Now showing 1 - 10 of 23
  • 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.
  • 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.
  • 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  482
  • Publication
    Long-period seismicity in the shallow volcanic edifice formed from slow-rupture earthquakes
    (Nature Publishing Group, 2013-12-15) ; ; ;
    Despite recent technological advances in volcano monitoring, eruption forecasting is still inadequate. Improved forecasting requires a deeper understanding of when unrest will lead to an actual eruption. Shallow Long Period (low spectral frequency) seismic events are routinely employed as a primary tool in forecasting strategies as they often precede eruptions. They are universally explained as resonating fluid-filled cracks or conduits, indicating the presence of mechanically active near-surface fluids. We undertake very high resolution seismic field experiments at Mt Etna, Italy; Turrialba, Costa Rica and Ubinas, Peru, in which we find that seismogram resonance is propagation path related whilst the seismic sources comprise short pulses. Data analysis and numerical modelling show that slow-rupture failure in unconsolidated volcanic materials reproduces all key aspects of these new observations. Contrary to current interpretations, here we show that our observed Long Period events are not direct indicators of fluid presence/migration, but rather are markers for upper edifice deformation. This finding encapsulates this seismicity within growing observations of a spectrum of deformation rates in other non-volcanic environments, from slow-slip earthquakes through fast dynamic rupture. It calls for a reassessment of how lowfrequency seismic signals are interpreted in their key role in eruption forecasting.
      1051Scopus© Citations 124
  • Publication
    Classification of long-term very long period (VLP) volcanic earthquakes at Whakaari/White Island volcano, New Zealand
    We have observed very long period earthquakes (VLPs) over the period 2007 to the end of 2019 at Whakaari/White Island volcano, New Zealand. The earthquakes exhibit similitude between waveforms which suggests repeating source locations and processes. VLPs recorded at two permanent stations were detected using waveform semblance and were then classified into two main families (F1 and F2) using a clustering analysis. The two families are characterized by ‘mirror image’ reverse waveform polarity suggesting that they are genetically related, but occur during different evolutionary phases of volcanic activity. F1 events occurred throughout the observation period, while F2 events mainly occurred as swarms that mark the onset of volcanic unrest. A detailed cluster analysis reveals possible sub-families implying slight temporal evolutions within a family. Our results add to our understanding of the volcanic magma–hydrothermal system at Whakaari/White Island indicating that relatively stable VLP sources may be exploited to improve monitoring for future unrest.[Figure not available: see fulltext.]
    Scopus© Citations 17  169
  • 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.
  • Publication
    Moment tensor inversion for the source location and mechanism of long period (LP) seismic events from 2009 at Turrialba volcano, Costa Rica
    Long-period (LP) seismic events were recorded during the temporary installation of a broadband seismic network of 13 stations from March to September 2009 on Turrialba volcano, Costa Rica. Over 6000 LPs were extracted using a modified STA/LTA method and a family consisting of 435 similar LP events has been identified. For the first time at Turrialba volcano, full-waveform moment tensor inversion is performed to jointly determine the location and source mechanism of the events. The LPs in the family are likely to be caused by crack mechanisms dipping towards the southwest at angles of approximately 10 to 20°, located at shallow depths (< 800 m) below the active Southwest and Central craters. As the locations are so shallow, the most probable causes of crack mechanisms are hydrothermal fluids resonating within or 'pulsing' through a crack. The waveforms observed at the summit stations suggest a 'pulsing' mechanism, but source resonance with a high degree of damping is also possible.
      495Scopus© Citations 15
  • Publication
    Helicopter vs. volcanic tremor: Characteristic features of seismic harmonic tremor on volcanoes
    We recorded high-frequency (> 10 Hz) harmonic tremor with spectral gliding at Hekla Volcano in Iceland. Particle motion plots indicated a shallow tremor source. We observed up to two overtones beneath our Nyquist frequency of 50 Hz and could resolve a source of closely spaced pulses of very short duration (0.03-0.1 s) on zoomed seismograms. Volcanic tremor with fundamental frequencies above 5 Hz, frequency gliding and/or repetitive sources similar to our observations were observed on different volcanoes around the world. However, this frequency content, duration and occurrence of volcano-related tremor was not observed in the last 35 years of seismic observations at Hekla. Detailed analysis reveals that this tremor was related to helicopters passing the volcano. This study relates the GPS track of a helicopter with seismic recordings of the helicopter at various distances. We show the effect the distance, number of rotor blades and velocity of the helicopter has on the observed up and down glidings at up to 40 km distance. We highlight similarities and differences between volcano-related and helicopter tremor in order to help avoid misinterpretations.
    Scopus© Citations 28  501
  • 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.
      147Scopus© Citations 1
  • Publication
    Source mechanism of long period events recorded by a high density seismic network during the 2008 eruption on Mount Etna
    129 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 degrees SE (Family 1) and 50 degrees NW (Family 2). For Family 1 events, the crack geometry is nearly orthogonal to the dike-like structure along which events are located, while for Family 2 the location gave two pipe-like bodies which 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 the 10th of May, 2008 as opposed to the flank lava flow.
    Scopus© Citations 34  336