Geology topics

Phillip B. Dawson

Publications and source records attributed to Phillip B. Dawson.

At least 19 recordsLinked to original sources

USGS2022

High-rate very-long-period seismicity at Yasur volcano, Vanuatu: source mechanism and decoupling from surficial explosions and infrasound

Yasur volcano, Vanuatu is a continuously active open-vent basaltic-andesite stratocone with persistent and long-lived eruptive activity. We present results from a seismo-acoustic field experiment at Yasur, providing locally dense broad-band seismic and infrasonic network coverage from 2016 July 27 to August 3. We corroborate our seismo-acoustic observations with coincident video data from cameras deployed at the crater and on an unoccupied aircraft system (UAS). The waveforms contain a profusion of signals reflecting Yasur’s rapidly occurring and persistent explosive activity. The typical infrasonic signature of Yasur explosions is a classic short-duration and often asymmetric explosion waveform characterized by a sharp compressive onset and wideband frequency content. The dominant seismic signals are numerous repetitive very-long-period (VLP) signals with periods of ∼2–10 s. The VLP seismic events are ‘high-rate’, reoccurring near-continuously throughout the data set with short interevent times (∼20–60 s). We observe variability in the synchronization of seismic VLP and acoustic sources. Explosion events clearly delineated by infrasonic waveforms are underlain by seismic VLPs. However, strong seismic VLPs also occur with only a weak infrasonic expression. Multiplet analysis of the seismic VLPs reveals a systematic progression in the seismo-acoustic source decoupling. The same dominant seismic VLP multiplet occurs with and without surficial explosions and infrasound, and these transitions occur over a timescale of a few days during our field campaign. We subsequently employ template matching, stacking, and full-waveform inversion to image the source mechanism of the dominant VLP multiplet. Inversion of the dominant VLP multiplet stack points to a composite source consisting of either a dual-crack (plus forces) or pipe-crack (plus forces) mechanism. The derived mechanisms correspond to a point-source directly beneath the summit vents with centroid depths in the range ∼900–1000 m below topography. All mechanisms suggest a northeast trending crack dipping relatively shallowly to the northwest and indicate a VLP source centroid and mechanism controlled by a stable structural geologic feature beneath Yasur. We interpret the results in the framework of gas slug ascent through the conduit responsible for Yasur explosions. The VLP mechanism and timing with infrasound (when present) are explained by a shallow-buffered top-down model in which slug ascent is relatively aseismic until reaching the base of a shallow section. Slug disruption in this shallow zone triggers a pressure disturbance that propagates downward and couples at the conduit base (VLP centroid). If the shallow section is open, an explosion propagates to the surface, producing infrasound. In the case of (the same multiplet) VLPs occurring without surficial explosions and weak or no infrasound, the decoupling of the dominant VLPs at ∼900–1000 m depth from surficial explosions and infrasound strongly indicates buffering of the terminal slug ascent. This buffering could be achieved by a variety of conditions at or directly beneath the vents, such as a high-viscosity layer of crystal-rich magma, a debris cap from backfill, a foam layer, or a combination of these. The dominant VLP at Yasur captured by our experiment has a source depth and mechanism separated from surface processes and is stable over time.

Geophysical Journal International↗

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↗

USGS2020

Origin and properties of hydrothermal tremor at Lone Star Geyser, Yellowstone National Park, USA

Geysers are rare geologic features that intermittently discharge liquid water and steam driven by heating and decompression boiling. The cause of variability in eruptive styles and the associated seismic signals are not well understood. Data collected from five broadband seismometers at Lone Star Geyser, Yellowstone National Park are used to determine the properties, location, and temporal patterns of hydrothermal tremor. The tremor is harmonic at some stages of the eruption cycle and is caused by near‐periodic repetition of discrete seismic events. Using the polarization of ground motion, we identify the location of tremor sources throughout several eruption cycles. During preplay episodes (smaller eruptions preceding the more vigorous major eruption), tremor occurs at depths of 7–10 m and is laterally offset from the geyser's cone by ~5 m. At the onset of the main eruption, tremor sources migrate laterally and become shallower. As the eruption progresses, tremor sources migrate along the same path but in the opposite direction, ending where preplay tremor originates. The upward and then downward migration of tremor sources during eruptions are consistent with warming of the conduit followed by evacuation of water during the main eruption. We identify systematic relations among the two types of preplays, discharge, and the main eruption. A point‐source moment tensor fit to low‐frequency waveforms of an individual tremor event using half‐space velocity models indicates average V S ≳ 0.8 km/s, source depths ~4–20 m, and moment tensors with primarily positive isotropic and compensated linear vector dipole moments.

Wyoming↗

USGS2020

Very‐long‐period (VLP) seismic artifacts during the 2018 caldera collapse at Kīlauea, Hawaii

Throughout the 2018 eruption of Kīlauea volcano (Hawai‘i), episodic collapses of a portion of the volcano’s summit caldera produced repeated M w "> M w 4.9–5.3 earthquakes. Each of these 62 events was characterized by a very‐long‐period (VLP) seismic signal ( ⁠ > 40    s "> > 40 s ⁠ ). Although collapses in the later stage of the eruption produced earthquakes with significant amplitude clipping on near‐summit broadband seismometers, the first 12 were accurately recorded. For these initial collapse events, we compare average VLP seismograms at six near‐summit locations to synthetic seismograms derived from displacements at collocated Global Positioning System stations. We show that the VLP seismic signal was generated by a radially outward and upward ramp function in displacement. We propose that at local distances the period of the VLP seismic signal is solely dependent on the duration of this ramp function and the instrument transfer function, that is, the seismic VLP is an artifact of the bandlimited instrument response and not representative of real ground motion. The displacement ramp function imposes a sinc‐function velocity amplitude spectrum that cannot be fully recovered through standard seismic instrument deconvolution. Any near‐summit VLP signals in instrument‐response‐corrected velocity or displacement seismograms from these collapse events are subject to severe band limitation. Similarly, the seismic amplitude response is not flat through the low‐frequency corner, for example, instrument‐response‐uncorrected seismograms scaled by instrument sensitivity are equally prone to band limitation. This observation is crucial when attempting to clarify the different contributions to the VLP source signature. Not accounting for this effect could lead to misunderstanding of the magmatic processes involved.

Hawaii↗

USGS2020

Observations of an extreme atmospheric river storm with a diverse sensor network

Observational networks enhance real‐time situational awareness for emergency and water resource management during extreme weather events. We present examples of how a diverse, multitiered observational network in California provided insights into hydrometeorological processes and impacts during a 3‐day atmospheric river storm centered on 14 February 2019. This network, which has been developed over the past two decades, aims to improve understanding and mitigation of effects from extreme storms influencing water resources and natural hazards. We combine atmospheric reanalysis output and additional observations to show how the network allows: (1) the validation of record cool season precipitable water observations over southern California; (2) the identification of phenomena that produce natural hazards and present difficulties for short‐term weather forecast models, such as extreme precipitation amounts and snow level variability; (3) the use of soil moisture data to improve hydrologic model forecast skill in northern California's Russian River basin; and (4) the combination of meteorological data with seismic observations to identify when a large avalanche occurred on Mount Shasta. This case study highlights the value of investments in diverse observational assets and the importance of continued support and synthesis of these networks to characterize climatological context and advance understanding of processes modulating extreme weather.

California↗

USGS2018

Microseismic events associated with the Oroville Dam spillway

On 14 February 2017, two small (equivalent M D 0.8 and 1.0) seismic events occurred in proximity to the Oroville Dam in the Sierra Nevada foothills, California. To examine possible causal relationships between these events and reservoir operations, including the spillway failure starting prior to these events, we applied a new optimized template matching approach to seismic data between May 1993 - April 2018. We identified more than 19,000 smaller-magnitude events that were similar in character to the February 14 events. These events are located in proximity to the Oroville spillway and occurred in tight temporal clusters that strongly correlate with periods of spillway discharge. Seismic source inversion is inconclusive, but we suggest that these events might be induced by rapid changes in pore pressure along a fracture (or fractures) near the spillway. Cavitation cannot be ruled out, but it is unlikely to be the primary cause of the signals observed because these events are intermittent, impulsive and of short duration. The inferred repetitive opening and closing of the fracture(s) occurred long before any damage to the spillway and is thus probably not directly associated with spillway failure in February 2017. These events were not related to the 1975 M L 5.7 earthquake sequence that may have been induced by the filling of the Oroville reservoir.

California↗

USGS2018

Seismic evidence for significant melt beneath the Long Valley Caldera, California, USA

A little more than 760 ka ago, a supervolcano on the eastern edge of California (United States) underwent one of North America's largest Quaternary explosive eruptions. Over this ~6-day-long eruption, pyroclastic flows blanketed the surrounding ~50 km with more than 1400 km 3 of the now-iconic Bishop Tuff, with ashfall reaching as far east as Nebraska. Collapse of the volcano's magma reservoir created the restless Long Valley Caldera. Although no rhyolitic eruptions have occurred in 100 k.y., beginning in 1978, ongoing uplift suggests new magma may have intruded into the reservoir. Alternatively, the reservoir could be approaching final crystallization, with present-day uplift related to the expulsion of fluid from the last vestiges of melt. Despite 40 years of diverse investigations, the presence of large volumes of melt in Long Valley's magma reservoir remain unresolved. Here we show, through full waveform seismic tomography, a mid-crustal zone of low shear-wave velocity. We estimate the reservoir contains considerable quantities of melt, >1000 km 3 , at melt fractions as high as ~27%. While supervolcanoes like Long Valley are rare, understanding the volume and concentration of melt in their magma reservoirs is critical for determining their potential hazard.

California↗

USGS2018

Deep fluid pathways beneath Mammoth Mountain, California, illuminated by migrating earthquake swarms

Although most volcanic seismicity is shallow (within several kilometers of the surface), some volcanoes exhibit deeper seismicity (10 to 30+ km) that may reflect active processes such as magma resupply and volatile transfer. One such volcano is Mammoth Mountain, California, which has also recently exhibited high rates of CO 2 discharge at the surface. We perform high-resolution earthquake detection and relocation to reveal punctuated episodes of rapidly propagating seismicity at mid-crustal depths along a narrow fracture zone surrounding a body of partial melt. We infer that these earthquakes track dike intrusions or fluid pressure pulses associated with CO 2 exsolution, suggesting that the deep plumbing system of Mammoth Mountain is an active conduit for fluid transport from the base of the crust to the surface.

California↗

USGS2016

Origin of the pulse-like signature of shallow long-period volcano seismicity

Short-duration, pulse-like long-period (LP) events are a characteristic type of seismicity accompanying eruptive activity at Mount Etna in Italy in 2004 and 2008 and at Turrialba Volcano in Costa Rica and Ubinas Volcano in Peru in 2009. We use the discrete wave number method to compute the free surface response in the near field of a rectangular tensile crack embedded in a homogeneous elastic half space and to gain insights into the origin of the LP pulses. Two source models are considered, including (1) a vertical fluid-driven crack and (2) a unilateral tensile rupture growing at a fixed sub-Rayleigh velocity with constant opening on a vertical crack. We apply cross correlation to the synthetics and data to demonstrate that a fluid-driven crack provides a natural explanation for these data with realistic source sizes and fluid properties. Our modeling points to shallow sources (<1 km depth), whose signatures are representative of the Rayleigh pulse sampled at epicentral distances >∼1 km. While a slow-rupture failure provides another potential model for these events, the synthetics and resulting fits to the data are not optimal in this model compared to a fluid-driven source. We infer that pulse-like LP signatures are parts of the continuum of responses produced by shallow fluid-driven sources in volcanoes.

Journal of Geophysical Research B: Solid Earth↗

USGS2016

Tomographic image of a seismically active volcano: Mammoth Mountain, California

High-resolution tomographic P wave, S wave, and V P / V S velocity structure models are derived for Mammoth Mountain, California, using phase data from the Northern California Seismic Network and a temporary deployment of broadband seismometers. An anomalous volume (5.1 × 10 9 to 5.9 × 10 10 m 3 ) of low P and low S wave velocities is imaged beneath Mammoth Mountain, extending from near the surface to a depth of &sim;2 km below sea level. We infer that the reduction in seismic wave velocities is due to the presence of CO 2 distributed in oblate spheroid pores with mean aspect ratio α = 1.6 × 10 −3 to 7.9 × 10 −3 (crack-like pores) and mean gas volume fraction ϕ = 8.1 × 10 −4 to 3.4 × 10 −3 . The pore density parameter κ = 3 ϕ /(4π α ) = na 3 =0.11, where n is the number of pores per cubic meter and a is the mean pore equatorial radius. The total mass of CO 2 is estimated to be 4.6 × 10 9 to 1.9 × 10 11 kg. The local geological structure indicates that the CO 2 contained in the pores is delivered to the surface through fractures controlled by faults and remnant foliation of the bedrock beneath Mammoth Mountain. The total volume of CO 2 contained in the reservoir suggests that given an emission rate of 500 tons day −1 , the reservoir could supply the emission of CO 2 for &sim;25–1040 years before depletion. Continued supply of CO 2 from an underlying magmatic system would significantly prolong the existence of the reservoir.

California↗

USGS2015

Source mechanism of small long-period events at Mount St. Helens in July 2005 using template matching, phase-weighted stacking, and full-waveform inversion

Long-period (LP, 0.5-5 Hz) seismicity, observed at volcanoes worldwide, is a recognized signature of unrest and eruption. Cyclic LP “drumbeating” was the characteristic seismicity accompanying the sustained dome-building phase of the 2004–2008 eruption of Mount St. Helens (MSH), WA. However, together with the LP drumbeating was a near-continuous, randomly occurring series of tiny LP seismic events (LP “subevents”), which may hold important additional information on the mechanism of seismogenesis at restless volcanoes. We employ template matching, phase-weighted stacking, and full-waveform inversion to image the source mechanism of one multiplet of these LP subevents at MSH in July 2005. The signal-to-noise ratios of the individual events are too low to produce reliable waveform-inversion results, but the events are repetitive and can be stacked. We apply network-based template matching to 8 days of continuous velocity waveform data from 29 June to 7 July 2005 using a master event to detect 822 network triggers. We stack waveforms for 359 high-quality triggers at each station and component, using a combination of linear and phase-weighted stacking to produce clean stacks for use in waveform inversion. The derived source mechanism pointsto the volumetric oscillation (~10 m 3 ) of a subhorizontal crack located at shallow depth (~30 m) in an area to the south of Crater Glacier in the southern portion of the breached MSH crater. A possible excitation mechanism is the sudden condensation of metastable steam from a shallow pressurized hydrothermal system as it encounters cool meteoric water in the outer parts of the edifice, perhaps supplied from snow melt.

Washington↗

USGS2015

Seismic source dynamics of gas-piston activity at Kı̄lauea Volcano, Hawai‘i

Since 2008, eruptive activity at the summit of Kı̄lauea Volcano, Hawai‘i has been confined to the new Overlook pit crater within the Halema‘uma‘u Crater. Among the broad range of magmatic processes observed in the new pit are recurring episodes of gas pistoning. The gas-piston activity is accompanied by seismic signals that are recorded by a broadband network deployed in the summit caldera. We use raw data recorded with this network to model the source mechanism of representative gas-piston events in a sequence that occurred on 20–25 August 2011 during a gentle inflation of the Kı̄lauea summit. To determine the source centroid location and source mechanism, we minimize the residual error between data and synthetics calculated by the finite difference method for a point source embedded in a homogeneous medium that takes topography into account. We apply a new waveform inversion method that accounts for the contributions from both translation and tilt in horizontal seismograms through the use of Green's functions representing the seismometer response to translation and tilt ground motions. This method enables a robust description of the source mechanism over the period range 1–10,000 s. Most of the seismic wavefield produced by gas-pistoning originates in a source region ∼1 km below the eastern perimeter of the Halema‘uma‘u pit crater. The observed waveforms are well explained by a simple volumetric source with geometry composed of two intersecting cracks featuring an east striking crack (dike) dipping 80°to the north, intersecting a north striking crack (another dike) dipping 65° to the east. Each gas-piston event is marked by a similar rapid inflation lasting a few minutes, trailed by a slower deflation ramp extending up to 15 min, attributed to the efficient coupling at the source centroid location of the pressure and momentum changes accompanying the growth and collapse of a layer of foam at the top of the lava column. Assuming a simple lumped parameter representation of the shallow magmatic system, the observed pressure and volume variations can be modeled with the following attributes : foam thickness (10–50 m), foam cell diameter (0.04–0.10 m), and gas-injection velocity (0.01–0.06 m s −1 ). Gas-piston activity occurs in a narrow pipe with diameter of 6 m connecting the Halema‘uma‘u pit crater to the subjacent dike system. The height of the magma column is estimated at ∼104 m at the start of the sequence based on the period of very long period (VLP) oscillations accompanying the onset of the gas-piston signal. Based on the change in the period of VLP oscillations and tilt evidence, the height of the magma column is inferred to have risen by up to ∼23 m by the end of the 5 day long sequence. A penny-shaped crack model of the dike geometry yields effective diameters of ∼1.2–2.9 km for the east dike and 0.7 km for the north dike. The shallower north dike segment is embedded in a relatively weak medium, compatible with expected mechanical properties in the hydrothermal environment of this dike.

Hawaii↗

USGS2014

Characterization of very-long-period seismicity accompanying summit activity at Kīlauea Volcano, Hawai'i: 2007-2013

Eruptive activity returned to the summit region of Kīlauea Volcano, Hawai'i with the formation of the “Overlook crater” within the Halema'uma'u Crater in March 2008. The new crater continued to grow through episodic collapse of the crater walls and as of late 2013 had grown into an approximately elliptical opening with dimensions of ~ 160 × 215 m extending to a depth of ~ 200 m. Occasional weak explosive events and a persistent gas plume continued to occur through 2013. Lava was first observed in the new crater in September 2008, and through 2009 the lava level remained deep in the crater and was only occasionally observed. Since early 2010 a lava lake with fluctuating level within the Overlook crater has been nearly continuously present, and has reached to within 22 m of the Overlook crater rim. Volcanic activity at Kīlauea Volcano is episodic at all time scales and the characterization of very-long-period seismicity in the band 2–100 s for the years 2007–2013 illuminates a portion of this broad spectrum of volcanic behavior. Three types of very-long-period events have been observed over this time and each is associated with distinct processes. Type 1 events are associated with vigorous degassing and occurred primarily between 2007 and 2009. Type 2 events are associated with rockfalls onto the lava lake and occurred primarily after early 2010. Both of these event types are induced by pressure and momentum changes at the top of the magma column that are transmitted downward to a source centroid ~ 1 km below the northeast corner of the Halema'uma'u Crater where the energy couples to the solid Earth at a geometrical discontinuity in the underlying dike system. Type 3 events are not related to surficial phenomena but are associated with transients in mass transfer that occur within the dike system. Very-long-period tremor has also accompanied the return of eruptive activity, with increasing amplitude associated with hours- to months-long changes in gas emission rates and summit deformation.

Hawaii↗

USGS2013

Very long period conduit oscillations induced by rockfalls at Kilauea Volcano, Hawaii

Eruptive activity at the summit of Kilauea Volcano, Hawaii, beginning in 2010 and continuing to the present time is characterized by transient outgassing bursts accompanied by very long period (VLP) seismic signals triggered by rockfalls from the vent walls impacting a lava lake in a pit within the Halemaumau pit crater. We use raw data recorded with an 11-station broadband network to model the source mechanism of signals accompanying two large rockfalls on 29 August 2012 and two smaller average rockfalls obtained by stacking over all events with similar waveforms to improve the signal-to-noise ratio. To determine the source centroid location and source mechanism, we minimize the residual error between data and synthetics calculated by the finite difference method for a point source embedded in a homogeneous medium that takes topography into account. We apply a new waveform inversion method that accounts for the contributions from both translation and tilt in horizontal seismograms through the use of Green's functions representing the seismometer response to translation and tilt ground motions. This method enables a robust description of the source mechanism over the period range 1–1000 s. The VLP signals associated with the rockfalls originate in a source region ∼1 km below the eastern perimeter of the Halemaumau pit crater. The observed waveforms are well explained by a simple volumetric source with geometry composed of two intersecting cracks including an east striking crack (dike) dipping 80° to the north, intersecting a north striking crack (another dike) dipping 65° to the east. Each rockfall is marked by a similar step-like inflation trailed by decaying oscillations of the volumetric source, attributed to the efficient coupling at the source centroid location of the pressure and momentum changes induced by the rock mass impacting the top of the lava column. Assuming a simple lumped parameter representation of the shallow magmatic system, the observed pressure and volume variations can be modeled with the following attributes: rockfall volume (200–4500 m 3 ), length of magma column (120–210 m), diameter of pipe connecting the Halemaumau pit crater to the subjacent dike system (6 m), average thickness of the two underlying dikes (3–6 m), and effective magma viscosity (30–210 Pa s). Most rockfalls occur during episodes of sustained deflation of the Kilauea summit. The mass loss rate in the shallow magmatic system is estimated to be 1400–15,000 kg s −1 based on measurements of the temporal variation of VLP period in the two large rockfalls that occurred on 29 August 2012.

Hawaii↗

USGS2013

Source characterization for an explosion during the 2009 eruption of Redoubt Volcano from very-long-period seismic waves

The 2009 eruption of Redoubt produced several very-long-period (VLP) signals associated with explosions. We invert for the source location and mechanism of an explosion at Redoubt volcano using waveform methods applied to broadband recordings. Such characterization of the source carries information on the geometry of the conduit and the physics of the explosion process. Inversions are carried out assuming the volcanic source can be modeled as a point source, with mechanisms described by a) a set of 3 orthogonal forces, b) a moment tensor consisting of force couples, and c) both forces and moment tensor components. We find that the source of the VLP seismic waves during the explosion is well-described by either a combined moment/force source located northeast of the crater and at an elevation of 1.6 km ASL or a moment source at an elevation of 800 m to the southwest of the crater. The moment tensors for the solutions with moment and force and moment-only share similar characteristics. The source time functions for both moment tensors begin with inflation (pressurization) and execute two cycles of deflation-reinflation (depressurization–repressurization). Although the moment/force source provides a better fit to the data, we find that owing to the limited coverage of the broadband stations at Redoubt the moment-only source is the more robust and reliable solution. Based on the moment-only solution, we estimate a volume change of 19,000 m 3 and a pressure change of 7 MPa in a dominant sill and an out-of-phase volume change of 5000 m 3 and pressure change of 1.8 MPa in a subdominant dike at the source location. These results shed new light on the magmatic plumbing system beneath Redoubt and complement previous studies on Vulcanian explosions at other volcanoes.

Alaska↗

USGS2012

Identifying bubble collapse in a hydrothermal system using hiddden Markov models

Beginning in July 2003 and lasting through September 2003, the Norris Geyser Basin in Yellowstone National Park exhibited an unusual increase in ground temperature and hydrothermal activity. Using hidden Markov model theory, we identify over five million high-frequency (>15 Hz) seismic events observed at a temporary seismic station deployed in the basin in response to the increase in hydrothermal activity. The source of these seismic events is constrained to within ~100 m of the station, and produced ~3500–5500 events per hour with mean durations of ~0.35–0.45 s. The seismic event rate, air temperature, hydrologic temperatures, and surficial water flow of the geyser basin exhibited a marked diurnal pattern that was closely associated with solar thermal radiance. We interpret the source of the seismicity to be due to the collapse of small steam bubbles in the hydrothermal system, with the rate of collapse being controlled by surficial temperatures and daytime evaporation rates.

Wyoming↗

USGS2010

Monitoring very-long-period seismicity at Kilauea Volcano, Hawaii

On 19 March, 2008 eruptive activity returned to the summit of Kilauea Volcano, Hawaii with the formation of a new vent within the Halemaumau pit crater. The new vent has been gradually increasing in size, and exhibiting sustained degassing and the episodic bursting of gas slugs at the surface of a lava pond ∼200 m below the floor of Halemaumau. The spectral characteristics, source location obtained by radial semblance, and Hidden Markov Model pattern recognition of the degassing burst signals are consistent with an increase in gas content in the magma transport system beginning in October, 2007. This increase plateaus between March – September 2008, and exhibits a fluctuating pattern until 31 January, 2010, suggesting that the release of gas is slowly diminishing over time.

Hawai'i↗

USGS2010

Seismic source mechanism of degassing bursts at Kilauea volcano, Hawaii: Results from waveform inversion in the 10–50 s band

The current (March 2008 to February 2009) summit eruptive activity at Kilauea Volcano is characterized by explosive degassing bursts accompanied by very long period (VLP) seismic signals. We model the source mechanisms of VLP signals in the 10–50 s band using data recorded for 15 bursts with a 10‐station broadband network deployed in the summit caldera. To determine the source centroid location and source mechanism, we minimize the residual error between data and synthetics calculated by the finite difference method for a point source embedded in a homogeneous medium that takes topography into account. The VLP signals associated with the bursts originate in a source region ∼1 km below the eastern perimeter of Halemaumau pit crater. The observed waveforms are well explained by the combination of a volumetric component and a vertical single force component. For the volumetric component, several source geometries are obtained which equally explain the observed waveforms. These geometries include (1) a pipe dipping 64° to the northeast; (2) two intersecting cracks including an east striking crack (dike) dipping 80° to the north, intersecting a north striking crack (another dike) dipping 65° to the east; (3) a pipe dipping 58° to the northeast, intersecting a crack dipping 48° to the west–southwest; and (4) a pipe dipping 57° to the northeast, intersecting a pipe dipping 58° to the west–southwest. Using the dual‐crack model as reference, the largest volume change obtained among the 15 bursts is ∼24,400 m 3 , and the maximum amplitude (peak to peak) of the force is ∼20 GN. Each burst is marked by a similar sequence of deflation and inflation, trailed by decaying oscillations of the volumetric source. The vertical force is initially upward, synchronous with source deflation, then downward, synchronous with source reinflation, followed by oscillations with polarity opposite to the volumetric oscillations. This combination of force and volume change is attributed to pressure and momentum changes induced during a fluid dynamic source mechanism involving the ascent, expansion, and burst of a large slug of gas within the upper ∼150 m of the magma conduit. As the slug expands upon approach to the surface and more liquid becomes wall supported by viscous shear forces, the pressure below the slug decreases, inducing conduit deflation and an upward force on the Earth. The final rapid slug expansion and burst stimulate VLP and LP oscillations of the conduit system, which slowly decay due to viscous dissipation and elastic radiation. Consideration of the fluid dynamic arguments leads us to prefer the dual‐crack VLP source model as it is the only candidate model capable of producing plausible values of length scales and pressure changes. The magnitudes of the vertical forces observed in the 15 bursts appear consistent with slug masses of 10 4 to 10 6 kg.

Hawaii↗