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Métaxian, Jean-Philippe
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- PublicationOpen AccessStructure of Masaya and Momotombo volcano, Nicaragua, investigated with a temporary seismic network(2019-05)
; ; ; ; ; ; ; ; ; ; ; Since the end of 2013, the region around the two volcanoes Masaya and Momotombo, which includes the Nicaraguan capital Managua, has shown an unusually high seismic and volcanic activity. In December 2015, the Momotombo volcano erupted after 110 years of quiescence. Since mid-December 2015, the Masaya volcano has also shown gradually increasing activity, including the formation of a lava lake in its main crater. By adding 30 broadband stations, we had temporarily (December 2016–March 2017) densified the permanent Nicaraguan seismic network around these volcanoes to study the local seismicity and image the subsurface struc- ture. During the observation period, we observed an overall low level of seismicity. Recorded events around Momotombo likely consist of aftershocks of the M5.5 earthquake that struck this area on September, 26th, 2016. At Masaya, we did not observe volcano-tectonic events. Using the continuous waveform recordings, we perform a 3D ambient seismic noise tomography that reveals a first image of the subsurface velocity structure below the Masaya and Momotombo volcanoes. While Momotombo shows a typical elongated low shear-wave velocity anomaly that reaches depths of about 8 km, Masaya does not show indications of a deep plumbing system. At Masaya, we have indications of a shallow (0–3 km) magmatic chamber, offset to the west and not directly below the active Santiago vent, where the crater lake is located At greater depth (3–8 km) a low velocity anomaly towards the northeast coincides in location with a modelled positive gravity anomaly and could indicate the presence of a former intrusive body. With this study we want to trigger further interest in the diverse tectonic and volcanic features of Nicaragua. Future, long-term seismic imaging and monitoring projects are of critical interest for the estimation of seismic and volcanic risks in Managua and the surroundings.643 262 - 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 - 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 - 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