Analysis of seismic and acoustic observations at Arenal Volcano, Costa Rica, 1995-1997

In November 1995, we installed five, three-component broadband seismometers and electronic tiltmeters around the circumference of Arenal Volcano, a yo ung stratovolcano in Costa Rica that exhibits strombolian activity. With th e addition of two continuous-recording GPS receivers deployed in May 1995, these instruments provide continuous monitoring of seismicity and ground de formation at an active volcano over a very wide bandwidth. In addition, dur ing April-May 1997, we deployed a small, linear array of co-located three-c omponent seismometers and broadband microphones. This paper presents a comp rehensive analysis of all the seismic and acoustic data collected thus far. Seismic signals are primarily of two types: (1) long period (1-3 Hz) trans ients associated with summit explosions: and (2) harmonic tremor that conta ins regularly spaced spectral peaks (0.9, 1.8, 2.7, 3.6, 4.5, 5.4, 6.3, and 7.1 Hz) and lasts up to several hours. The explosion signals appear to ori ginate in a small volume that is located at shallow depth beneath the vent and does not migrate with time. No unambiguous long-period seismic signals (T > 5 s)associated with volcanic processes at Arenal have been observed du ring the three-year deployment period. The spectra of summit explosions sho w distinct signatures at each site, suggesting significant path and/or site modification of the waveforms. In contrast, the harmonic tremor signals sh ow no variation in the frequency content at the five sites, nor on the thre e seismic components at each site (Hagerty et al., 1997). This, and the fac t that harmonic tremor is recorded in the acoustic channels as well, demons trates that harmonic tremor is not a seismic propagation effect and that pr essure disturbances propagate within the magma-gas mixture inside of volcan ic conduits. These pressure waves are sensitive to the how velocity and to small changes in the gas content of the magma-gas mixture. Observations and synthetic tests are presented that challenge the notion that harmonic trem or is a superposition of repeated gas explosions at shallow depth. We propo se that equilibrium degassing of the melt dentes a stable, stratified magma column where the void fraction increases with decreasing depth and that di sruption of this equilibrium stratification is responsible for observed var iations in the seismic efficiency of explosions. (C) 2000 Elsevier Science B.V. All rights reserved.