Tilt and translation of rapid volumetric deformations at volcanoes

Abstract

The August 2012 to October 2013 White Island unrest sequence included 5 explosive volcanic eruptions and the emplacement of a small dome. These events were linked to an overall increase in SO2 and H2S gas and the seismic tremor which began in late 2011. Before this eruption period, a small swarm of 25 events was observed on 19-21 August 2011 and captured on a temporary seismic network including 14 sensors. Each earthquake included linked sub-earthquakes having distinct high frequency (HF =2-5 Hz), long-period (LP = 0.5-1.1 Hz) and very long period (VLP = 0.04-0.125 Hz or 8-25 s) seismic signals.

For each linked set of earthquake, we found the locations, starting times by using standard location methods. Results suggest that the events are centred beneath the active vent at depths generally less than 1.5 km. The HF and LP earthquakes have shallow depths (less than1 km), while VLP have slightly deeper source locations (0.8-1.5 km). An analysis of the onset of the waveforms suggest that the main VLP pulse precedes the HF and LP part of the earthquake.

Waveform inversion for the VLP source is consistent with the rupture of a steep East-West oriented crack opening either as a crack with a gap or a crack with a gap that also has slippage between its two sides. The moment (or size) of the isotropic component is estimated at 1.2x1012 Nm and the corresponding volumetric change is about 145-450 m3. The inversion analysis was done with and without the effect of tilting and this change in the model produced modest changes of the source motion and moment. The tilting results are poorly constrained due to the significant ocean and wind noise in the data. The results are interpreted as an upward migration of fluids which first excite the VLP from a high angle crack in the hot magma, followed by the excitation of LP and HF source processes in the overlying hydrothermal system. 

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Technical Abstract

The August 2012 to October 2013 White Island unrest sequence included 5 explosive volcanic eruptions and the 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 process. 

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 1.2x1012 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. 

 

Insights into fluid transport mechanisms at White Island from analysis of coupled very long-period (VLP), long-period (LP) and high-frequency (HF) earthquakes(external link)

Journal of Volcanology and Geothermal Research
AD Jolly, I Lokmer, J Thun,J Salichon, B Fry, L Chardot