SEISMO-ACOUSTIC ARRAY OBSERVATIONS OF SHALLOW CONDUIT PROCESSES AT FUEGO VOLCANO, GUATEMALA

We deployed small antennas of six broadband seismic and five acoustic sensors 900 m north of the active vent of Fuego volcano during January 2008 to investigate the source of explosions and background tremor. The L-shaped seismic array had stations spaced 30 m apart with one axis parallel to the ridge that runs north from the summit and the other axis down to the west for a total aperture of 150 m. The infrasound sensors were deployed in a similar array, but with an average station spacing of 50 m. There was no lava effusion during the four-day deployment, but explosions were clearly recorded with the seismic and acoustic arrays approximately once per hour with varied amounts of ash, and with durations from ~20-150 s. In addition to the explosions, our seismic array recorded constant volcanic tremor at 1.9 Hz and various discrete events that were not generally detected by the acoustic array. The dominant class of such events, which repeated approximately 10-15 times per hour, had an impulsive onset with first motion toward the vent, a short duration of <5 s, dominant frequencies from 1-3 Hz, and no infrasound component. All of the seismic signals are predominately surface waves radiating from the direction of the vent. Apparent velocities from overlapping 1 or 2 s windows of explosions begin at 1-2 km/s and decrease with time to about 500 m/s until the arrival of the ground-coupled airwave. Events with no apparent infrasound also have low apparent velocities of 0.5 - 2 km/s, suggesting they are occurring at shallow depths; a weak P-wave is typically observed about 200 ms before the shear- and surface-wave train. We also recorded some explosions that have very little seismic signal until the arrival of the ground-coupled airwave. Source inversion is not possible due to the limited array geometry, but we use forward modeling of candidate source geometries to infer differences between the sources of the dominant seismic signals. Constraints from visual, thermal and degassing data are integrated with the seismic data to infer characteristics of the shallow magmatic system.