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School of Geological Sciences, University College Dublin, Dublin, Ireland
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- PublicationRestrictedSource mechanism of long-period events recorded by a high-density seismic network during the 2008 eruption on Mount Etna(2011)
; ; ; ; ; ; ; ; ;De Barros, L.; School of Geological Sciences, University College Dublin, Dublin, Ireland ;Lokmer, I.; School of Geological Sciences, University College Dublin, Dublin, Ireland ;Bean, C. J.; School of Geological Sciences, University College Dublin, Dublin, Ireland ;O'Brien, G. S.; School of Geological Sciences, University College Dublin, Dublin, Ireland ;Saccorotti, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia ;Métaxian, J.-P.; LGIT, Université de Savoie-IRD-CNRS, Chambéry, France ;Zuccarello, L.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia ;Patanè, D.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia; ; ; ; ; ; ; One hundred twenty-nine long-period (LP) events, divided into two families of similar events, were recorded by the 50 stations deployed on Mount Etna in the second half of June 2008. During this period lava was flowing from a lateral fracture after a summit Strombolian eruption. In order to understand the mechanisms of these events, we perform moment tensor inversions. Inversions are initially kept unconstrained to estimate the most likely mechanism. Numerical tests show that unconstrained inversion leads to reliable moment tensor solutions because of the close proximity of numerous stations to the source positions. However, single forces cannot be accurately determined as they are very sensitive to uncertainties in the velocity model. Constrained inversions for a crack, a pipe or an explosion then allow us to accurately determine the structural orientations of the source mechanisms. Both numerical tests and LP event inversions emphasise the importance of using stations located as close as possible to the source. Inversions for both families show mechanisms with a strong volumetric component. These events are most likely generated by cracks striking SW–NE for both families and dipping 70° SE (family 1) and 50° NW (family 2). For family 1 events, the crack geometry is nearly orthogonal to the dikelike structure along which events are located, while for family 2 the location gave two pipelike bodies that belong to the same plane as the crack mechanism. The orientations of the cracks are consistent with local tectonics, which shows a SW–NE weakness direction. The LP events appear to be a response to the lava fountain occurring on 10 May 2008 as opposed to the flank lava flow.168 17 - PublicationOpen AccessQuantifying strong seismic propagation effects in the upper volcanic edifice using sensitivity kernelsIn volcanic environments, the correct interpretation of the signals recorded by a seismic station is critical for a determination of the internal state of the volcano. Those signals contain information about both the seismic source and the properties of the path travelled by the seismic wave. Therefore, understanding the path effect is necessary for both source inversions and geophysical investigation of the volcanoes' properties at depth. We present an application of the seismic adjoint methodology and sensitivity kernel analysis to investigate seismic wave propagation effects in the upper volcanic edifice. We do this by performing systematic numerical simulations to calculate synthetic seismograms in two-dimensional models of Mount Etna, Italy, considering different wave velocity properties. We investigate the relationship between different portions of a seismogram and different parts of the structural volcano model. In particular, we examine the influence of known near-surface low-velocity volcanic structure on the recorded seismic signals. Results improve our ability to understand path effects highlighting the importance of the shallowest velocity structure in shaping the recorded seismograms and support recent studies that show that, although long-period seismic events are commonly associated with magma movements in resonant conduits, these events can be reproduced without the presence of fluids. We conclude that edifice heterogeneities impart key signatures on volcano seismic traces that must be considered when investigating volcano seismic sources.
29 10 - PublicationOpen AccessAn investigation of high frequency seismic tremor on Mt Etna(Copernicus {GmbH}, 2023-04-23)
; ; ; ; ; ; ; ; ; ; ;; ; High frequency seismic data (> 10 Hz) on volcanoes have traditionally been less studied as precursory seismicity to volcanic eruptions is dominated by lower frequency signals. However, inspection of newly acquired data during a field campaign between July and September 2022 from individual high sampling rate seismic stations on Mt. Etna reveals the presence of high frequency (10-90Hz) signals, which are poorly understood. In an attempt to determine their location, mechanisms and wavefield properties, we deployed 104 nodal seismic sensors, mainly in 6 tuned circular array configurations consisting of several rings with increasing radius and number of nodes per ring around a central station. The nodes record at a sampling rate of 250Hz (125Hz Nyquist) and the frequency content of the recorded seismicity shows signals up to about 100 Hz. In addition to the high frequency nodes, we also deployed a profile consisting of 11 elements (infrasound, short period) as well as four broad band sensors.A variety of signals were recorded, with coherent signals on different stations across the full spectral range. Thus far initial multi-array beamforming has been applied to the data, demonstrating a range of locations which varies depending on the frequency range looked at. Whilst sources near the summit region are most common (especially at frequencies below 5 Hz), there are also other locations from which tremor emanates, opening questions about their origin. Comparisons with infrasound, gas and weather data are ongoing, in an effort to shed light on the sources of these unusual signals.16 1 - PublicationOpen AccessSource geometry from exceptionally high resolution Long Period event observations at Mt Etna during the 2008 eruption.(2009-11-30)
; ; ; ; ; ; ; ; ;De Barros, L.; School of Geological Sciences, University College Dublin, Belfield, Dublin 4 ;Bean, C. J.; School of Geological Sciences, University College Dublin, Belfield, Dublin 4 ;Lokmer, I.; School of Geological Sciences, University College Dublin, Belfield, Dublin 4 ;Saccorotti, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia ;Zuccarello, L.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia ;O'Brien, G. S.; School of Geological Sciences, University College Dublin, Belfield, Dublin 4 ;Metaxian, J. P.; Universite de Savoie-IRD-CNRS, 73376 Chambery, France ;Patanè, D.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia; ; ; ; ; ; ; During the second half of June, 2008, 50 broadband seismic stations were deployed on Etna volcano in close proximity to the summit, allowing us to observe seismic activity with exceptionally high resolution. 129 Long Period events (LP) with dominant frequencies ranging between 0.3 and 1.2 Hz, were extracted from this dataset. These events form two families of similar waveforms with different temporal distributions. Event locations are performed by cross-correlating signals for all pairs of stations in a two-step scheme. In the first step, the absolute location of the centre of the clusters was found. In the second step, all events are located using this position. The hypocentres are found at shallow depths (0 to 700 m deep) below the summit craters. The very high location resolution allows us to detect the temporal migration of the events along a dike-like structure and 2 pipe shaped bodies, yielding an unprecedented view of some elements of the shallow plumbing system at Mount Etna. These events do not seem to be a direct indicator of the ongoing lava flow or magma upwelling.125 170 - PublicationRestrictedAn integrated method to model volcanic processes and associated geophysical signals(2009)
; ; ; ; ; ; ; ; ;Vassalli, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia ;Longo, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia ;Montagna, C. P.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia ;O'Brien, G. S.; School of Geological Sciences, University College Dublin, Belfield, Dublin 4, Ireland ;Bean, C. J.; School of Geological Sciences, University College Dublin, Belfield, Dublin 4, Ireland ;Bisconti, L.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia ;Papale, P.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia ;Saccorotti, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia; ; ; ; ; ; ; We present a numerical approach for modelling the complex sub-surface volcanic processes and associated geophysical signals. This method is based on the one-way coupling of the dynamics of a magmatic system and the response of the host rocks. The two systems are modelled independently, by two different numerical codes, that solve the equations of motion for the magmatic fluid and the equation of elasto-dynamics for wave propagation in the surrounding medium, respectively. Synthetic geophysical signals can be obtained and compared with those recorded by monitoring networks. The final aim is to understand how the complex physics of magma dynamics, coupled to its hosting medium, translates into geophysical data that can be measured and interpreted in order to understand sub-surface magma dynamics and forecast the short-term volcanic hazard. We applied this method to the Campi Flegrei volcanic system (southern Italy) and investigated the convection and mixing dynamics induced by the arrival of new CO2-rich magma into a hypothetical shallow magma chamber. The pressure waves originated by this system are propagated in the surrounding rocks, and the associated broad-band ground displacement and gravity anomalies are evaluated at the Earth’s surface.266 64 - PublicationRestrictedAnalysis of sustained long-period activity at Etna Volcano, Italy(2007-02-15)
; ; ; ; ; ;Saccorotti, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia ;Lokmer, I.; School of Geological Sciences, University College Dublin, Dublin, Ireland ;Bean, C. J.; School of Geological Sciences, University College Dublin, Dublin, Ireland ;Di Grazia, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia ;Patanè, D.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia; ; ; ; Following the installation of a broadband network on Mt. Etna, sustained Long-Period (LP) activity was recorded accompanying a period of total quiescence and the subsequent onset of the 2004–2005 effusive episode. From about 56000 events detected by an automatic classification procedure, we analyse a subset of about 3000 signals spanning the December 17th, 2003–September 25th, 2004, time interval. LP spectra are characterised by several, unevenly-spaced narrow peaks spanning the 0.5–10 Hz frequency band. These peaks are common to all the recording sites of the network, and different from those associated with tremor signals. Throughout the analysed time interval, LP spectra and waveforms maintain significant similarity, thus indicating the involvement of a non-destructive source process that we interpret in terms of the resonance of a fluid-filled buried cavity. Polarisation analysis indicates radiation from a non-isotropic source involving large amounts of shear. Concurrently with LP signals, recordings from the summit station also depict Very-Long-Period (VLP) pulses whose rectilinear motion points to a region located beneath the summit craters at depths ranging between 800 and 1100 m beneath the surface. Based on a refined repicking of similar waveforms, we obtain robust locations for a selected subset of the most energetic LP events from probabilistic inversion of travel-times calculated for a 3D heterogenous structure. LP sources cluster in a narrow volume located beneath the summit craters, and extending to a maximum depth of ≈ 800 m beneath the surface. No causal relationships are observed between LP, VLP and tremor activities and the onset of the 2004–2005 lava effusions, thus indicating that magmatic overpressure played a limited role in triggering this eruption. These data represent the very first observation of LP and VLP activity at Etna during non-eruptive periods, and open the way to the quantitative modelling of the geometry and dynamics of the shallow plumbing system.217 27 - PublicationRestrictedMoment-tensor inversion of LP events recorded on Etna in 2004 using constraints obtained from wave simulation tests(2007-11-29)
; ; ; ; ;Lokmer, I.; Seismology and Computational Rock Physics Laboratory, School of Geological Sciences, University College Dublin, Dublin, Ireland ;Bean, C. J.; Seismology and Computational Rock Physics Laboratory, School of Geological Sciences, University College Dublin, Dublin, Ireland ;Saccorotti, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia ;Patanè, D.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia; ; ; The persistent occurrence of long period (LP) events at Mt Etna became apparent with the installation of the first fixed broad-band seismic network in late 2003. Repeating similar LP events from Nov. ‘03 to Sept. ‘04 indicate a non-destructive source process. We perform moment tensor (MT) inversions on a stacked high S/N ratio representative LP signal, conducting a grid search for the source geometry and L2-inversion for the source time function. Results indicate a NNW-SSE oriented resonating sub-vertical crack as the most probable source. This result is consistent with deformation and GPS observations. Crucial to this result are constraints imposed by detailed 3D full waveform numerical simulations in a heterogeneous tomographic model with topography, and in particular a detailed assessment of the influence of very near surface velocity structure on LP signals. Pulsating gas injection is hypothesised as the most likely LP trigger.127 17 - PublicationOpen AccessCharacterisation and locations of volcanic high frequency tremor above 10 Hz on Mount Etna(2024-04)
; ; ; ; ; ; ; ; ; ; ; ;; ; ;When it comes to volcanic tremor, low frequency signals (below 5 Hz) are well investigated. Such tremor signals can usually be linked to magma movement or gas fluctuations. However, little is known about seismic tremor signals on Mount Etna above 10 Hz. Hence, a large field campaign targeting high frequencies was undertaken in the summer of 2022. It consisted of the deployment of six dense circular arrays ranging from 30 to 200 m apertures of seismic nodes installed around the summit craters. It led to the detection of tremor bands between 10 and around 20 Hz as well as the typical tremor signals below 5 Hz. The tremor is detected with good coherency at stations within one array (despite an extreme level of scattering) in good agreement with the energy distribution in the average amplitude spectra of the array. The high frequency tremor varies strongly in intensity over time periods of one hour and re-occurs several times throughout the deployment period of almost a week. In contrast the tremor below 5 Hz is relatively constant. This suggests that the high frequency tremor could be a separate signal due to a process that may not yet be fully understood. Localisations of these tremor episodes point to or near the Bocca Nuova Summit Crater which was actively degassing at the time. Interestingly, high frequency seismic tremor is matched in time very well by a narrow 3.5-5 Hz acoustic band. While the match in time clearly suggests a connection between the two signals, the different frequencies indicate two different but linked processes happening simultaneously. The acoustic signal implies degassing processes. Later during the deployment tremor episodes are found which are accompanied by much weaker acoustic signals (if at all present) suggesting gases might not necessarily be involved in generating the detected seismic tremor at all. In summer 2023 we undertook a complementary second deployment of seismic, acoustic and optical camera data in the Bocca Nuova summit area. Once again, we find tremor below 5 Hz, however high frequency characteristics are different to the previous year with tremor bands less dominant than before and much more constant over time. More than one acoustic band is found as well, also constant over time. In this second data set we use camera recordings of the crater activity as a proxy for degassing activity to try and understand the precise origin of these seismic and acoustic volcanic signals.12 1 - PublicationRestrictedTime reverse location of seismic long-period events recorded on Mt Etna(2011-01)
; ; ; ; ; ; ; ;O’Brien, G. S.; School of Geological Sciences, University College Dublin, Dublin, Ireland ;Lokmer, I.; School of Geological Sciences, University College Dublin, Dublin, Ireland ;De Barros, L.; School of Geological Sciences, University College Dublin, Dublin, Ireland ;Bean, C. J.; School of Geological Sciences, University College Dublin, Dublin, Ireland ;Saccorotti, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia ;Metaxian, J. P.; LGIT, Universite de Savoie, IRD, CNRS, Chambery, France ;Patanè, D.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia; ; ; ; ; ; We present the first application of a time reverse location method in a volcanic setting, for a family of long-period (LP) events recorded on Mt Etna. Results are compared with locations determined using a full moment tensor grid search inversion and cross-correlation method. From 2008 June 18 to July 3, 50 broad-band seismic stations were deployed on Mt Etna, Italy, in close proximity to the summit. Two families of LP events were detected with dominant spectral peaks around 0.9 Hz. The large number of stations close to the summit allowed us to locate all events in both families using a time reversal location method. The method involves taking the seismic signal, reversing it in time, and using it as a seismic source in a numerical seismic wave simulator where the reversed signals propagate through the numerical model, interfere constructively and destructively, and focus on the original source location. The source location is the computational cell with the largest displacement magnitude at the time of maximum energy current density inside the grid. Before we located the two LP families we first applied the method to two synthetic data sets and found a good fit between the time reverse location and true synthetic location for a known velocity model. The time reverse location results of the two families show a shallow seismic region close to the summit in agreement with the locations using a moment tensor full waveform inversion method and a cross-correlation location method.144 21 - PublicationRestrictedA review of seismic methods for monitoring and understanding active volcanoesA review of seismic methods for monitoring and understanding active volcanoes
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