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Lokmer, Ivan
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Lokmer, Ivan
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- 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 - PublicationRestrictedA review of seismic methods for monitoring and understanding active volcanoesA review of seismic methods for monitoring and understanding active volcanoes
44 7 - 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 - 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 - 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 Access2D Synthetic dataset of numerical simulations of long-period seismicity in a volcanic edifice and related sensitivity kernelsThis work describes the data used in the EPSL research article "Quantifying strong seismic propagation effects in the upper volcanic edifice using sensitivity kernels". The dataset is generated in order to investigate to what extent the seismic signals recorded on volcanoes are affected by near surface velocity structure. Data were calculated using the computational spectral elements scheme SPECFEM2D, where the wave propagation beneath Mount Etna volcano, Italy, was simulated in both homogeneous and heterogeneous models. The heterogeneous model comprises a low-velocity superficial structure (top several hundred meters) based on the previously published studies. Several different source mechanisms and locations were used in the simulations. The seismic wavefield was "recorded" by 15 surface receivers distributed along the surface of the volcano. The associated sensitivity kernels were also computed. These kernels highlight the region of the velocity model that affects the recorded seismogram within a desired time window. The text files describing the velocity models used in the simulations are also provided. The data may be of interest to volcano seismologists, as well as earthquake seismologists studying path effects and wave propagation through complex media.
30 10 - 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 - 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 - PublicationRestrictedTemporal evolution of long-period seismicity at Etna Volcano, Italy, and its relationships with the 2004–2005 eruption(2008-02-01)
; ; ; ; ;Lokmer, I.; Seismology and Computational Rock Physics Laboratory, School of Geological Sciences, University College Dublin, Belfield, Ireland ;Saccorotti, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia ;Di Lieto, B.; Dipartimento di Fisica “E. R. Caianiello”, Universita degli Studi di Salerno, Salerno, Italia ;Bean, C. J.; Seismology and Computational Rock Physics Laboratory, School of Geological Sciences, University College Dublin, Belfield, Ireland; ; ; Between December 2004 and August 2005, more than 50,000 long-period events (LP) accompanied by very-long period pulses (VLP) were recorded at Mt. Etna, encompassing the effusive eruption which started in September 2004. The observed activity can be explained by the injection of a gas slug formed within the magmatic column into an overlying cavity filled by either magmatic or hydrothermal fluids, thus triggering cavity resonance. Although a large number of LP events exhibit similar waveforms before the eruption, they change significantly during and after the eruption. We study the temporal evolution of the LP-VLP activity in terms of the source movement, change of the waveforms, temporal evolution of the dominant resonance frequencies and the source Q factor and changes in the polarization of the signal. The LP source locations before and after the eruption, respectively, do not move significantly, while a slight movement of the VLP source is found. The intensity of the LP events increases after the eruption as well as their dominant frequency and Q factor, while the polarization of the signals changes from predominantly transversal to pure radial motion. Although in previous studies a link between the observed LP activity and the eruption was not found, these observations suggest that such a link was established at the latter end of the eruptive sequence, most likely as a consequence of a reestablishment of the pressure balance in the plumbing system, after it was undermined due to the discharge of large amounts of resident magma during the eruption. Based on the polarization properties of the signal and geological setting of the area, a fluid-filled crack is proposed as the most likely source geometry. The spectral analysis based on the autoregressive-models (SOMPI) is applied to the signals in order to analyse the resonance frequencies and the source Q-factors. The results suggest water and basalt at low gas volume fraction as the most likely fluids involved in the source process. Using theoretical relations for the “slow waves” radiated from the fluid-filled crack, we also estimate the crack size for both fluids, respectively.322 15