 5.7 earthquake. They also find that the recovery of the velocity reduction is slow, compared with those identified from previous studies at other faulting areas. They suggest that a large volume of steam-heated water from the LVC hydrothermal system surrounding the cracks might sustain opening of cracks over several months, leading to this slow recovery of seismic velocity.
By combining geodetic measurements, Taira and Brenguier (2016) also reveal that the long-term seismic velocity fluctuation closely tracks snow-induced vertical deformation without time delay (Figure 1b). They suggest that this observation is most consistent with a hydrologic load model (either elastic or poroelastic response). Their working hypothesis is that an increased surface loading instantaneously induces hydrothermal-fluid pressure diffusion serving to open cracks and subsequently reduce seismic velocity.
Taira and Brenguier (2016) demonstrate that seismic interferometry is an effective seismological tool for time-lapse monitoring of stress and pore-pressure transients at volcanic areas. They have been continuously monitoring the seismic velocity at the LVC. An updated time series of seismic velocity change can be found at http://earthquakes.berkeley.edu/~taira/LP/LP_Noise.html.