Very Long Period Events Research Articles

Tracking changes in magma transport from very-long-period seismic signals at Piton de la Fournaise volcano

Changes in magma properties and transport geometry can have a direct impact on volcanic activity. However, such variations can be difficult to track during eruptions. We report previously undetected very-long-period (VLP) signals at Piton de la Fournaise that can be used to probe changes in magma transport. Source analysis of VLP events during the August-October 2015 eruption indicates a source depth of about 0.9-1.2 km and points to the resonance of the magma dike feeding the eruption. The evolution of the resonance period reveals a shortening of the dike when the magma flux decreases at the end of the eruption. VLP events are actually quite frequent at Piton de la Fournaise: all eruptions analyzed in this study exhibit VLP signals that are indicative of rapid drops in lava discharge. This work encourages the detection of VLP signals to monitor changes in magma flow during volcanic eruptions, and anticipate the corresponding evolution in effusive activity.

Amplitude Source Location Method With Depth‐Dependent Scattering and Attenuation Structures: Application at Nevado del Ruiz Volcano, Colombia

AbstractThe amplitude source location (ASL) method, which uses high‐frequency amplitudes based on the assumption of isotropic radiation of S waves, was tested with envelope waveforms simulated from depth‐dependent scattering and attenuation structures at Nevado del Ruiz volcano, Colombia. Our test results indicate that estimated ASLs are more accurate when a two‐layer attenuation model is used than when a homogenous attenuation model is used, as has commonly been done in previous studies. We applied the ASL method with a two‐layer attenuation model to the high‐frequency (5‐10 Hz) band of volcanotectonic (VT) earthquakes, long‐period (LP) and very long period (VLP) events, and tremor recorded in 2016 at Nevado del Ruiz. Results indicate that the differences between ASLs and VT earthquake hypocenters (determined using onset arrival times) were smaller for a two‐layer model compared to a homogeneous model. Our determined ASLs for LP and VLP events and tremor were distributed along a NW‐dipping plane beneath the summit crater, near and above a region with high P to S wave velocity ratios that has been interpreted as shallow magma storage. This plane may represent weak zones along the Palestina fault system. Our results suggest that LP and VLP events and tremor occur during magma ascent along the fault system. This study demonstrates that the ASL method with a depth‐dependent attenuation model is useful to improve the accuracy of estimated source locations for volcanoseismic signals.

The 2007 caldera collapse of Piton de la Fournaise volcano: Source process from very-long-period seismic signals

In April 2007, Piton de la Fournaise volcano experienced its largest caldera collapse in at least 300 yr. This event consisted of a series of 48 subsidence increments characterized by very-long-period (VLP) seismic signals equivalent to Mw 4.4 to 5.4. Source analysis of VLP events suggests a piston-like mechanism with a collapsing crack source corresponding to the contraction of the magma reservoir and a single force representing the collapse of the above rock column. We show that the collapse dynamics is primarily controlled by magma withdrawal from the reservoir, which was likely in a bubbly state at the time of the event. Similar to the 2018 Kilauea collapse, we observe a reduction of the time-interval between successive subsidence increments, which results from the acceleration of magma withdrawal and a progressive weakening of the edifice at the beginning of the sequence.

Migration of Very Long Period Seismicity at Aso Volcano, Japan, Associated With the 2016 Kumamoto Earthquake

AbstractEarthquakes are known to precede volcanic activity, including long‐period or very long period (VLP) volcanic seismicity. However, the relationships among earthquakes, VLP seismicity, and volcanic eruptions are not well understood. Here we present the locations of VLP seismicity at Aso volcano, Japan, between January 2015 and December 2016, a period that includes the Mw 7.0 Kumamoto earthquake and phreatomagmatic eruptions. By using a differential‐time backprojection method developed in this study to accurately locate VLP events, we clearly identified two distinct VLP clusters. Whereas the eastern cluster was active during eruptions, the western cluster displayed intense VLP seismicity only for a few months after the earthquake. The western cluster may be associated with opening of new fractures during the earthquake. This study explores the mechanisms that can relate earthquake to volcanic activities and provides a new approach to analyze the dynamic behaviors inside volcanoes that may yield useful information for hazard evaluation.

Comparison of high- and low-frequency signal sources for very-long-period seismic events at Asama volcano, Japan

Very-long-period (VLP) seismic events at Asama volcano in central Japan are characterized by a transient signal of 10–20 s duration. Associated with the transient motion, a high-frequency (HF) oscillation within the 5–10 Hz band is observed. We investigated the location and size of the oscillation source in the HF band (HF source) using an amplitude source location method and compared our results with those for the VLP band (VLP source) deduced in our previous waveform inversion study. We analysed 1437 VLP events recorded during an intense observation campaign during 2008–2009, and additional 571 events surrounding an eruptive activity in 2015 (including a VLP event that immediately preceded an eruption). The HF source locations of most events were deeper than the VLP source locations by ∼150 m, although there was almost no time lag between the signals. This suggests a strong connection between the two sources, with one source immediately responding to the other. The eruptive VLP event had an HF source location close to normal VLP events but had a greater event size. We surmise that the HF signal is caused by an inflow of volcanic gas from depth into a semi-vertical crack-like cavity (as imaged in our previous waveform inversion study), and the VLP signal is caused by resultant inflation of the same cavity. The centroid of inflation (VLP source) is near the roof of the cavity, where gas accumulates, whereas the HF oscillation is emitted more intensely from lower and narrower portions of the cavity. This results in source depth differences between the two signal bands. In this model, the inflation rate of the cavity is controlled by the volume flux of the gas inflow, producing similar temporal variations between the VLP and HF signals. According to this model, the eruptive VLP is associated with a greater amount of gas inflow and accumulation, which likely played a key role in the eruption.

Origin of transient self‐potential signals associated with very long period seismic pulses observed during the 2000 activity of Miyakejima volcano

AbstractOrigin of the previously reported transient geoelectrical (self‐potential, SP) signals in the Miyakejima 2000 activity, that repeatedly occurred concurrently with very long period (VLP) seismic pulses, was investigated. SP waveforms stacked across repeated VLP events showed a step‐like rise followed by a gradual decay at all stations spread over the island of 8 km diameter. Within a realistic range of hydrological diffusivity, the short time constants of the SP signals cannot be explained by the electrokinetic effect caused by fluid flow within a limited volume, proposed earlier as a fluid injection hypothesis. On the other hand, poroelasticity predicts an island‐wide distributed flow field to occur almost instantaneously upon VLP events due to the step of strain field imposed by the mechanical event. We propose that the observed SP signals resulted from the streaming current by this island‐wide flow field. Our quantitative model, assuming a vertical tensile crack as a mechanical source, which has been suggested by preceding seismic studies, can explain the time constants and the amplitude of the SP signals (both spatial pattern and absolute amplitude), within a reasonable range of rock properties and the scalar moment of the mechanical source (VLP event). Location and attitude of the mechanical source were well constrained by grid search and are consistent with those estimated earlier from other types of data.

Variability in eruption style and associated very long period events at Fuego volcano, Guatemala

AbstractRepeated short‐term deployments of seismic, infrasound, video, and gas‐emission instruments at Fuego volcano, Guatemala have revealed three types of very long period (VLP) events associated with conduit sealing, pressure accumulation, and release. In 2008, ash‐rich explosions issued from a vent on the western flank and produced one type of VLP (Type 1). Impulsive, bomb‐rich explosions from the summit vent in 2009 produced a shorter period VLP (Type 2), but also generated ash release. Type 3 VLP events occurred during ash‐free exhalations from the summit in 2008 and had waveform shapes similar to Type 2 events. Weak infrasound records for Type 1 explosions compared to Type 2 suggest lower pressures and higher magma porosity for Type 1. Type 3 events correlate with spikes in SO2 emission rate and are driven by partial sealing and rapid release of ash‐free gas at the summit vent. Variations in the VLP period may provide a new tool for monitoring conditions within the conduit.

Intermittent inflations recorded by broadband seismometers prior to caldera formation at Miyake-jima volcano in 2000

Very-long-period (VLP) pulses with widths of 20s on velocity seismograms were observed during volcanic activity at Miyake-jima Volcano, Japan in 2000. The VLP events occurred repeatedly during a few days prior to caldera formation and essentially vanished following the onset of caldera collapse. Waveform inversions of the pulse-like signals point to a source offset 3.5km beneath and 1km south of the summit. A candidate for the source mechanism is the inflation of an elliptical cylinder with axis tilted 20–30° from vertical and major axis of the elliptical cross section oriented northeast-southwest. The inferred mechanism appears consistent with a step-like pressurization of a magma reservoir impacted by a falling rock mass in response to gravitational instability. The repeated occurrences of the rock collapses lead to the caldera formation at Miyake-jima.

Long period and very long period events at Mt. Etna volcano: Characteristics, variability and causality, and implications for their sources

Almost 50,000 long period (LP) events were recorded at Mt. Etna from November 2003 to May 2006. We analysed these events, as well as very long period (VLP) events which were associated with some of them. During some intervals the spectral and wavefield features of LP events remained steady, with significant changes occurring between the intervals. Based on the times of the changes, we distinguish five different sub-periods. In particular, during sub-period III (June–November 2005) the wavefield at the stations nearest to the summit area was composed of P waves. Locations for 150 LP events occurring in sub-period III, determined using radial semblance, changed, at the same time as the events' spectral features changed. It was during this sub-period that many of the LP events were associated with VLP events. Based on similarity of the waveforms, we distinguished two families of VLP events, with gradually evolving waveforms. The two families are located in slightly different places, but near the sources of the LP events. The change between the families occurred at the same time as the spectra of the LP events changed. Finally the source of the VLP events was investigated by performing complete waveform moment tensor inversion of stacks of the two families. Synthetic Green's functions for the full moment tensor were calculated for a homogeneous half-space. For both families, the source region with the highest variance reduction lies approximately beneath the active craters, at 500 m below the altitudes of the stations. The solutions for both families are very similar with sources that are between 60 and 70% isotropic. Attempts to determine deviatoric moment tensors produced consistently poorer fits. The remaining energy is poorly constrained and is likely to be noise. In conclusion, these results highlight changes in the LP and VLP events at Mt. Etna over time, and the causal relationship between them.

Changes in the VLP seismic source during the 2007 Stromboli eruption

Aim of this paper is to identify variations in Very-Long-Period (VLP) source associated with eruptive style changes at Stromboli volcano (Italy) and to retrieve information about the shallow plumbing system that sustains the eruptive activity. We have considered a dataset of 74493 VLP events recorded during the period from January through August 2007, when an effusive eruption occurred (February 27–April 2). We performed a polarization analysis of the entire dataset and divided the considered period into four sub-periods on the basis of polarization characteristics. We then located the events and selected a subset of these events by applying a location quality threshold. The high quality locations demonstrate that during the effusive eruption the VLP sources first moved downward and then moved southwestward. To retrieve information about the geometry of the structures where the source processes take place, we further consider a subset of events and estimate their source mechanisms by using a moment tensor source function (MTSF) inversion technique. Inversion of the waveforms of the VLP events that occurred on February 27 allows us to obtain information about the dynamics of different source centroids distributed along different portions of the shallow magmatic conduits. The structure defined by the locations and source mechanisms shows a greater complexity compared with previous studies and their time variations give an insight into the kinematics of the eruption.

Unsupervised Neural Analysis of Very-Long-Period Events at Stromboli Volcano Using the Self-Organizing Maps

We have implemented a method based on an unsupervised neural network to cluster the waveforms of very-long-period (VLP) events associated with explosive activity at the Stromboli volcano (southern Italy). Stromboli has several active vents in the summit area producing together more than 200 explosions/day. We applied this method to investigate the relationship between each vent and its associated VLP explosive waveform. We selected 147 VLP events recorded between November and December 2005, when digital infrared camera recordings were available. From a visual inspection of the infrared camera images, we classified the VLPs on the basis of which vent produced each explosion. We then applied the self-organizing map (SOM), an unsupervised neural technique widely applied in data exploratory analysis, to cluster the VLPs on the basis of their waveform similarity. Our analysis demonstrates that the most recurrent VLP waveforms are usually generated by the same vent. Some exceptions occurred, however, in which different waveforms are associated with the same vent, as well as different vents generating similar waveforms. This suggests that the geometry of the upper conduit-vent system plays a role in shaping the recurring VLP events, whereas occasional modest changes in the source process dynamics produce the observed exceptions.

Moment tensor inversion of very long period seismic signals from Strombolian eruptions of Erebus Volcano

Strombolian eruptions from the long-lived lava lake of Erebus volcano, Ross Island, Antarctica, generate repeating Very Long Period (VLP) signals, containing energy between approximately 30 and 5 s, that persist for several minutes and through the post-eruptive refilling of the lava lake. The initial approximately 10 s of this signal is moderately variable, particularly with respect to its initial polarity, while the following VLP coda has been observed to be stable since the earliest VLP observations were made (1996). To estimate forces and force couples consistent with the Erebus VLP signature, we perform moment tensor inversions for point sources using high signal-to-noise data stacks from the six-station, 18-component broadband seismographic network and Green's function forward calculations that incorporate topography. We infer a shallow (approximate depth of less than 400 m below the lava lake surface) source centroid that underlies the center to the northwestern rim of the main crater, east and north of the lava lake. Integrated M ii functions over the predominant (180 s) signal duration of VLP events show that the net scalar moments for these events are on the order of 4 × 10 13 N m (corresponding to a moment magnitude mw ≈ 3) for typical sized VLP events. Moment rate tensors which characterize force couple components are dominated (85–97% of variance) by dilatational components. Approximately 25% of the data variance is attributable to single forces that are attributable to oscillatory reaction forces caused by fluid transport, however, the relative contributions of vertical forces and couples with this sparse network is poorly resolved for these shallow sources. The generally high degree of repeatability in the VLP signal across thousands of eruptions over the past decade indicates that the response of the conduit system to gas slug ascent and subsequent gravitational disequilibrium is stable, consistent with the generally unchanging surface manifestation of the convecting lava lake system, and arguing for a thermally and dynamically stable conduit system beneath the lava lake.

Eruption dynamics at Mount St. Helens imaged from broadband seismic waveforms: Interaction of the shallow magmatic and hydrothermal systems

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.

Source process of very-long-period events accompanying long-period signals at Cotopaxi Volcano, Ecuador

Renewed seismic activity of Cotopaxi, Ecuador, began in January 2001 with the increased number of long-period (LP) events, followed by a swarm of volcano-tectonic (VT) earthquakes in November 2001. In late June 2002, the activity of very-long-period (VLP) (2 s) events accompanying LP (0.5–1 s) signals began beneath the volcano. The VLP waveform was characterized by an impulsive signature, which was accompanied by the LP signal showing non-harmonic oscillations. We observed temporal changes of both the VLP and LP signals from the beginning until September 2003: The VLP signal gradually disappeared and the LP signal characterized by decaying harmonic oscillations became dominant. Assuming possible source geometries, we applied a waveform inversion method to the observed waveforms of the largest VLP event. Our inversion and particle motion analyses point to volumetric changes of a sub-vertical crack as the VLP source, which is located at a depth of 2–3 km beneath the northeastern flank. The spectral analysis of the decaying harmonic oscillations of LP events shows frequencies between 2.0 and 3.5 Hz, with quality factors significantly above 100. The increased VT activity and deformation data suggest an intrusion of magma beneath the volcano. A release of gases with small magma particles may have repetitively occurred due to the pressurization, which was caused by sustained bubble growth at the magma ceiling. The released particle-laden gases opened a crack above the magma system and triggered the resonance of the crack. We interpret the VLP and LP events as the gas-release process and the resonance of the crack, respectively.

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 com- bines 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 receiv- ers. 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.

Broadband seismic signals associated with the 2000 volcanic unrest of Mount Bandai, northeastern Japan

We have observed volcano-tectonic (VT) earthquakes, volcanic tremor and very long-period events (VLPEs) associated with the volcanic unrest of Mount Bandai since 2000 using a broadband seismic network closely deployed around the volcano. VT earthquakes are characterized by high-frequency (>10 Hz) signals with clear onsets of P and S phases, whereas volcanic tremor is characterized by a long coda comprising frequencies ranging from a few hertz to more than 10 Hz. In contrast, waveforms of VLPEs consist of very long-period (∼10 s) signals preceded by short-period (<1 s) components. Hypocenters of the VT earthquakes are concentrated in two clusters at depths of 0–2 km beneath the summit of Mount Bandai and to the northwest of the volcano. Conversely, the hypocenters of the volcanic tremor and the source of the short-period signals associated with VLPEs are located in the gap between the two clusters of VT earthquakes. Since this gap lies near a low-resistivity zone detected by magnetotelluric soundings, the source mechanisms of the tremor and short-period VLPE precursors are considered to be related to magmatic fluids consisting of hot water and/or magma. The short-period source of VLPEs is inferred to trigger inflation and deflation at a depth of about 5 km beneath the volcano’s north-northwestern flank and to thereby excite the subsequent very long period signals. Our analysis of these broadband seismic signals clarifies the recent activity of magmatic fluid beneath Mount Bandai, a volcano previously quiescent for more than 110 years.

Broadband seismic experiment at Merapi Volcano, Java, Indonesia: very-long-period pulses embedded in multiphase earthquakes

Multiphase (MP) and low frequency (LF) earthquakes with spectral peak amplitudes at 3–4 and 1 Hz, respectively, are two common types of shallow volcanic earthquakes previously recognized at Merapi Volcano. Their mechanisms are poorly understood but MPs have been temporally associated with lava dome growth. We conducted a seismic experiment in January–February 1998, using four broadband seismographs to investigate the nature of seismic activity associated with dome growth. During our experiment, Merapi experienced mild dome growth with low-level seismic activity. We compare our data to that recorded on a local short-period (SP) network, with the following preliminary results. MP and LF events as recorded and classified on the short-period network instruments were recognized on the broadband network. Frequency spectrograms revealed similar patterns in the near summit region at widely separated broadband stations. Higher frequency spectra than previously recognized were identified for both MP and LF events, and were strongly attenuated as a function of radial distance from the source. Thus the spectral characteristics of these events as recorded on far-field stations are not fully indicative of the source processes. In particular, many events classified as LF-type appear to have much high frequency energy near the source. This aspect of these so-called LF earthquakes, and their association with very-long-period (VLP) pulses, suggests that many events identified in the far-field as LF events are in actuality a variety of the MP event and involve similar source processes. Broadband records indicated that simple large-amplitude VLP pulses were embedded in MP and LF wavetrains. From event to event these pulses were similar in their waveforms and had periods of 4 s. VLP events embedded in LF and MP earthquakes were located using particle motions. The epicenters were clustered in a central region of the dome complex, and preliminary source depths were within about 100 m of the dome surface, suggesting a source region deep within the dome or the uppermost conduit. A similar source location was established by study of MP high-frequency onsets. Our broadband data suggests that we have recorded both elastic seismic waves and a simple embedded pulse that is interpreted to represent a surface tilt at the seismometer site. The inferred tilt indicates an inflation and subsequent deflation, possibly caused by a gas pressure pulse or episodic shallow magma transport with stick-slip movement of the conduit wall.