With the emergence of portable broadband seismic instrumentation, availability of digital networks with wide dynamic range, and development of new powerful analysis techniques made possible by ...greatly increased computer capacity, volcano seismology has now reached a mature stage where insights are rapidly being gained on the role played by magmatic and hydrothermal fluids in the generation of seismic waves. Volcanoes produce a wide variety of signals originating in the transport of magma and related hydrothermal fluids and their interaction with solid rock. Typical signals include (1) brittle failure earthquakes that reflect the response of the rock to stress changes induced by magma movement; (2) pressure oscillations accompanying the dynamics of liquids and gases in conduits and cracks; and (3) magma fracturing and fragmentation. Oscillatory behaviors within magmatic and hydrothermal systems are the norm and are the expressions of the complex rheologies of these fluids and nonlinear characteristics of associated processes underlying the release of thermo-chemical and gravitational energy from volcanic fluids along their ascent path. The interpretation of these signals and quantification of their source mechanisms form the core of modern volcano seismology. The accuracy to which the forces operating at the source can be resolved depends on the degree of resolution achieved for the volcanic structure. High-resolution tomography based on iterative inversions of seismic travel-time data can image three-dimensional structures at a scale of a few hundred meters provided adequate local short-period earthquake data are available. Hence, forces in a volcano are potentially resolvable for periods longer than ~1s. In concert with techniques aimed at the interpretation of processes occurring in the fluid, novel seismic methods have emerged that are allowing the detection of stress changes in volcanic structures induced by magma movement. These methods include (1) ambient noise interferometry, in which the ambient seismic noise is used to probe temporal changes in volcanic structures; (2) the measurement of seismic anisotropy, where changes in the alignment of fluid-filled microcracks and pore space are monitored to assess the response of the crust to pressurization of a magmatic system; and (3) the detection of systematic changes in fault plane solutions of volcano-tectonic earthquakes caused by local stress perturbations during conduit pressurization. As new seismic methods refine our understanding of seismic sources and behavior of volcanic structures, we face new challenges in elucidating the physico-chemical processes that cause volcanic unrest and its seismic and gas-discharge manifestations. Future important goals toward meeting those challenges must include a better understanding of the key types of magma movement, degassing and boiling events that produce characteristic seismic phenomena, along with a quantitative understanding of multiphase fluid behavior under dynamic volcanic conditions. Realizing these goals will be essential for the development of an integrated model of volcanic behavior and will require multidisciplinary research involving detailed field measurements, laboratory experiments, and numerical modeling.
► Seismology provides a unique tool to study magma transport mechanics and volcanic structures. ► Seismicity in volcanoes reflects dynamic interactions between gas, liquid and solid. ► Seismic source mechanisms can be quantified from waveform inversions. ► Seismic source mechanisms yield clues about fluid dynamics.
At an active volcano, long-period seismicity reflects pressure changes resulting from unsteady mass transport in the sub-surface plumbing system and shows a bit of the internal dynamics of a volcano. ...At shallow depths, this activity can be an indicator of impending eruption.
One of the most striking aspects of seismicity during the 2004–2008 eruption of Mount St. Helens (MSH) was the precise regularity in occurrence of repetitive long‐period (LP) or “drumbeat” events ...over sustained time periods. However, this precise regularity was not always observed, and at times the temporal occurrence of LP events became more random. In addition, accompanying the dominant LP class of events during the 2004–2008 MSH eruption, there was a near‐continuous, randomly occurring series of smaller seismic events. These subevents are not always simply small‐amplitude versions of the dominant LP class of events but appear instead to result from a separate random process only loosely coupled to the main LP source mechanism. We present an analysis of the interevent time and amplitude distributions of the subevents, using waveform cross correlation to separate LP events from the subevents. We also discuss seismic tremor that accompanied the 8 March 2005 phreatic explosion event at MSH. This tremor consists of a rapid succession of LPs and subevents triggered during the explosion, in addition to broadband noise from the sustained degassing. Immediately afterward, seismicity returned to the pre‐explosion occurrence pattern. This triggering in relation to the rapid ejection of steam from the system, and subsequent return to pre‐explosion seismicity, suggests that both seismic event types originated in a region of the subsurface hydrothermal system that was (1) in contact with the reservoir feeding the 8 March 2005 phreatic explosion but (2) not destroyed or drained by the explosion event. Finally, we discuss possible thermodynamic conditions in a pressurized hydrothermal crack that could give rise to seismicity. Pressure drop estimates for typical LP events are not generally large enough to perturb pure water in a shallow hydrothermal crack into an unstable state. However, dissolved volatiles such as CO2 may lead to a more unstable system, increasing the seismogenic potential of a hydrothermal crack subject to rapid heat flux. The interaction of hydrothermal and magmatic systems beneath MSH in 2004–2008 thus appears able to explain a wide range of observed phenomena, including subevents, LP events, larger (Md > 2) events, and phreatic explosions.
We present high‐broadband infrasound (0.01–100 Hz; 200‐Hz sample rate) observations of Vulcanian explosions at Popocatépetl volcano, Mexico. Popocatépetl is a highly active andesitic stratovolcano ...with regular violent explosions, making it a promising target for seismoacoustic observations. We deployed a four‐element broadband infrasound array (aperture 50 m) colocated with a compact broadband (120 s) seismometer at a site (ATLI) 15.8 km to the east‐southeast of Popocatépetl's summit. We highlight waveform examples from five powerful explosions during October to December 2017 that produced infrasound zero‐to‐peak pressure amplitudes ranging from 30 to 100 Pa at ATLI. The infrasound waveforms are highly asymmetric and are associated with clear air‐ground‐coupled arrivals on seismometers, with inverted vertical displacement waveforms tracking infrasonic pressure waveforms. Popocatépetl is close to major population centers, and array processing reveals persistent background infrasound from multiple directions, presumably of anthropogenic origin; our results have implications for infrasound monitoring at populated volcanoes.
Plain Language Summary
Seismology and acoustics are complementary methods for quantifying volcanic eruption processes, corresponding to elastic wavefields propagating through the solid Earth and acoustic wavefields propagating through the fluid atmosphere, respectively. Seismic data currently form the backbone of most volcano‐monitoring systems. Seismic signals at erupting volcanoes capture subsurface magma transport and rapid depressurization associated with explosive eruptions. Infrasound (acoustic waves with frequencies below 20 Hz, the lower‐frequency limit of human hearing) is a newer technology; infrasound data record subaerial degassing and allow physical quantification of explosive eruption mechanisms. Popocatépetl is one of the two most active volcanoes in Mexico (together with Volcán de Colima) and a prodigious source of explosive activity, making it an obvious target for combined seismic and infrasound (seismoacoustic) observations. We recorded continuous infrasound and seismic waveform data at a site 15.8 km to the east‐southeast of Popocatépetl for several months, capturing five powerful explosions. Our data were collected at a location where local people report hearing sounds associated with visual observations of explosions from Popocatépetl. Part of the motivation of this work is to investigate the capability of infrasound stations at distances greater than 5 km to monitor Popocatépetl with significantly reduced risk exposure to field personnel and instrumentation.
Key Points
Infrasound array study of Popocatépetl; high broadband (200‐Hz sample rate) includes sub‐bass range
Vulcanian explosions produce high‐amplitude asymmetric infrasound, which is air‐ground coupled
Inverted vertical displacement seismic waveforms track infrasonic pressure waveforms
The current eruption at Mount St. Helens is characterized by dome building and shallow, repetitive, long‐period (LP) earthquakes. Waveform cross‐correlation reveals remarkable similarity for a ...majority of the earthquakes over periods of several weeks. Stacked spectra of these events display multiple peaks between 0.5 and 2 Hz that are common to most stations. Lower‐amplitude very‐long‐period (VLP) events commonly accompany the LP events. We model the source mechanisms of LP and VLP events in the 0.5–4 s and 8–40 s bands, respectively, using data recorded in July 2005 with a 19‐station temporary broadband network. The source mechanism of the LP events includes: 1) a volumetric component modeled as resonance of a gently NNW‐dipping, steam‐filled crack located directly beneath the actively extruding part of the new dome and within 100 m of the crater floor and 2) a vertical single force attributed to movement of the overlying dome. The VLP source, which also includes volumetric and single‐force components, is 250 m deeper and NNW of the LP source, at the SW edge of the 1980s lava dome. The volumetric component points to the compression and expansion of a shallow, magma‐filled sill, which is subparallel to the hydrothermal crack imaged at the LP source, coupled with a smaller component of expansion and compression of a dike. The single‐force components are due to mass advection in the magma conduit. The location, geometry and timing of the sources suggest the VLP and LP events are caused by perturbations of a common crack system.
SUMMARY
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.
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 m3, 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 104 to 106 kg.
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
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
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.
We estimate the acoustic properties of a crack containing magmatic or hydrothermal fluids to quantify the source properties of long‐period (LP) events observed in volcanic areas assuming that a ...crack‐like structure is the source of LP events. The tails of synthetic waveforms obtained from a model of a fluid‐driven crack are analyzed by the Sompi method to determine the complex frequencies of one of the modes of crack resonance over a wide range of the model parameters α/a and ρf/ρs, where αis the P wave velocity of the rock matrix, a is the sound speed of the fluid, and ρf and ps are the densities of the fluid and rock matrix, respectively. The quality factor due to radiation loss (Qr) for the selected mode almost monotonically increases with increasing α/a, while the dimensionless frequency (v) of the mode decreases with increasing α/a and ρf/ρs. These results are used to estimate Q and v for a crack containing various types of fluids (gas‐gas mixtures, liquid‐gas mixtures, and dusty and misty gases) for values of a, ρf, and quality factor due to intrinsic losses (Qi) appropriate for these types of fluids, in which Q is given by Q−l = Qr−l + Qi−1. For a crack containing such fluids, we obtain Q ranging from almost unity to several hundred, which consistently explains the wide variety of quality factors measured in LP events observed at various volcanoes. We underscore the importance of dusty and misty gases containing small‐size particles with radii around 1 μm to explain long‐lasting oscillations with Q significantly larger than 100. Our results may provide a basis for the interpretation of spatial and temporal variations in the observed complex frequencies of LP events in terms of fluid compositions beneath volcanoes.
The seismicity of Popocatépetl is dominated by long‐period and very‐long period signals associated with hydrothermal processes and magmatic degassing. We model the source mechanism of repetitive ...long‐period signals in the 0.4–2 s band from a 15‐station broadband network by stacking long‐period events with similar waveforms to improve the signal‐to‐noise ratio. The data are well fitted by a point source located within the summit crater ∼250 m below the crater floor and ∼200 m from the inferred magma conduit. The inferred source includes a volumetric component that can be modeled as resonance of a horizontal steam‐filled crack and a vertical single force component. The long‐period events are thought to be related to the interaction between the magmatic system and a perched hydrothermal system. Repetitive injection of fluid into the horizontal fracture and subsequent sudden discharge when a critical pressure threshold is met provides a non‐destructive source process.
Key Points
Source properties quantification of long‐period signals at Popocatepetl volcano
Imagining seismic source mechanism associated with LP and VLP seismicity
LP and VLP seismicity obey interaction of magmatic and hydrothermal processes
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