Observations of volcanic tremor during the January–February 2005 eruption of Mt. Veniaminof, Alaska

Abstract

Mt. Veniaminof, Alaska Peninsula, is a stratovolcano with a summit ice-filled caldera containing a small intracaldera cone and active vent. From January 2 to February 21, 2005, Mt. Veniaminof erupted. The eruption was characterized by numerous small ash emissions (VEI 0 to 1) and accompanied by low-frequency earthquake activity and volcanic tremor. We have performed spectral analyses of the seismic signals in order to characterize them and to constrain their source. Continuous tremor has durations of minutes to hours with dominant energy in the band 0.5-4.0 Hz, and spectra characterized by narrow peaks either irregularly (non-harmonic tremor) or regularly spaced (harmonic tremor). The spectra of non-harmonic tremor resemble those of low-frequency events recorded simultaneously with surface ash explosions, suggesting that the source mechanisms might be similar or related. We propose that non-harmonic tremor at Mt. Veniaminof results from the coalescence of gas bubbles while low-frequency events are related to the disruption of large gas pockets within the conduit. Harmonic tremor, characterized by regular and quasisinusoidal waveforms, has duration of hours. Spectra containing up to five harmonics suggest the presence of a resonating source volume that vibrates in a longitudinal acoustic mode. An interesting feature of harmonic tremor is that frequency is observed to change over time; spectral lines move towards higher or lower values while the harmonic nature of the spectra is maintained. Factors controlling the variable characteristics of harmonic tremor include changes in acoustic velocity at the source and variations of the effective size of the resonator.

Seismic and infrasound signals at Mt. Etna: Modeling the North-East crater conduit and its relation with the 2008–2009 eruption feeding system

“…Nevertheless, the lack of an obstruction does not imply that it is an acoustically opened termination, as observed by several authors (e.g. Garces and McNutt, 1997;De Angelis and McNutt, 2007). Analyses of NEC infrasonic events , recorded during the entire 2007-2009 period, highlighted the presence of integer harmonics of the fundamental mode.…”

“…Garces and McNutt, 1997;Fee et al, 2010b). Resonance is a propagation path effect inside the conduit and is generated by multiple reflections of pressure disturbances in the fluid against boundaries (Garces and McNutt, 1997;De Angelis and McNutt, 2007). Thus, information on conduit geometry and physical properties of the fluid filling the conduit can be inferred.…”

Seismoacoustic investigations of paroxysmal activity at Mt. Etna volcano: New insights into the 16 November 2006 eruption

“…Such a mechanism of acoustic generation has also been demonstrated with laboratory experiments [e.g., Vidal et al , 2006]. We assumed that NEC infrasound events are generated by longitudinal acoustic resonance, and did not take into account radial modes [e.g., Hagerty et al , 2000; De Angelis and McNutt , 2007]. In fact, at the frequency considered, the wavelengths of the acoustic waves in the fluid are greater than the width of the conduit and consequently the longitudinal modes are dominant [ Garcés and McNutt , 1997].…”

Towards an Automatic Monitoring System of Infrasonic Events at Mt. Etna: Strategies for Source Location and Modeling

“…In fact, if the conduit is an open-open or closed-closed system has nk/2 (k = wavelength, n = 1,2,3,…) waves as longitudinal resonance modes whereas a system with one open and one closed end has (2n -1) k/4 waves. Therefore, open-open or closed-closed systems produce signals with spectra characterized by evenly spaced peaks consisting of the fundamental mode, f 0 , and a set of n integer harmonics which are multiples of f 0 (f 0 , f 1 = 2 f 0 , f 2 = 3 f 0 ,…, f n = n f 0 ); on the other hand spectra that have equally spaced peaks and contain only odd harmonics (f 0 , f 1 = 3f 0 , f 2 = 5f 0 , …, f n = (2n -1) f 0 ) are linked to resonant systems with one open and one closed end (DE ANGELIS and MCNUTT, 2007). Finally, the frequency is also directly related to the wave velocity of the fluid in the conduit (c).…”

“…Considering these prospective fluids filling the conduit, the lower end of the resonating system can consist in the interface between bubbly-magma or gas and nonvesciculated magma. Because of the strong impedance contrast between the source fluid and the underlying non-vesciculated magma, this termination acts like a closed termination (DE ANGELIS and MCNUTT, 2007). The upper end of the conduit can be either open to the atmosphere and act as an open termination, or obstructed by a relatively viscous plug at the vent acting as a closed boundary.…”