Geology topics

B. Chouet

Publications and source records attributed to B. Chouet.

At least 19 recordsLinked to original sources

USGS2026

Bulk location and source mechanisms of repetitive long-period seismicity at Mount St. Helens in July 2005

The 2004–2008 eruption of Mount St. Helens exemplified the practical challenges of processing high seismic event rates and data volumes. This protracted multi-year eruptive sequence was accompanied by a near-continuous occurrence of long-period (LP, 0.5–5 Hz) seismicity, but network-based picks and locations have previously been performed for only a relatively small subset of the overall seismicity, and full-waveform source–mechanism inversions have previously been performed for just two event types during the eruption. We present a new workflow for the bulk detection, cluster picking, 3-D location, and full-waveform source–mechanism inversion of repetitive LP volcanic seismicity recorded by a local broadband seismic network. Our emphasis is on methods that: exploit waveform similarity (multiplets) commonly observed for volcanic LP seismicity swarms; can take into account 3-D topographic and velocity structure heterogeneity typical of volcanoes; can process high event rates with relatively low analyst burden; and do not require a priori catalogue picks or known waveform templates. We develop and illustrate the workflow using data from Mount St. Helens from 2005 June 29 to July 31, a time-window characterized by high-rate LP seismicity, including numerous lower amplitude small LPs, and for which relatively dense seismic network coverage is available. Approximately 40 per cent of seismicity (20 561 events out of 52 141 triggers) during 2005 June 29 to July 31 is repetitive enough to be grouped into 17 multiplets of at least 300 events. We present 3-D locations of the 17 multiplets, representing 20 076 events having picks from at least eight stations, along with full-waveform source mechanism inversions of the 17 separate multiplet stacks. Collectively, our seismic 3-D hypocentre distributions for 2005 June 29 to July 31 are compatible with sources around the margins of the upper conduit section that is associated with plug solidification. All location results indicate compact seismicity clusters, with the 17 multiplets spatially connected and collectively located at shallow depth, approximately ~ 775 m depth beneath the 1980s crater floor and directly beneath, or immediately adjacent to, the extruding dacite spine. The full-waveform source–mechanism inversions for the 17 multiplets generally indicate mechanisms dominated by volumetric components consistent with subhorizontal cracks along with minor contributions from subvertical cracks. The overall picture is that of a stack or plexus of subhorizontal cracks located at shallow depth beneath the extruding dacite spines, consistent with subhorizontal hydrofractures extending from the conduit margins into the surrounding wall rock; minor subvertical crack components of the mechanisms could possibly arise from surfaces parallel to the conduit wall, such as within the damage halo associated with degassing pathways.

Washington↗

USGS2021

Sources of volcanic tremor associated with the summit caldera collapse during the 2018 east rift eruption of Kīlauea Volcano, Hawai'i

Volcanic tremor occurring at the beginning of the 2018 Kīlauea eruption is characterized using both seismic and tilt data recorded at the Kīlauea summit. An automatic seismic network-based approach detects several types of tremor including (a) 0.5–1 Hz long-period tremor preceding the eruption, located at the south-southwest edge of Halema'uma'u Crater and attributed to the quasi-steady radiation from a shallow hydrothermal system and (b) two sequences of gliding tremor at the beginning of the eruption, both with locations on the edges of the crater and within it. The first sequence is attributed to two swarms of low-amplitude regularly repeating earthquakes induced by the jerky motions of a cylindrical rock piston with radius of 325 m, height of 250 m, and mass of 2.07 × 10 11 kg, progressively intruding 12.3 m into the shallow hydrothermal system with volume of 10 8 m 3 and depth extent of 300 m. The second sequence is attributed to a gradual evolution in the properties of a bubbly magma within an east-striking dike below Halema'uma'u Crater, impacted by repeated roof collapses. A fluid-filled crack model points to a decrease in gas volume fraction from 4.22% to 1.6 × 10 −2 % in the magma filling the dike, and a model of gas retro-diffusion within the melt suggests a two orders of magnitude decrease in bubble number density from 7 × 10 8 m −3 down to 4 × 10 6 m −3 . Both models feature a quasi to totally degassed magma by May 26.

Hawaii↗

USGS2012

Singularity spectrum of intermittent seismic tremor at Kilauea Volcano, Hawaii

Fractal singularity analysis (FSA) is used to study a 22-year record of deep seismic tremor (30–60 km depth) for regions below Kilauea Volcano on the assumption that magma transport and fracture can be treated as a system of coupled nonlinear oscillators. Tremor episodes range from 1 to 100 min (cumulative duration = 1.60×10 4 min; yearly average = 727 min yr −1 ; mean gradient = 24.2 min yr −1 km −1 ). Partitioning of probabilities, p i , in the phase space of normalized durations, x i , are expressed in terms of a function f(α), where α is a variable exponent of a length scale, ℓ. Plots of f(α) vs. α are called multifractal singularity spectra. The spectrum for deep tremor durations is bounded by α values of about 0.4 and 1.9 at f = 0; f max ≃ 1.0 for α ≃ 1. Results for tremor are similar to those found for systems transitional between complete mode locking and chaos.

Geophysical Research Letters↗

USGS2012

Fractal properties of tremor and gas piston events observed at Kilauea Volcano, Hawaii

Studies the fractal properties of shallow volcanic tremor and gas piston events associated with magma degassing at Kilauea Volcano, Hawaii, using data from two dense short-baseline arrays of seismographs deployed near the active crater of Puu Oo on the east rift of the volcano. The existence of a categorically stable attractor characterizing both types of activities strongly suggests that the excitation mechanism of tremor is similar to that of gas piston events, which in turn are correlated with visual observations at the volcanic vent. Low values derived for the dimension of the attractor in phase space points to significant self-organization in the process of generation of tremor and offers general constraints on the dimensionality of attractors derived from models of acoustic emission associated with magma flow, vesiculation, and degassing. -from Authors

Hawaii↗

USGS2012

Resonance of a fluid-driven crack: Radiation properties and implications for the source of long-period events and harmonic tremor

A dynamic source model is presented, in which a three-dimensional crack containing a viscous compressible fluid is excited into resonance by an impulsive pressure transient applied over a small area ΔS of the crack surface. The crack excitation depends critically on two dimensionless parameters called the crack stiffness, C = ( b /μ)( L / d ), and viscous damping loss, F = (12η L )/(ρ ƒ d 2 α), where b is the bulk modulus, η is the viscosity, ρ ƒ is the density of the fluid, μ is the rigidity, α is the compressional velocity of the solid, L is the crack length, and d is the crack thickness. The first parameter characterizes the ability of the crack to vibrate and shapes the spectral signature of the source, and the second quantifies the effect of fluid viscosity on the duration of resonance. Resonance is sustained by a very slow wave trapped in the fluid-filled crack. This guided wave, called the crack wave, is similar to the tube wave propagating in a fluid-filled borehole; it is inversely dispersive, showing a phase velocity that decreases with increasing wavelength, and its wave speed is always lower than the acoustic velocity of the fluid, decreasing rapidly as the crack stiffness increases. The source spectrum shows many sharp peaks characterizing the individual modes of vibration of the crack; the variation of spectral shape, both in the number and width of peaks, is surprisingly complex, reflecting the interference between the lateral and longitudinal modes of resonance, as well as nodes for these modes. The far-field spectrum is marked by narrow-band dominant and subdominant peaks that reflect the interaction of the various source modes. The frequency of the dominant spectral peak radiated by the source is independent of the radiation direction. The frequency, bandwidth, and spacing of the resonant peaks are strongly dependent on the crack stiffness, larger values of the stiffness factor shifting these peaks to lower frequencies and decreasing their bandwidth. The excitation of a particular mode depends on the position of the trigger and on the extent of the crack surface affected by the pressure transient. Fluid viscosity decreases the amplitudes of the main spectral peaks, smears out the finer structure of the spectrum, and greatly reduces the duration of the radiated signal. The energy loss by radiation is stronger for high frequencies, producing a seismic signature that is marked by a high-frequency content near the onset of the signal and dominated by a longer-period component of much longer duration in the signal coda. Such signature is in harmony with those displayed by long-period events observed on active volcanoes and in hydrofracture experiments. The very low velocity which is possible in a crack with high stiffness ( C ≥ 100) also provides an attractive explanation for very long period tremor, such as type 2 tremor at Aso volcano, Japan, without the requirement of an unrealistically large magma container. The standing wave pattern set up on the crack surface by the sustained resonance in the fluid is observable in the near field of the crack, suggesting that the location and extent of the source may be estimated from the mapping of the pattern of nodes and antinodes seen in its vicinity. According to the model, the long-period event and harmonic tremor share the same source but differ in the boundary conditions for fluid flow and in the triggering mechanism setting up the resonance of the source, the former being viewed as the impulse response of the tremor generating system and the latter representing the excitation due to more complex forcing functions.

Journal of Geophysical Research Solid Earth↗

USGS2011

Pressure waves in a supersaturated bubbly magma

We study the interaction of acoustic pressure waves with an expanding bubbly magma. The expansion of magma is the result of bubble growth during or following magma decompression and leads to two competing processes that affect pressure waves. On the one hand, growth in vesicularity leads to increased damping and decreased wave amplitudes, and on the other hand, a decrease in the effective bulk modulus of the bubbly mixture reduces wave velocity, which in turn, reduces damping and may lead to wave amplification. The additional acoustic energy originates from the chemical energy released during bubble growth. We examine this phenomenon analytically to identify conditions under which amplification of pressure waves is possible. These conditions are further examined numerically to shed light on the frequency and phase dependencies in relation to the interaction of waves and growing bubbles. Amplification is possible at low frequencies and when the growth rate of bubbles reaches an optimum value for which the wave velocity decreases sufficiently to overcome the increased damping of the vesicular material. We examine two amplification phase-dependent effects: (1) a tensile-phase effect in which the inserted wave adds to the process of bubble growth, utilizing the energy associated with the gas overpressure in the bubble and therefore converting a large proportion of this energy into additional acoustic energy, and (2) a compressive-phase effect in which the pressure wave works against the growing bubbles and a large amount of its acoustic energy is dissipated during the first cycle, but later enough energy is gained to amplify the second cycle. These two effects provide additional new possible mechanisms for the amplification phase seen in Long-Period (LP) and Very-Long-Period (VLP) seismic signals originating in magma-filled cracks.

Geophysical Journal International↗

USGS2007

Waveform inversion of volcano-seismic signals for an extended source

[1] We propose a method to investigate the dimensions and oscillation characteristics of the source of volcano-seismic signals based on waveform inversion for an extended source. An extended source is realized by a set of point sources distributed on a grid surrounding the centroid of the source in accordance with the source geometry and orientation. The source-time functions for all point sources are estimated simultaneously by waveform inversion carried out in the frequency domain. We apply a smoothing constraint to suppress short-scale noisy fluctuations of source-time functions between adjacent sources. The strength of the smoothing constraint we select is that which minimizes the Akaike Bayesian Information Criterion (ABIC). We perform a series of numerical tests to investigate the capability of our method to recover the dimensions of the source and reconstruct its oscillation characteristics. First, we use synthesized waveforms radiated by a kinematic source model that mimics the radiation from an oscillating crack. Our results demonstrate almost complete recovery of the input source dimensions and source-time function of each point source, but also point to a weaker resolution of the higher modes of crack oscillation. Second, we use synthetic waveforms generated by the acoustic resonance of a fluid-filled crack, and consider two sets of waveforms dominated by the modes with wavelengths 2 L /3 and 2 W /3, or L and 2 L /5, where W and L are the crack width and length, respectively. Results from these tests indicate that the oscillating signature of the 2 L /3 and 2 W /3 modes are successfully reconstructed. The oscillating signature of the L mode is also well recovered, in contrast to results obtained for a point source for which the moment tensor description is inadequate. However, the oscillating signature of the 2 L /5 mode is poorly recovered owing to weaker resolution of short-scale crack wall motions. The triggering excitations of the oscillating cracks are successfully reconstructed.

Journal of Geophysical Research B: Solid Earth↗

USGS2006

Real-time monitoring and massive inversion of source parameters of very long period seismic signals: An application to Stromboli Volcano, Italy

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 volcano.

Stromboli volcano↗

USGS2004

Application of near real-time radial semblance to locate the shallow magmatic conduit at Kilauea Volcano, Hawaii

Radial Semblance is applied to broadband seismic network data to provide source locations of Very-Long-Period (VLP) seismic energy in near real time. With an efficient algorithm and adequate network coverage, accurate source locations of VLP energy are derived to quickly locate the shallow magmatic conduit system at Kilauea Volcano, Hawaii. During a restart in magma flow following a brief pause in the current eruption, the shallow magmatic conduit is pressurized, resulting in elastic radiation from various parts of the conduit system. A steeply dipping distribution of VLP hypocenters outlines a region extending from sea level to about 550 m elevation below and just east of the Halemaumau Pit Crater. The distinct hypocenters suggest the shallow plumbing system beneath Halemaumau consists of a complex plexus of sills and dikes. An unconstrained location for a section of the conduit is also observed beneath the region between Kilauea Caldera and Kilauea Iki Crater.

Hawaii↗

USGS2003

Volcano seismology

A fundamental goal of volcano seismology is to understand active magmatic systems, to characterize the configuration of such systems, and to determine the extent and evolution of source regions of magmatic energy. Such understanding is critical to our assessment of eruptive behavior and its hazardous impacts. With the emergence of portable broadband seismic instrumentation, availability of digital networks with wide dynamic range, and development of new powerful analysis techniques, rapid progress is being made toward a synthesis of high-quality seismic data to develop a coherent model of eruption mechanics. Examples of recent advances are: (1) high-resolution tomography to image subsurface volcanic structures at scales of a few hundred meters; (2) use of small-aperture seismic antennas to map the spatio-temporal properties of long-period (LP) seismicity; (3) moment tensor inversions of very-long-period (VLP) data to derive the source geometry and mass-transport budget of magmatic fluids; (4) spectral analyses of LP events to determine the acoustic properties of magmatic and associated hydrothermal fluids; and (5) experimental modeling of the source dynamics of volcanic tremor. These promising advances provide new insights into the mechanical properties of volcanic fluids and subvolcanic mass-transport dynamics. As new seismic methods refine our understanding of seismic sources, and geochemical methods better constrain mass balance and magma behavior, we face new challenges in elucidating the physico-chemical processes that cause volcanic unrest and its seismic and gas-discharge manifestations. Much work remains to be done toward a synthesis of seismological, geochemical, and petrological observations into an integrated model of volcanic behavior. Future important goals must include: (1) interpreting the key types of magma movement, degassing and boiling events that produce characteristic seismic phenomena; (2) characterizing multiphase fluids in subvolcanic regimes and determining their physical and chemical properties; and (3) quantitatively understanding multiphase fluid flow behavior under dynamic volcanic conditions. To realize these goals, not only must we learn how to translate seismic observations into quantitative information about fluid dynamics, but we also must determine the underlying physics that governs vesiculation, fragmentation, and the collapse of bubble-rich suspensions to form separate melt and vapor. Refined understanding of such processes—essential for quantitative short-term eruption forecasts—will require multidisciplinary research involving detailed field measurements, laboratory experiments, and numerical modeling.

Pure and Applied Geophysics↗

USGS2003

Shallow-velocity models at the Kilauea Volcano, Hawaii, determined from array analyses of tremor wavefields

The properties of the surface wavefield at Kilauea Volcano are analysed using data from small-aperture arrays of short-period seismometers deployed in and around the Kilauea caldera. Tremor recordings were obtained during two Japan-US cooperative experiments conducted in 1996 and 1997. The seismometers were deployed in three semi-circular arrays with apertures of 300, 300 and 400 m, and a linear array with length of 1680 m. Data are analysed using a spatio-temporal correlation technique well suited for the study of the stationary stochastic wavefields of Rayleigh and Love waves associated with volcanic activity and scattering sources distributed in and around the summit caldera. Spatial autocorrelation coefficients are obtained as a function of frequency and are inverted for the dispersion characteristics of Rayleigh and Love waves using a grid search that seeks phase velocities for which the L-2 norm between data and forward modelling operators is minimized. Within the caldera, the phase velocities of Rayleigh waves range from 1400 to 1800 m s -1 at 1 Hz down to 300-400 m s -1 at 10 Hz, and the phase velocities of Love waves range from 2600 to 400 m s -1 within the same frequency band. Outside the caldera, Rayleigh wave velocities range from 1800 to 1600 m s -1 at 1 Hz down to 260-360 m s -1 at 10 Hz, and Love wave velocities range from 600 to 150 m s -1 within the same frequency band. The dispersion curves are inverted for velocity structure beneath each array, assuming these dispersions represent the fundamental modes of Rayleigh and Love waves. The velocity structures observed at different array sites are consistent with results from a recent 3-D traveltime tomography of the caldera region, and point to a marked velocity discontinuity associated with the southern caldera boundary.

Geophysical Journal International↗

USGS2003

A numerical simulation of magma motion, crustal deformation, and seismic radiation associated with volcanic eruptions

The finite difference method is used to calculate the magma dynamics, seismic radiation, and crustal deformation associated with a volcanic eruption. The model geometry consists of a cylindrical reservoir and narrow cylindrical conduit embedded in a homogeneous crust. We consider two models of eruption. In the first model, a lid caps the vent and the magma is overpressurized prior to the eruption. The eruption is triggered by the instantaneous removal of the lid, at which point the exit pressure becomes equal to the atmospheric pressure. In the second model, a plug at the reservoir outlet allows pressurization of only the magmatic fluid in the reservoir before the eruption. Magma transfer between the reservoir and conduit is triggered by the instantaneous removal of the plug, and the eruption occurs when the pressure at the conduit orifice exceeds the material strength of the lid capping the vent. In both models, magma dynamics are expressed by the equations of mass and momentum conservation in a compressible fluid, in which fluid expansion associated with depressurization is accounted for by a constitutive law relating pressure and density. Crustal motions are calculated from the equations of elastodynamics. The fluid and solid are dynamically coupled by applying the continuity of wall velocities and normal stresses across the conduit and reservoir boundaries. Free slip is allowed at the fluid-solid boundary. Both models predict the gradual depletion of the magma reservoir, which causes crustal deformation observed as a long-duration dilatational signal. Superimposed on this very-long-period (VLP) signal generated by mass transport are long-period (LP) oscillations of the magma reservoir and conduit excited by the acoustic resonance of the reservoir-conduit system during the eruption. The volume of the reservoir, vent size, and magma properties control the duration of VLP waves and dominant periods of LP oscillations. The second model predicts that when the magmatic fluid reaches the vent, a high-pressure pulse occurs at this location in accordance with the basic theory of compressible fluid dynamics. This abrupt pressure increase just beneath the vent is consistent with observed seismograms in which pulse-like Rayleigh waves excited by a shallow source are dominant. The strength of the lid plays an important role in the character of the seismograms and in defining the type of eruption observed.

Geophysical Journal International↗

USGS2003

Performance of the radial semblance method for the location of very long period volcanic signals

We investigate the performance of a source location method that combines multichannel semblance and particle motions and is being increasingly used to obtain estimates of the source locations of very long period (VLP) seismic signals recorded on volcanoes. The method makes use of the radial particle motions and large wavelengths that characterize the VLP events. To assess the capabilities of this radial semblance method , and to better understand its limitations, we quantify the effects of window length, noise contents of the signal , inaccurate velocity models, receiver coverage, and orientation errors in the horizontal components of the receivers. Our results show that the semblance method performs best when (1) the noise level is low enough to allow a good characterization of the waveforms, (2) the sources are located at distances between one half of the average receiver spacing and about two times the network aperture, and (3) the orientations of the horizontal components of the seismometers are known with relative accuracy. When these requirements are met, the radial semblance method constitutes an adequate tool to obtain preliminary locations of VLP volcanic signals recorded by broadband networks. Moreover, we provide a formula to determine the radial semblance level that should be used to define error regions associated to the estimated source locations.

Bulletin of the Seismological Society of America↗

USGS2002

Source mechanism of very-long-period signals accompanying dome growth activity at Merapi volcano, Indonesia

Very-long-period (VLP) pulses with period of 6–7s, displaying similar waveforms, were identified in 1998 from broadband seismographs around the summit crater. These pulses accompanied most of multiphase (MP) earthquakes, a type of long-period event locally defined at Merapi Volcano. Source mechanisms for several VLP pulses were examined by applying moment tensor inversion to the waveform data. Solutions were consistent with a crack striking ∼70° and dipping ∼50° SW, 100m under the active dome, suggest pressurized gas transport involving accumulation and sudden release of 10–60 m 3 of gas in the crack over a 6s interval.

Merapi volcano↗

USGS2002

Long-period events and tremor at Popocatepetl volcano (1994–2000) and their broadband characteristics

Following an initial phreatic eruption on 21 December 1994, activity at Popocatepetl has been dominated by fumarolic emissions interspersed with more energetic emissions of ashes and gases. A phase of repetitive dome-building and dome-destroying episodes began in March 1996 and is still ongoing at present. We describe the long-period (LP) seismicity accompanying eruptive activity at Popocatepetl from December 1994 through May 2000, using data from a three-component broadband seismometer located 5 km from the summit crater. The broadband records display a variety of signals, with periods ranging in the band 0.04–90 s. Long-period events and tremor with typical dominant periods in the range 0.3–2.0 s are the most characteristic signals observed at Popocatepetl. These signals appear to reflect volumetric sources driven by pressure fluctuations associated with the unsteady transport of gases beneath the crater. Very-long-period (VLP) signals are also observed in association with LP events and tremor. The VLP signals which accompany LP events display Ricker-like wavelets with periods near 36 s, whereas VLP signals associated with tremor waveforms typically show sustained oscillations at periods ranging up to 90 s. The spectra and particle motion patterns remain similar from event to event for the majority of LP and tremor signals analyzed during the time span of this study, suggesting a repeated, non-destructive activation of a common source. Hypocenters determined by phase pick analyses of selected LP events recorded by the seven-station, permanent Popocatepetl short-period network suggest that the majority of these events are confined to a source region in the top 1.5 km below the crater floor. The repetitive occurrences of VLP signals with closely matched waveform characteristics are consistent with a non-destructive reactivation of at least two sources. One source appears to coincide with the main source region of LP seismicity, whereas the second is a deeper source whose activity appears to be intimately linked with episodes of monochromatic tremor.

Bulletin of Volcanology↗

USGS2002

Identifying elements of the plumbing system beneath Kilauea Volcano, Hawaii, from the source locations of very-long-period signals

We analyzed 16 seismic events recorded by the Hawaiian broad-band seismic network at Kilauca Volcano during the period September 9-26, 1999. Two distinct types of event are identified based on their spectral content, very-long-period (VLP) waveform, amplitude decay pattern and particle motion. We locate the VLP signals with a method based on analyses of semblance and particle motion. Different source regions are identified for the two event types. One source region is located at depths of ~1 km beneath the northeast edge of the Halemaumau pit crater. A second region is located at depths of ~8 km below the northwest quadrant of Kilauea caldera. Our study represents the first time that such deep sources have been identified in VLP data at Kilauea. This discovery opens the possibility of obtaining a detailed image of the location and geometry of the magma plumbing system beneath this volcano based on source locations and moment tensor inversions of VLP signals recorded by a permanent, large-aperture broad-band network.

Geophysical Journal International↗

USGS2002

Mapping the sources of the seismic wave field at Kilauea volcano, Hawaii, using data recorded on multiple seismic Antennas

Seismic antennas constitute a powerful tool for the analysis of complex wave fields. Well-designed antennas can identify and separate components of a complex wave field based on their distinct propagation properties. The combination of several antennas provides the basis for a more complete understanding of volcanic wave fields, including an estimate of the location of each individual wave-field component identified simultaneously by at least two antennas. We used frequency-slowness analyses of data from three antennas to identify and locate the different components contributing to the wave fields recorded at Kilauea volcano, Hawaii, in February 1997. The wave-field components identified are (1) a sustained background volcanic tremor in the form of body waves generated in a shallow hydrothermal system located below the northeastern edge of the Halemaumau pit crater; (2) surface waves generated along the path between this hydrothermal source and the antennas; (3) back-scattered surface wave energy from a shallow reflector located near the southeastern rim of Kilauea caldera; (4) evidence for diffracted wave components originating at the southeastern edge of Halemaumau; and (5) body waves reflecting the activation of a deeper tremor source between 02 hr 00 min and 16 hr 00 min Hawaii Standard Time on 11 February.

Hawaii↗

USGS2001

Spatial extent of a hydrothermal system at Kilauea Volcano, Hawaii, determined from array analyses of shallow long-period seismicity 2. Results

Array data from a seismic experiment carried out at Kilauea Volcano, Hawaii, in February 1997, are analyzed by the frequency-slowness method. The slowness vectors are determined at each of three small-aperture seismic antennas for the first arrivals of 1129 long-period (LP) events and 147 samples of volcanic tremor. The source locations are determined by using a probabilistic method which compares the event azimuths and slownesses with a slowness vector model. The results show that all the LP seismicity, including both discrete LP events and tremor, was generated in the same source region along the east flank of the Halemaumau pit crater, demonstrating the strong relation that exists between the two types of activities. The dimensions of the source region are approximately 0.6×1.0×0.5 km. For LP events we are able to resolve at least three different clusters of events. The most active cluster is centered ∼200 m northeast of Halemaumau at depths shallower than 200 m beneath the caldera floor. A second cluster is located beneath the northeast quadrant of Halemaumau at a depth of ∼400 m. The third cluster is <200 m deep and extends southeastward from the northeast quadrant of Halemaumau. Only one source zone is resolved for tremor. This zone is coincident with the most active source zone of LP events, northeast of Halemaumau. The location, depth, and size of the source region suggest a hydrothermal origin for all the analyzed LP seismicity.

Hawai'i↗