Now showing 1 - 4 of 4
  • 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
      294Scopus© Citations 27
  • 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.
      262Scopus© Citations 10
  • Publication
    Relating gas ascent to eruption triggering for the April 27, 2016, White Island (Whakaari), New Zealand eruption sequence
    The April 27, 2016 eruption sequence at White Island was comprised of 6 discrete eruptive events that occurred over a 35-min period. Seismicity included three episodes of VLP activity: the first occurring ~ 2 h and a second occurring 10 min prior to the first eruption. A third larger VLP event occurred just prior to the fourth eruption. A VLP source depth of 800–1000 m below the vent is obtained from an analysis of the waveform semblance, and a volumetric source is obtained from waveform inversion of the largest VLP event. Lag times between VLP occurrence and eruption onsets provide an opportunity to examine gas migration and stress transfer models as potential triggers to the eruptive activity. Plausible lag times for a deep gas pulse to the surface are obtained by application of a TOUGH2 computational model which suggests propagation times of 0.25–1.9 m/s and are informed by previously measured White Island rock porosities and permeabilities. Results suggest that pre-eruption VLP may be plausibly linked to advection of gas from the VLP source at a magmatic carapace located ~ 800–1000 m depth. Alternatively, the large VLP that occurred just prior to the fourth eruption may be linked to a quasi-dynamic or quasi-static stress perturbation.
      288Scopus© Citations 33
  • Publication
    Micrometre-scale deformation observations reveal fundamental controls on geological rifting
    Many of the world's largest volcanic eruptions are associated with geological rifting where major fractures open at the Earth's surface, yet fundamental controls on the near-surface response to the rifting process are lacking. New high resolution observations gleaned from seismometer data during the 2014 Bároarbunga basaltic dyke intrusion in Iceland allow us unprecedented access to the associated graben formation process on both sub-second and micrometre scales. We find that what appears as quasi steady-state near-surface rifting on lower resolution GPS observation comprises discrete staccato-like deformation steps as the upper crust unzips through repetitive low magnitude (M < 0) failures on fracture patches estimated between 300 m and 1200 m in size. Stress drops for these events are one to two orders of magnitude smaller than expected for tectonic earthquakes, demonstrating that the uppermost crust in the rift zone is exceptionally weak.
      51Scopus© Citations 12