V13D-01 INVITED 13:40h
Acoustic Oscillations in Volcanoes
The intensity of infrasonic waves produced by volcanic activity ranges from very low amplitude pressure signals (mPa) to violent shock waves produced during explosive eruptions (MPa). Recorded waveforms vary from simple single pulses to complicated, long lasting signals where echoes and/or multiple pulses may be present. Whether echoes occur, are sustained, and are recorded depends on the elasticity of the surrounding walls, the attenuation of the fluid, the depth of the source, and the relative position of the sensor. A shallow explosion would release most of its energy to the atmosphere. In this case, echoes would be primarily associated with reflections from crater walls or nearby mountains. A deep explosion in a vesiculated magma column may not be multiply reflected (and thus maintain resonance) in a conduit if it has to propagate through a heavily attenuating magma-gas mixture. Yet highly vesiculated foams, with their low sound speeds and their sensitive dependence of gas exsolution and viscosity on ambient pressure, are extremely unstable under any fluid flow conditions. Due to the decrease in density and sound speed with increased vesiculation, an acoustic pulse arriving from some depth in a moving magma column would encounter an increase in Mach number as it approaches a highly vesiculated region. When this pulse reaches the foam, the pressure perturbation and its associated streaming may induce rapid exsolution and trigger a fragmentation-enhanced explosive eruption that could lower the fragmentation void fraction threshold and enhance jet flow. Lowering of the fragmentation threshold may permit conduit reverberation. Cavitation may occur when a fluid is excessively tensed. Flow acceleration through a constriction (choked flow), or the passage of an intense sound pulse can induce cavitation and produce a bubble oscillation. The precondition of existing bubbles for cavitation lend vesiculated foams particularly vulnerable to collapse. Sound from periodic turbulent vortices induced by surface discontinuities or shear (Aeolian tones, edge tones, vortex sheets) may occur at depth in the melt or at the ground-air interface. Avalanches, landslides, and pyroclastic flows would also generate acoustically active turbulent structures, as well as a sound from impact and explosive gas release. Jet noise can be produced by fumaroles, lava tubes, and eruptions. Jet flow resonance, known as screech, may occur within a supersonic jet and be observable during vigorous eruptions. Vigorous lava fountaining events radiate discrete infrasonic pulses which may be indicative of oscillations in the pressure driving the fluid flow. Infrasound from the oscillation of a lava tube or lava lake may be produced by the movement of the magma. Sound from lava falls, as seen through skylights in Pu'u O'o, may be enhanced by ringing of the air in a lava tube. As in the ocean, standing waves in a molten lava lake may generate sound efficiently if they slam into walls or if they entrain periodic flow into confined regions. As in a furnace, pressure and thermal oscillations may be induced in a lava tube when the gas in the tube is overburned, leading to a low pressure with gas overdrawing, followed by a fiery pressure increase during subsequent overburning.
V13D-02 13:55h
Tidal Modulation of Diffuse CO2 Degassing and Seismic Activity at Teide Volcano,Tenerife, Canary Islands.
A seismic swarm started in April, 2004, to the west-northwest of Teide volcano, in Tenerife, Canary Islands. Teide volcano (3715 m high) is located at the northern scarp of Las Cañadas caldera, a large depression produced by multiple episodes of caldera collapse and giant landslides. The basanite-phonolite magmatic system and the hydrothermal system associated with Teide volcano is emitting gases that reach the summit producing weak fumaroles. The efflux of CO2 is measured every hour at a continuous monitoring station that uses the accumulation chamber method. The average daily vertical component of the Earth tide at this volcano has been cross-correlated with the daily seismic energy released from April to August, 2004, and the low frequency component of the CO2 efflux emitted at the submit. This comparison suggests that seismic events and higher release of CO2 efflux occur when the vertical tide component has maximum values. Gravity forces are probably triggering seismic activity of a system that is already unstable. At the same time, variations in gravity forces can generate variations in fluid confining pressure and produce a higher release of CO2 from the hydrothermal system when the pressure decreases. Tidal effects seem to modulate the release of seismic energy and the low frequency CO2 efflux.
V13D-03 14:10h
Source Mechanism of Vulcanian Degassing at Popocat\'{e}petl Volcano, Mexico, Determined From Moment-Tensor Inversion of Very-long-period Seismic Waveforms
The source mechanism of very-long-period (VLP) signals accompanying degassing exhalations at Popocat\'{e}petl is analyzed in the $15-70$~s band by minimizing the residual error between data and synthetics calculated for a point source embedded in a homogeneous medium. The waveforms of two events (04/23/00, 05/23/00) representative of mild Vulcanian eruptions are well reproduced by our inversion, which takes into account volcano topography. The source centroid is positioned 1500~m below the western perimeter of the summit crater, and the modeled source is composed of a shallow-dipping crack (sill with easterly dip of $10\deg$) intersecting a steeply-dipping crack (northeast striking dike with northwest dip of $83\deg$), whose surface trace bisects the vent. Both cracks undergo a similar sequence of inflation, deflation, and reinflation --- reflecting a cycle of pressurization, depressurization, and repressurization within a time interval of $3-5$~min. The largest moment release occurs in the sill, showing a maximum volume change of $500-1000\:{\rm m^3}$, pressure drop of $3-5$~MPa, and amplitude of recovered pressure equal to 1.2 times the amplitude of the pressure drop. In contrast, the maximum volume change in the dike is $200-300\:{\rm m^3}$, with a corresponding pressure drop of $1-2$~MPa and pressure recovery equal to the pressure drop. Accompanying these volumetric sources is a single force with magnitude of $5 \times 10^8$~N, consistent with melt advection in response to the pressure transients. The source-time history of the three components of this force confirms that significant mass movement starts in the sill and triggers a mass movement response in the dike within $\sim 5$~s. Such source behavior is consistent with the opening of an escape pathway for accumulated gases from slow pressurization of the sill driven by magma crystallization. The opening of a pathway for pent-up gases in the sill and rapid evacuation of this separated gas phase induces the pressure drop. Pressure recovery in the magma filling the sill is driven by diffusion of gases from the resulting supersaturated melt into bubbles. Assuming a penny-shaped crack at ambient pressure of 40~MPa, the observed pressure and volume variations can be modeled with the following attributes: crack radius, (100~m), crack aperture, (5~m), bubble number density, ($10^{10} - 10^{12}\:{\rm m^{-3}}$), initial bubble radius, ($10^{-6}\:{\rm m}$), final bubble radius, ($\sim 10^{-5}\:{\rm m}$), and net decrease of gas concentration in the melt, (0.01~wt%).
V13D-04 14:25h
Coupling Between Fluid Flow and Heat Transfer - A Mechanism for Quasi-Periodic Variations in CO2 Discharges from Deep Underground Sources
Leakage of CO2 from underground sources is of interest in connection with volcanic hazards assessment, and with the integrity and safety of geologic disposal reservoirs for CO2 that have been proposed as a means for mitigating global warming from atmospheric emissions. Underground accumulations of CO2, whether naturally occurring or man-made, store vast amounts of compressional energy. At subsurface temperature and pressure conditions, CO2 is always buoyant relative to aqueous fluids, and its upward migration may conceivably give rise to a self-enhancing runaway release due to decompression and the much lower viscosity as compared to water. Natural occurrences of CO2 have been implicated in hydrothermal eruptions, and may be capable of causing "pneumatic" eruptions that are not powered by thermal energy. We have performed numerical simulations of CO2 release through fracture zones and faults in order to determine under what conditions, if any, a self-enhancing, eruptive release may be possible. Our simulations include coupling between multiphase fluid flow and associated heat transfer effects, and accurately represent the thermophysical properties of CO2 in sub-critical (liquid or gaseous) and supercritical conditions, as well as transitions between different phase compositions, and phase partitioning between CO2-rich and aqueous phases. The behavior of rising CO2 plumes is found to be strongly affected by heat transfer effects. As supercritical CO2 migrates upward it cools due to expansion. Much stronger cooling may arise from boiling of liquid CO2 that may occur after temperatures and pressures drop below critical values (Tcrit = 31.04 deg-C, Pcrit = 73.82 bar). Our simulations of CO2 migration up a fault zone produce quasi-periodic cycling of thermodynamic conditions and substantial variations of CO2 fluxes discharged at the land surface on a time scale of order 1 year. This behavior is explained in terms of an interplay between multiphase flow in the fault zone and conductive heat exchange with surrounding country rock of low permeability. CO2 upflow rates are reduced by heat transfer limitations, which give rise to substantial increase in fluid density as temperatures decline. A better understanding of natural hydrothermal and pneumatic eruptions is necessary in order that the effectiveness and safety of geologic disposal systems for CO2 may be evaluated. This work was supported by the Office of Basic Energy Sciences of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098.
V13D-05 INVITED 14:40h
Temporal evolution of a magmatic dike system inferred from the complex frequencies of very-long-period seismic signals
Very-long-period (VLP) seismic signals showing decaying harmonic oscillations with oscillation periods near 10 s and lasting for 300 s were observed in association with an earthquake swarm that occurred beneath Hachijo Island, Japan. The VLP activity began beneath this island on 20 August 2002. After the heightened VLP activity between 20 and 26 August, in which a maximum daily number of 40 VLP events was recorded on 26 August, the VLP signals were observed until April 2003. Spectral analysis of the decaying harmonic oscillations in the tail of the VLP waveforms identifies two spectral peaks (hereafter referred to as Peaks 1 and 2). The complex frequencies (frequency and quality factor, Q) of Peaks 1 and 2 show systematic trends in the period of the heightened VLP activity between 20 and 26 August. The Q values of both Peaks 1 and 2 show increasing trends from roughly 15 to 35 in this period. On the other hand, the frequencies of Peaks 1 and 2 show opposite trends in this period. The frequency of Peak 1 shows a decreasing trend from 0.1 to 0.09 Hz, while the frequency of Peak 2 shows an increasing trend from 0.128 to 0.136 Hz. After these systematic variations, the frequencies and Q values of the two peaks remain roughly constant, although scatters exist especially in the Q values. Numerical simulations based on the fluid-filled crack model indicate that the observed systematic temporal variations can be reasonably explained by changes in geometry of a crack containing a bubbly basalt. Assuming two crack resonance modes with wavelengths 2L/7 and 2W/5, where L and W are the length and width of a crack, the frequencies and Q values calculated for the crack with gradual increases in the length ranging from 1500 to 4000 m, width ranging from 1000 to 2000 m, and aperture ranging from 0.1 to 0.4 m reproduce the systematic temporal variations observed between 20 and 26 August. This result suggests an expansion of a dike during the heightened VLP activity. The roughly constant frequencies and Q values of the two peaks after these systematic variations suggest that the magmatic dike stably existed beneath Hachijo Island for about eight months. Since such a thin dike solidifies within several days if we only consider conduction of heat, the long lifetime of the dike suggested by the VLP signals implies a circulation of magma between the dike and a possible magma chamber beneath this island.
V13D-06 14:55h
Oscillatory phenomena in the geyser system at Onikobe, Japan
We conducted experimental observation at the Onikobe geyser, Japan, to understand hot-water effusion mechanisms, which are considered to be similar to the volcanic eruptions. In this study, we present simplicities and complexities in oscillatory phenomena found in geyser system by examining ground tilt motions and acoustic waves excited by effusion. The Onikobe geyser effuses hot water every about 10 minutes for about 1 to 1.5 minute during our observation in April 2004. Such cyclic signals are clearly recorded as saw-teethed time sequence in tilt motions. After an effusion, the tilt meters start to record a gradual increase of the pressure in chambers until the following effusion, which indicate a constant supply of hot-water to the geyser system. Carefully examining the data, we find that the gradual increase of the pressure tends to become weak about 1 min before the following effusion, and that about 10 percent of pressure loss is observed just before effusion. This suggests that a bulk density of the hot-water decreases before effusion and vesiculation process proceeds in chambers of the geyser system. During the pressure decrease, hot water escapes from a side of the vent, and the temperature of water increase from 80 to 100 C. And then, the effusion starts and the tilt motion start to show a de-pressurization of the chambers. Since the tilt vectors indicate a directional change about 20 s after the start of effusion, we infer two chambers beneath the vent. The geyser system sometimes becomes unstable. Time interval between nearby effusions changes from 10 min to 6 min and vice versa, and the two time intervals are irregularly repeated. In these unstable periods, as duration time of the effusion becomes shorter, interval time to the following effusion becomes longer. This implies that the Onikobe geyser system is a _gtime predictable system_h, that is, we can predict the time of next effusion by measuring the duration time of effusion. It should be noted that the tilt vectors does not change its direction when effusions with a short duration occur. However, significant differences are not recognized in tilt motions before each effusion, hence duration of effusion cannot be easily predicted although more accurate measurements may find out some difference in the data. Our acoustic sensor installed close to the vent records large signals associated with a start of effusion as well as short period signals of hot water falling to the ground. The observed acoustic signal is also characterized by a long-period oscillation with a period of a few to ten seconds. This long-period signals are often observed a few to tens of seconds after the beginning of effusion, but the waveforms are not always the same. We infer that the periodic oscillations in air-waves are caused by a Helmholtz resonance in the chamber system. Our observation revealed that chamber system beneath the vent significant role on oscillatory and cyclic phenomena in the geyser system, which may help us to understand the complexity found in active volcanoes.
V13D-07 15:10h
Dynamics of Gas-Rock Interaction at Erupting Volcanoes
Multi-parameter waveform observations of volcanic activity can provide new and detailed information on the volcano-atmosphere interface and illuminate the physical processes governing the interaction of thermal fluids escaping the earth. Recent investigations at several volcanoes utilize seismic, acoustic and visual observations to constrain the evolution of explosive activity. We present evidence of non-linear explosive chugging activity at Karymsky Volcano, Russia, and Sangay Volcano, Ecuador, and present a model showing how oscillatory behavior can be modeled by a choked vent. New, high quality measurements of acoustic arrival times (at Sangay) and detailed error analysis shows a strong correlation of acoustic amplitude with time between oscillatory pulses at two active volcanoes. Time-domain analysis of numerous episodes of oscillatory tremor shows a statistically significant positive correlation between amplitudes and interval times, indicating a feed back system near the vent. Specifically, we can show that, in general, larger pulses in the oscillatory sequence produce longer the time intervals before the next pulse. In particular, we show that activity can be modeled as choked fluid flow through narrow conduits near surface vents, analogous to a common kitchen pressure cooker.
V13D-08 15:25h
Analogue Experiments on the Generation of Flow-Induced Tremor
We explore the generation and properties of vibrations induced by fluid flow through a crack in an elastic medium by sending either air or water through a fracture in concentrated gelatin. The ambient stress field is varied by changing the orientation of both the crack and the fluid flow direction with respect to the gravitational force. Stresses near the crack walls are examined qualitatively by making use of the photoelastic properties of gelatin. Motions of the crack walls are imaged with high speed photography and acoustic energy is recorded with a microphone. Preliminary results indicate that the system is non-linear and that the ambient stress field affects both the onset of instabilities and the power spectrum of the acoustic signals. Fluid pressure opens the crack, and flow-induced oscillations of the walls are generated if the fluid flows fast enough. These vibrations generate relatively low frequency non-linear acoustic signals, which under some experimental conditions are punctuated by short duration higher frequency sounds produced by crack walls clapping together. The results of the experiments are compared with predictions from theoretical linear stability analysis, numerical simulations, and observations of volcanic tremor.