Now showing 1 - 10 of 29
  • Publication
    Diffraction imaging of sedimentary basins: An example from the Porcupine Basin 
    iffraction imaging is the technique of separating diffraction energy from the source wavefield and processing it independently. As diffractions are formed from objects and discontinuities, or diffractors, which are small in comparison to the wavelength, if the diffraction energy is imaged, so too are the diffractors. These diffractors take many forms such as faults, fractures, and pinch-out points, and are therefore geologically significant. Diffraction imaging has been applied here to the Porcupine Basin; a hyperextended basin located 200km to the southwest of Ireland with a rich geological history. The basin has seen interest both academically and industrially as a study on hyperextension and a potential source of hydrocarbons. The data is characterised by two distinct, basin-wide, fractured carbonates nestled between faulted sandstones and mudstones. Additionally, there are both mass-transport deposits and fans present throughout the data, which pose a further challenge for diffraction imaging. Here, we propose the usage of diffraction imaging to better image structures both within the carbonate, such as fractures, and below.
      514
  • Publication
    Wave height quantification using land based seismic data with grammatical evolution
    Accurate, real time, continuous ocean wave height measurements are required for the initialisation of ocean wave forecast models, model hindcasting, and climate studies. These measurements are usually obtained using in situ ocean buoys or by satellite altimetry, but are sometimes incomplete due to instrument failure or routine network upgrades. In such situations, a reliable gap filling technique is desirable to provide a continuous and accurate ocean wave field record. Recorded on a land based seismic network are continuous seismic signals known as microseisms. These microseisms are generated by the interactions of ocean waves and will be used in the estimation of ocean wave heights. Grammatical Evolution is applied in this study to generate symbolic models that best estimate ocean wave height from terrestrial seismic data, and the best model is validated against an Artificial Neural Network. Both models are tested over a five month period of 2013, and an analysis of the results obtained indicates that the approach is robust and that it is possible to estimate ocean wave heights from land based seismic data.
      446
  • Publication
    Coseismic velocity variations caused by static stress changes associated with the 2001 Mw=4.3 Agios Ionis earthquake in the Gulf of Corinth, Greece
    (American Geophysical Union, 2010-07) ; ; ;
    The analysis of temporal variations in the seismic velocity across faults can be used to estimate in situ stress changes. Seismic velocity of propagation depends on the fault stiffness, which is a function of stress. The coda wave interferometry technique is applied to seven families of repeating earthquakes (multiplets) recorded on the southern shore of the Gulf of Corinth, Greece, to estimate high precision velocity changes in the Earth ’ s crust associated with the M w = 4.3 Agios Ioanis earthquake. Results show that the Agios Ioanis event causes a perturbation in elastic properties at seismogenic depth, resulting in a reduction of 0.2% in the seismic velocity. The results are not consistent with either damage induced by dynamic stresses nor a fluid transient origin. In contrast, both the spatial distribution and magnitude of the velocity perturbation correlate well with modeled static stress variations. This suggests that the measured changes in the mechanical properties of the seismogenic crust can be attributed to a change in static stress field associated with the M w = 4.3 Agios Ioanis earthquake. The velocity changes indicate an unclamping of the Pyrgaki fault at depth, which has local hazard implications
      283
  • Publication
    Quantifying strong seismic propagation effects in the upper volcanic edifice using sensitivity kernels
    In 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.
      153Scopus© Citations 2
  • Publication
    Persistent Shallow Background Microseismicity on Hekla Volcano, Iceland: A Potential Monitoring Tool
    Hekla is one of Iceland's most active volcanoes. Since 1970 it has erupted four times with a period of quiescence of 14 years since the last eruption. We detected persistent levels of background microseismicity with a temporary seismic network in autumn 2012. An amplitude based as well as an arrival-time based location method was applied to two populations of events and located them at shallow depths on the northern flank, close to the summit. This seismicity has not been identified previously by the permanent seismic network in Iceland as it is below its detection threshold. The detected events were either short, higher frequency events with distinct arrivals located beneath the summit on the northern flank of Hekla or longer, emergent, lower frequency events about 4 km northeast of the summit at 200¿400 m depth below the surface. Estimated moment magnitudes were MW = -1.1 to -0.1 and MW = -0.9 to -0.0 and local magnitudes ML = -0.5 to +0.3 and ML = -0.3 to +0.3, respectively. This seismicity does not show any correlation with gas output but is located at the steepest slopes of the edifice. Hence we suggest that the current shallow microseismicity at Hekla is structurally controlled. This offers a possible opportunity of using near summit microseismicity as a tool for monitoring emerging unrest at Hekla. Microseismicity rates will be very sensitive to small stress perturbations due to magma migration at depth. Currently in the absence of microseismicity monitoring, Hekla switches from apparently quiescent to fully eruptive on the order of only 1 h.
      441Scopus© Citations 6
  • Publication
    Separation and location of microseism sources
    Microseisms are ground vibrations caused largely by ocean gravity waves. Multiple spatially separate noise sources may be coincidentally active. A method for source separation and individual wavefield retrieval of microseisms using a single pair of seismic stations is introduced, and a method of back azimuth estimation assuming Rayleigh-wave arrivals of microseisms is described. These methods are combined to separate and locate sources of microseisms in a synthetic model and then applied to field microseismic recordings from Ireland in the Northeast Atlantic. It is shown that source separation is an important step prior to location for both accurate microseism locations and microseisms wavefield studies.
      459Scopus© Citations 7
  • Publication
    New observations of displacement steps associated with volcano seismic long-period events, constrained by step-table experiments
    (American Geophysical Union (AGU), 2015-05-28) ; ;
    Long-period (LP) volcano seismic events often precede volcanic eruptions and are viewed with considerable interest in hazard assessment. They are usually thought to be associated with resonating fluid-filled conduits although alternative models involving material failure have recently been proposed. Through recent field experiments, we uncovered a step-like displacement component associated with some LP events, outside the spectral range of the typically narrow-band analysis for this kind of event. Bespoke laboratory experiments with step tables show that steps of the order of a few micrometers can be extracted from seismograms, where long-period noise is estimated and removed with moving median filters. Using these constraints, we observe step-like ground deformation in LP recordings near the summits of Turrialba and Etna Volcanoes. This represents a previously unobserved static component in the source time history of LP events, with implications for the underlying source process.
      374Scopus© Citations 10
  • Publication
    Investigating the source characteristics of long-period (LP) seismic events recorded on Piton de la Fournaise volcano, La Réunion
    Magmatic and hydrothermal processes play a significant role in generating seismicity at active volcanoes. These signals can be recorded at the surface and can be used to obtain an insight into the volcano's internal dynamics. Long period (LP) events are of particular interest as they often accompany or precede volcanic eruptions, but they are still not well understood. Piton de la Fournaise volcano, La Réunion Island, is one of the most active volcanoes in the world however LP events are rarely recorded there. A seismic network of 20 broadband seismometers has been operational on Piton de la Fournaise volcano since November 2009. Between November 2009 and January 2011 the volcano erupted five times, but only 15 LP events were recorded. Three of these eruptions were preceded by LP events, and several LP events were recorded during an intrusive phase. A family of three repeating LP events exists within the dataset. In order to characterize these events we locate and perform moment tensor inversion on the LP family. The LP events are located within the summit crater at shallow depths (< 200 m below the surface). Inversions show that the source mechanism is best represented by a tensile crack with horizontal crack geometry. We also investigate the relationship between LP occurrence and eruptive characteristics (size of the eruption, deformation of the edifice, etc.), and we find that the events exist only during flank eruptions and can be generated by the activity of the hydrothermal system and/or by the deformation inside the crater.
    Scopus© Citations 13  612
  • Publication
    Statistical selection of the 'best' seismic source mechanisms from inversions of synthetic volcanic long-period events
    (American Geophysical Union, 2010-09) ; ;
    Inversions for the source mechanism of long‐period (LP) seismicity recorded on volcanoes have become increasingly common and are used to interpret fluid‐driven processes. The source mechanism considered for LP inversions usually consists of a symmetric moment tensor with and without single forces. Also, constrained inversions have been performed where one presumes a specific source geometry that reduces the degrees of freedom in the inversion. To select the correct solution from the different possible mechanisms, the Akaike information criterion (AIC) has been used. However, since AIC performs well only if the inverted model is close to the true model, we tested its ability to select the correct model in LP inversions. Using synthetic data sets generated on Mt. Etna, Italy, with a tomography velocity model and the Green’s functions calculated for a simplified, homogeneous velocity model, we have investigated (1) if any of the inversion source models can recover the true mechanism and (2) the ability of the Akaike and Bayesian information criteria (BIC) to select the correct model. Results show that in some cases it is possible to recover the mechanism but never the source magnitude and that the BIC is a better measure than the AIC in selecting the true source model, although in numerous cases both criteria fail to select the correct solution. Therefore, the BIC should be used as opposed to the AIC if it is necessary to select an appropriate source. Caution should be used when using the statistical measure in any seismic inversion application.
      294Scopus© Citations 14
  • Publication
    Time reverse location of seismic long-period events recorded on Mt Etna
    We present the first application of a time reverse location method in a volcanic setting, for a family of long-period events recorded on Mt Etna. Results are compared with locations determined using a full moment tensor grid search inversion and cross-correlation method. From June 18th to July 3rd, 2008, 50 broadband seismic stations were deployed on Mt Etna, Italy, in close proximity to the summit. Two families of long-period 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 long-period families we first applied the method to two synthetic datasets 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.
      662Scopus© Citations 43