Now showing 1 - 10 of 23
  • Publication
    Efficacy of Seismic Interferometry in Removing Surface Waves from Active Seismic Records
    (Copernicus, 2021-04-30) ;
    While there are seismic techniques which make use of surface waves in imaging the subsurface, there are also those where these types of waves are considered coherent noise. Important examples where the surface waves may significantly degrade the obtained images include different types of reflection seismic surveys (e.g., shallow surveys for engineering, environmental and groundwater investigations, and deep surveys for imaging hydrocarbon reservoirs). In a strongly heterogeneous medium (encountered typically in onshore surveys), the conventional methods for attenuating these waves (such as f-k "velocity" filtering) often do not give satisfactory results.
      150
  • Publication
    2D Synthetic dataset of numerical simulations of long-period seismicity in a volcanic edifice and related sensitivity kernels
    This 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.
      159Scopus© Citations 1
  • 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.
    Scopus© Citations 2  117
  • 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.
      453
  • Publication
    The state of stress in the shallow crust of the Hikurangi Subduction Margin hangingwall, New Zealand
    Knowledge of in situ stress fields is critical for a better understanding of deformation, faulting regime, and earthquake processes in seismically active margins such as the Hikurangi Subduction Margin (HSM), North Island, New Zealand. In this study, we utilize Leak-off Test (LOTs) data, borehole breakout widths measured from borehole image logs, and rock unconfined compressive strengths (UCS) derived from empirical P-wave velocity log relationships to estimate vertical (Sv), minimum (Shmin), and maximum horizontal stress magnitudes (SHmax) and interpret the likely faulting regime experienced in four boreholes (Kauhauroa-2, Kauhauroa-5, Titihaoa-1, and Tawatawa-1). Using the standard Anderson’s stress regime classification, relative stress magnitudes in Kauhauroa-5 at 1200-1700 m depth and Kauhauroa-2 at 1800-2100 m and indicate that the stress state in the shallow crust of the central and northern part of HSM is predominantly strike-slip (SHmax≥Sv≥Shmin) and normal Sv≥SHmax> Shmin respectively. Moving to the offshore, southern HSM a dominant compressional stress regime (SHmax> Shmin >Sv), with some possible strike slip stress states are observed in Titihaoa-1 from 2240-2660 m and Tawatawa-1 from 750-1350 m. The observed normal/strike-slip stress state in Kauhauroa-2 and Kauhauroa-5 is consistent with the average SHmax orientation of 64° ± 18° (NE-SW) determined from borehole breakouts and dominantly NE–SW striking normal faults interpreted from seismic reflection data. The normal/ strike-slip regime in this area suggests that the stress regime here is probably influenced by the effect of the clockwise rotation of the HSM hangingwall associated with oblique Pacific-Australia plate convergence (ENE-WSW). Alternatively, these stress states could be the result of gravitational collapse due to rapid uplift of the subducting plate during the mid-Miocene. The compressional stress regime in the southern HSM in Titihaoa-1 and Tawatawa-1 is in agreement with the SHmax orientations of 148° ± 14° (NW-SE ) and 102° ± 16° (WNW-ESE) obtained from image logs and mapped NE–SW striking reverse faults in this region. This observation suggests that the tectonics here are strongly linked to the subduction of Hikurangi plateau under Australian Plate (NW-SE) or active frontal thrusts in the overriding plate.
      222
  • Publication
    Helicopter location and tracking using seismometer recordings
    We use frequency domain methods usually applied to volcanic tremor to analyse ground based seismic recordings of a helicopter. We preclude misinterpretations of tremor sources and show alternative applications of our seismological methods. On a volcano, the seismic source can consist of repeating, closely spaced, small earthquakes. Interestingly, similar signals are generated by helicopters, due to repeating pressure pulses from the rotor blades. In both cases the seismic signals are continuous and referred to as tremor. As frequency gliding is in this case merely caused by the Doppler effect, not a change in the source, we can use its shape to deduce properties of the helicopter and its flight path. We show in this analysis that the number of rotor blades, rotor revolutions per minute (RPM), helicopter speed, flight direction, altitude and location can be deduced from seismometer recordings. Access to GPS determined flight path data from the helicopter offers us a robust way to test our location method.
    Scopus© Citations 12  490
  • Publication
    Simulation of High-Frequency Rotational Motion in a Two-Dimensional Laterally Heterogeneous Half-Space
    The seismic waves responsible for vibrating civil engineering structures undergo interference, focusing, scattering, and diffraction by the inhomogeneous medium encountered along the sourceto-site propagation path. The subsurface heterogeneities at a site can particularly alter the local seismic wave field and amplify the ground rotations, thereby increasing the seismic hazard. The conventional techniques to carry out full wave field simulations (such as finite-difference or spectral finite element methods) at high frequencies (e.g., 15 Hz) are computationally expensive, particularly when the size of the heterogeneities is small (e.g., <100 m). This study proposes an alternative technique that is based on the first-order perturbation theory for wave propagation. In this technique, the total wave field due to a particular source is obtained as a superposition of the ‘mean’ and ‘scattered’ wave fields. Whereas the ‘mean’ wave field is the response of the background (i.e., heterogeneity-free) medium due to the given source, the ‘scattered’ wave is the response of the background medium excited by fictitious body forces. For a two-dimensional laterally heterogeneous elastic medium, these body forces can be conveniently evaluated as a function of the material properties of the heterogeneities and the mean wave field. Since the problem of simulating high-frequency rotations in a laterally heterogeneous medium reduces to that of calculating rotations in the background medium subjected to the (1) given seismic source and (2) body forces that mathematically replace the small-scale heterogeneities, the original problem can be easily solved in a computationally accurate and efficient manner by using the classical (analytical) wavenumber-integration method. The workflow is illustrated for the case of a laterally heterogenous layer embedded in a homogeneous half-space excited by plane bodywaves.
      175
  • 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.
    Scopus© Citations 10  354
  • Publication
    Tremor-rich shallow dyke formation followed by silent magma flow at Bárðarbunga in Iceland
    The Bárðarbunga eruption in Iceland in 2014 and 2015 produced about 1.6 km3 of lava. Magma propagated away from Bárðarbunga to a distance of 48 km in the subsurface beneath Vatnajökull glacier, emerging a few kilometres beyond the glacier's northern rim. A puzzling observation is the lack of shallow (<3 km deep), high-frequency earthquakes associated Q.1 with shallow dyke formation near the subaerial and subglacial eruptive sites, suggesting that near-surface dyke formation is seismically quiet. However, seismic array observations and seismic full wavefield simulations reveal the presence and nature of shallow, pre-eruptive, long-duration seismic tremor activity. Here we use analyses of seismic data to constrain therelationships between seismicity, tremor, dyke propagation and magma flow during the Bárðarbunga eruption. We show that although tremor is usually associated with magma flow in volcanic settings, pre-eruptive tremor at Bárðarbunga was probably caused by swarms of microseismic events during dyke formation, and hence is directly associated with fracturing of the upper 2-3 km of the crust. Subsequent magma flow in the newly formed shallow dyke was seismically silent, with almost a complete absence of seismicity or tremor. Hence, we suggest that the transition from temporarily isolated, large, deep earthquakes to many smaller, shallower, temporally overlapping earthquakes (< magnitude 2) that appear as continuous tremor announces the arrival of a dyke opening in the shallow crust, forming a pathway for silent magma flow to the Earth's surface. 
      778Scopus© Citations 36
  • Publication
    Insights into fluid transport mechanisms at White Island from analysis of coupled very long-period (VLP), long-period (LP) and high-frequency (HF) earthquakes
    The August 2012 to October 2013 White Island unrest sequence included 5 explosive volcanic eruptions and emplacement of a small dome. These events were linked to an overall increase in SO2 and H2S gas flux and RSAM seismic tremor which began in late 2011. Prior to this unrest, a small swarm of 25 events was observed on 19–21 August 2011 and captured on a temporary seismic array including 14 broadband sensors. Each event comprised coupled pulses having distinct high frequency (HF = 2–5 Hz), long-period (LP = 0.5–1.1 Hz) and very long period (VLP = 0.03–0.125 Hz or 8–30 s) earthquakes. For each coupled event, we compute the source locations, origin times and related uncertainties by application of standard arrival time locations for the HF earthquakes and waveform semblance for the LP and VLP earthquakes. Results suggest that the events are centred beneath the active vent at depths generally < 1.5 km. The HF and LP earthquakes have shallow depths (< 1 km), while VLP have slightly deeper source locations (0.8–1.5 km). Emergent onsets for LP and VLP sources make an analysis of the absolute origin times problematic but waveform matching of VLP to LP and HF components suggests that the main VLP pulse precedes the HF and LP source processes. Waveform inversion for the VLP source is consistent with the rupture of a high angle East-West oriented crack opening either in a purely tensile or shear-tensile manner. The moment of the isotropic component is estimated at 1.2 × 1012 Nm and the corresponding volumetric change is in the range 145–450 m3. Results are interpreted as an upward migration of fluids which first excite the VLP from a high angle crack in the magma carapace followed by the excitation of LP and HF source processes in the overlying hydrothermal system
    Scopus© Citations 31  339