Now showing 1 - 3 of 3
  • 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.]
      54Scopus© Citations 11
  • 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.
      257Scopus© Citations 33
  • 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
      284Scopus© Citations 27