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Chouet, B. A.
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- PublicationRestrictedThe source of infrasound associated with long-period events at Mount(2009)
; ; ; ; ; ; ; ;Matoza, R. S.; Laboratory for Atmospheric Acoustics, Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California ;Garcés, M. A.; Infrasound Laboratory, Hawai’i Institute of Geophysics andPlanetology, University of Hawai’i at Manoa ;Chouet, B. A.; U.S. Geological Survey, Menlo Park, California ;D'Auria, L.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Hedlin, M. A. H.; Laboratory for Atmospheric Acoustics, Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California ;De Groot-Hedlin, E.; Laboratory for Atmospheric Acoustics, Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California ;Waite, G. P.; U.S. Geological Survey, Menlo Park, California; ; ; ; ; ; During the early stages of the 2004–2008 Mount St. Helens eruption, the source process that produced a sustained sequence of repetitive long-period (LP) seismic events also produced impulsive broadband infrasonic signals in the atmosphere. To assess whether the signals could be generated simply by seismic-acoustic coupling from the shallow LP events, we perform finite difference simulation of the seismo-acoustic wavefield using a single numerical scheme for the elastic ground and atmosphere. The effects of topography, velocity structure, wind, and source configuration are considered. The simulations show that a shallow source buried in a homogeneous elastic solid produces a complex wave train in the atmosphere consisting of P/SV and Rayleigh wave energy converted locally along the propagation path, and acoustic energy originating from the source epicenter. Although the horizontal acoustic velocity of the latter is consistent with our data, the modeled amplitude ratios of pressure to vertical seismic velocity are too low in comparison with observations, and the characteristic differences in seismic and acoustic waveforms and spectra cannot be reproduced from a common point source. The observations therefore require a more complex source process in which the infrasonic signals are a record of only the broadband pressure excitation mechanism of the seismic LP events. The observations and numerical results can be explained by a model involving the repeated rapid pressure loss from a hydrothermal crack by venting into a shallow layer of loosely consolidated, highly permeable material. Heating by magmatic activity causes pressure to rise, periodically reaching the pressure threshold for rupture of the ‘‘valve’’ sealing the crack. Sudden opening of the valve generates the broadband infrasonic signal and simultaneously triggers the collapse of the crack, initiating resonance of the remaining fluid. Subtle waveform and amplitude variability of the infrasonic signals as recorded at an array 13.4 km to the NW of the volcano are attributed primarily to atmospheric boundary layer propagation effects, superimposed upon amplitude changes at the source.200 34 - PublicationRestrictedReal-time monitoring and massive inversion of source parameters of very long period seismic signals:An application to Stromboli Volcano, Italy.(2006)
; ; ; ; ; ;Auger, E.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;D'Auria, L.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Martini, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Chouet, B.; U.S. Geological Survey, Menlo Park, California, USA. ;Dawson, P.; U.S. Geological Survey, Menlo Park, California, USA.; ; ; ; We present a comprehensive processing tool for the real-time analysis of the source mechanism of very long period (VLP) seismic data based on waveform inversions performed in the frequency domain for a point source. A search for the source providing the best-fitting solution is conducted over a three-dimensional grid of assumed source locations, in which the Green’s functions associated with each point source are calculated by finite differences using the reciprocal relation between source and receiver. Tests performed on 62 nodes of a Linux cluster indicate that the waveform inversion and search for the best-fitting signal over 100,000 point sources require roughly 30 s of processing time for a 2-min-long record. The procedure is applied to post-processing of a data archive and to continuous automatic inversion of real-time data at Stromboli, providing insights into different modes of degassing at this volcano177 30 - PublicationRestrictedShallow velocity structure of Stromboli volcano, Italy, derived from small-aperture array measurements of Strombolian tremor(1998-06)
; ; ; ; ; ; ;Chouet, B.; U.S. Geological Survey ;De Luca, G.; Dipartimento di fisica, Università degli studi dell'Aquila ;Milana, G.; Servizio Sismico Nanionale ;Dawson, P.; U.S. Geological Survey ;Martini, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Scarpa, R.; Dipartimento di fisica, Università degli studi dell'Aquila; ; ; ; ; The properties of the tremor wave field at Stromboli are analyzed using data from small-aperture arrays of short-period seismometers deployed on the north flank of the volcano. The seismometers are configured in two semi-circular arrays with radii of 60 and 150 m and a linear array with length of 600 m. The data are analyzed using a spatiotemporal correlation technique specifically designed for the study of the stationary stochastic wave field of Rayleigh and Love waves generated by volcanic activity and by scattering sources distributed within the island. The correlation coefficients derived as a function of frequency for the three components of motion clearly define the dispersion characteristics for both Rayleigh and Love waves. Love and Rayleigh waves contribute 70% and 30%, respectively, of the surface-wave power. The phase velocities of Rayleigh waves range from 1000 m/sec at 2 Hz to 350 m/sec at 9 Hz, and those for Love waves range from 800 to 400 m/sec over the same frequency band. These velocities are similar to those measured near Puu Oo on the east rift of Kilauea Volcano, Hawaii, although the dispersion characteristics of Rayleigh waves at Stromboli show a stronger dependence on frequency. Such low velocities are consistent with values expected for densely cracked solidified basalt. The dispersion curves are inverted for a velocity model beneath the arrays, assuming those dispersions represent the fundamental modes of Rayleigh and Love waves.268 28 - PublicationRestrictedShallow-conduit dynamics at Stromboli Volcano, Italy, imaged from waveform inversions(2008)
; ; ; ;Chouet, B.; US Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025, USA ;Dawson, P.; US Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025, USA ;Martini, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia; ; Modelling of Very-Long-Period (VLP) seismic data recorded during explosive activity at Stromboli in 1997 provides an image of the uppermost 1 km of its volcanic plumbing system. Two distinct dyke-like conduit structures are identified, each representative of explosive eruptions from two different vents located near the northern and southern perimeters of the summit crater. Observed volumetric changes in the dykes are viewed as the result of a piston-like action of the magma associated with the disruption of a gas slug transiting through discontinuities in the dyke apertures. Accompanying these volumetric source components are single vertical forces resulting from an exchange of linear momentum between the source and the Earth. In the dyke system underlying the northern vent, a primary disruption site is observed at an elevation near 440 m where a bifurcation in the conduit occurs. At a depth of 80 m below sea level, a sharp corner in the conduit marks another location where the elastic response of the solid to the action of the upper source induces pressure and momentum changes in the magma. In the conduit underlying the southern vent, the junction of two inclined dykes with a sub-vertical dyke at 520 m elevation is a primary site of gas slug disruption, and another conduit corner 280 m below sea level represents a coupling location between the elastic response of the solid and fluid motion.194 49