ALBERT

Description: SUMMARY Tenerife (Canary Islands, Spain) is a volcanic island dominated by the Teide-Pico Viejo complex, with a summit height of 3718 m. After renewed signs of activity starting in 2004, an active seismic experiment was performed in 2007 to derive a tomographic model and identify seismic anomalies possibly associated with the magmatic system. To complement the tomography, a double beam-forming analysis is applied on two orthogonal 2-D profiles crossing the island to look for evidence of the existence of a magma chamber. Numerical tests allow us to investigate the best measure of coherency between traces, and show that the correlation and n th root semblance methods give better results than the classical semblance. They also demonstrate that the technique is reliable for locating scattering structures at depth, even when the velocity model is imperfect. Applying this technique to the Tenerife data set, two main anomalies can be identified: one at approximately 7–9 km b.s.l. depth in the northern part of the island, and one shallower (1–4 km b.s.l.) beneath the main summit. These structures could be linked to the magmatic system, in good agreement with previous studies. The shallowest one may be the phonolitic storage area feeding the Teide-Pico Viejo complex, while the deepest structure may be related to the basaltic system.

Description: ABSTRACT A passive seismic experiment was conducted in April/May 2010 in the Albertine Graben region in Uganda to record low-frequency seismic signals and explore the possibility of their exploitation in this area as a direct hydrocarbon indicator (DHI). Recordings were made at locations directly overlying both hydrocarbon and water-bearing strata within the sedimentary basin as well as reference sites external to the basin, directly on the basement. Contrary to findings published in some literature to date, we found that spatial variations in the analysed wavefield parameters correlate with the underlying geology rather than the presence or absence of hydrocarbons. Inversion of the surface-wave (fundamental mode) dispersion curve as well as the observed horizontal-to-vertical spectral ratio of both surface and body waves provide evidence that the observed spectral variations can be explained solely by a simple layered/gradient velocity model, without the presence of any kind of anomaly that could be attributed exclusively to a hydrocarbon reservoir. Consequently, it is recommended that knowledge of the geological and velocity structure is sought when analysing passive low-frequency seismic data sets. This is a fundamental prerequisite in order to guard against misinterpretation of the spatial variation of seismic derived attributes as DHIs.

Description: 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.

Description: Crust at many divergent plate boundaries forms primarily by the injection of vertical sheet-like dykes, some tens of kilometres long. Previous models of rifting events indicate either lateral dyke growth away from a feeding source, with propagation rates decreasing as the dyke lengthens, or magma flowing vertically into dykes from an underlying source, with the role of topography on the evolution of lateral dykes not clear. Here we show how a recent segmented dyke intrusion in the Baretharbunga volcanic system grew laterally for more than 45 kilometres at a variable rate, with topography influencing the direction of propagation. Barriers at the ends of each segment were overcome by the build-up of pressure in the dyke end; then a new segment formed and dyke lengthening temporarily peaked. The dyke evolution, which occurred primarily over 14 days, was revealed by propagating seismicity, ground deformation mapped by Global Positioning System (GPS), interferometric analysis of satellite radar images (InSAR), and graben formation. The strike of the dyke segments varies from an initially radial direction away from the Baretharbunga caldera, towards alignment with that expected from regional stress at the distal end. A model minimizing the combined strain and gravitational potential energy explains the propagation path. Dyke opening and seismicity focused at the most distal segment at any given time, and were simultaneous with magma source deflation and slow collapse at the Baretharbunga caldera, accompanied by a series of magnitude M 〉 5 earthquakes. Dyke growth was slowed down by an effusive fissure eruption near the end of the dyke. Lateral dyke growth with segment barrier breaking by pressure build-up in the dyke distal end explains how focused upwelling of magma under central volcanoes is effectively redistributed over long distances to create new upper crust at divergent plate boundaries.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Sigmundsson, Freysteinn -- Hooper, Andrew -- Hreinsdottir, Sigrun -- Vogfjord, Kristin S -- Ofeigsson, Benedikt G -- Heimisson, Elias Rafn -- Dumont, Stephanie -- Parks, Michelle -- Spaans, Karsten -- Gudmundsson, Gunnar B -- Drouin, Vincent -- Arnadottir, Thora -- Jonsdottir, Kristin -- Gudmundsson, Magnus T -- Hognadottir, Thordis -- Fridriksdottir, Hildur Maria -- Hensch, Martin -- Einarsson, Pall -- Magnusson, Eyjolfur -- Samsonov, Sergey -- Brandsdottir, Bryndis -- White, Robert S -- Agustsdottir, Thorbjorg -- Greenfield, Tim -- Green, Robert G -- Hjartardottir, Asta Rut -- Pedersen, Rikke -- Bennett, Richard A -- Geirsson, Halldor -- La Femina, Peter C -- Bjornsson, Helgi -- Palsson, Finnur -- Sturkell, Erik -- Bean, Christopher J -- Mollhoff, Martin -- Braiden, Aoife K -- Eibl, Eva P S -- England -- Nature. 2015 Jan 8;517(7533):191-5. doi: 10.1038/nature14111. Epub 2014 Dec 15.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland, IS-101 Reykjavik, Iceland. ; Centre for the Observation and Modelling of Earthquakes and Tectonics (COMET), School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK. ; GNS Science, PO Box 30368, Lower Hutt 5040, New Zealand. ; Icelandic Meteorological Office, IS-150 Reykjavik, Iceland. ; 1] Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland, IS-101 Reykjavik, Iceland [2] Icelandic Meteorological Office, IS-150 Reykjavik, Iceland. ; Canada Centre for Mapping and Earth Observation, Natural Resources Canada, 560 Rochester Street, Ottawa, Ontario K1A 0E4, Canada. ; Department of Earth Sciences, University of Cambridge, Madingley Road, Cambridge CB3 0EZ, UK. ; Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA. ; Department of Geosciences, The Pennsylvania State University, University Park, Pennsylvania 16802, USA. ; Department of Earth Sciences, University of Gothenburg, SE-405 30 Gothenburg, Sweden. ; Seismology Laboratory, School of Geological Sciences, University College Dublin, Belfield, Dublin 4, Ireland.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/25517098" target="_blank"〉PubMed〈/a〉

Description: We present a numerical investigation of the effect that static stress perturbations due to fluid injection have on a nearby active fault where the fluid does not come in physical contact with the fault. Our modelling employs a lattice Boltzmann pore diffusion model coupled with a quasi-dynamic earthquake rupture model. As diffusivities and frictional parameters can be defined independently at individual nodes/cells this allows us to replicate complex 3-D geological media in our simulations. We demonstrate the effect an injection can have on an active nearby fault. Compared with our control catalogue (identical to the original simulation but without the injection), the injection not only altered the timing of the next earthquake sequence, it also changed its size, producing a M w 6.7 event, the largest observed earthquake on the fault. This large event pushes the fault into a subcritical state from which it took roughly 200 yr of continuous tectonic loading for the fault to return to a critical state.

Description: Fogo volcano is an active central volcano, with a lake filled caldera, in the central part of São Miguel Island, Azores, whose current activity is limited to hydrothermal manifestations such as active fumarolic fields, thermal and CO2 cold springs and soil diffuse degassing areas. It is affected by important active tectonic structures, with high seismic activity and practically continuous micro-seismicity. A recurrent feature from the seismicity observed in volcanic regions is the occurrence of clusters of similar earthquakes, whose origin can be attributed to the repeated action of a similar source mechanism at the same focal area. Doublets/multiplets were identified in this study within a catalogue of small magnitude (usually 〈 3) volcano tectonic events recorded in 2003–2004 by a selection of stations around Fogo volcano. All events have been cross-correlated and pairs whose waveforms exhibited a cross-correlation coefficient equal to or higher than 0.9 were analysed using the coda-wave interferometry technique. Subtle velocity variations found between events highlight a seasonal cycle of the velocity patterns, with lower velocity in winter time and higher velocity during summer months. Those results, together with quantitative differences between the same doublets at different stations, exhibit an excellent correlation with rainfall. A seasonal effect can also be broadly seen in the seismicity occurrence, and some of the swarms recorded over the two year period occur during the wettest season or close to episodes of abundant (above average) rainfall. Moreover, temporal and spatial analysis of several swarms highlighted the lack of any mainshock–aftershock sequence and organized migration of the hypocenters. This is suggestive of a very heterogeneous stress field. Vp/Vs is found to be lower than usually observed in volcanic areas, an occurrence likely related to the presence of steamy fluid associated with the geothermal system. Taken together, these observations suggest that pore pressurisation plays a major role in controlling a considerable part of the recorded seismicity. The geothermal fluids around Fogo massif have been identified as derived from meteoric water, which infiltrates through Fogo Lake and the volcano flanks and flows from south to north on the northern flank. All those elements seem to point to a role played by rainfall in triggering seismicity at São Miguel, possibly through pressure changes at depth in response to surface rain and/or an interaction with the geothermal system.

Description: Published

Description: 231-246

Description: 1.4. TTC - Sorveglianza sismologica delle aree vulcaniche attive

Description: JCR Journal

Description: reserved

Description: Stress can undergo rapid temporal changes in volcanic environments, and this is particularly true during eruptions. We use two independent methods, coda wave interferometry (CWI) and shear wave splitting (SWS) analysis to track stress related wave propagation effects during the waning phase of the 2002 NE fissure eruption at Mt Etna. CWI is used to estimate temporal changes in seismic wave velocity, while SWS is employed to monitor changes in elastic anisotropy. We analyse seismic doublets, detecting temporal changes both in wave velocities and anisotropy, consistent with observed eruptive activity. In particular, syn-eruptive wave propagation changes indicate a depressurization of the system, heralding the termination of the eruption, which occurs three days later.

Description: Published

Description: 1779-1788

Description: 1.4. TTC - Sorveglianza sismologica delle aree vulcaniche attive

Description: JCR Journal

Description: reserved

Description: 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.

Description: Published

Description: L22316

Description: 1.4. TTC - Sorveglianza sismologica delle aree vulcaniche attive

Description: JCR Journal

Description: reserved

Description: 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 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 with the 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.

Description: In press

Description: 3.6. Fisica del vulcanismo

Description: JCR Journal

Description: open

Description: 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.

Description: Published

Description: 340-354

Description: 1.4. TTC - Sorveglianza sismologica delle aree vulcaniche attive

Description: JCR Journal

Description: reserved