Seismic source dynamics of gas‐piston activity at Kı̄lauea Volcano, Hawai‘i

Abstract

Since 2008, eruptive activity at the summit of Kı̄lauea Volcano, Hawai‘i has been confined to the new Overlook pit crater within the Halema‘uma‘u Crater. Among the broad range of magmatic processes observed in the new pit are recurring episodes of gas pistoning. The gas‐piston activity is accompanied by seismic signals that are recorded by a broadband network deployed in the summit caldera. We use raw data recorded with this network to model the source mechanism of representative gas‐piston events in a sequence that occurred on 20–25 August 2011 during a gentle inflation of the Kı̄lauea summit. To determine the source centroid location and source mechanism, we minimize the residual error between data and synthetics calculated by the finite difference method for a point source embedded in a homogeneous medium that takes topography into account. We apply a new waveform inversion method that accounts for the contributions from both translation and tilt in horizontal seismograms through the use of Green's functions representing the seismometer response to translation and tilt ground motions. This method enables a robust description of the source mechanism over the period range 1–10,000 s. Most of the seismic wavefield produced by gas‐pistoning originates in a source region ∼1 km below the eastern perimeter of the Halema‘uma‘u pit crater. The observed waveforms are well explained by a simple volumetric source with geometry composed of two intersecting cracks featuring an east striking crack (dike) dipping 80°to the north, intersecting a north striking crack (another dike) dipping 65° to the east. Each gas‐piston event is marked by a similar rapid inflation lasting a few minutes, trailed by a slower deflation ramp extending up to 15 min, attributed to the efficient coupling at the source centroid location of the pressure and momentum changes accompanying the growth and collapse of a layer of foam at the top of the lava column. Assuming a simple lumped parameter representation of the shallow magmatic system, the observed pressure and volume variations can be modeled with the following attributes : foam thickness (10–50 m), foam cell diameter (0.04–0.10 m), and gas‐injection velocity (0.01–0.06 m s−1). Gas‐piston activity occurs in a narrow pipe with diameter of 6 m connecting the Halema‘uma‘u pit crater to the subjacent dike system. The height of the magma column is estimated at ∼104 m at the start of the sequence based on the period of very long period (VLP) oscillations accompanying the onset of the gas‐piston signal. Based on the change in the period of VLP oscillations and tilt evidence, the height of the magma column is inferred to have risen by up to ∼23 m by the end of the 5 day long sequence. A penny‐shaped crack model of the dike geometry yields effective diameters of ∼1.2–2.9 km for the east dike and 0.7 km for the north dike. The shallower north dike segment is embedded in a relatively weak medium, compatible with expected mechanical properties in the hydrothermal environment of this dike.

Shallowly driven fluctuations in lava lake outgassing (gas pistoning), Kīlauea Volcano

“…Orr and Rea (2012) analyzed gas piston events in a lava pond at Pu'u 'Ō'ō and attributed them to episodic growth and failure of a shallow foam layer at the top of the lava column, irrespective of the presence of a surface crust. Chouet and Dawson (2015) presented a model for Halema'uma'u gas pistoning in which foam formation occurred within a narrow conduit, which is different than the lava lake scenario we describe here.…”

“…Thus, three general models have been presented to explain gas pistoning: 1) shallow gas accumulation, either due to gas trapped by the surface crust (Swanson et al, 1979) or due to gas trapped within a near-surface foam layer (Orr and Rea, 2012;Chouet and Dawson, 2015), 2) deeply sourced gas slugs (Jaupart and Vergniolle, 1988) and 3) dynamic pressure balance (Whitham et al, 2006).…”

Continuous Gravity and Tilt Reveal Anomalous Pressure and Density Changes Associated With Gas Pistoning Within the Summit Lava Lake of Kīlauea Volcano, Hawai‘i

“…One of the two largest lava lakes on Earth (Patrick et al, ), the lava lake within a pit in the floor of Halema‘uma‘u Crater has been the subject of intensive multidisciplinary investigations that have taken advantage of detailed records of gas geochemistry and emissions, tephra deposition, lava level fluctuations, seismicity, and deformation. Such work has substantially elucidated knowledge of lava lake dynamics by providing insights into, for example, the mechanisms of very low‐frequency earthquakes (Chouet & Dawson, , ; Dawson & Chouet, ; Patrick et al, ), characteristics and driving forces of small explosive eruptions (Houghton et al, , ; Orr et al, ), the occurrence of gas pistoning (Nadeau et al, ; Patrick et al, ), the dynamics of bubble nucleation and resorption (Carey et al, , , ), volatile content and degassing (Edmonds et al, ; Mather et al, ), and the relation between lava lake activity and the volcano's East Rift Zone (ERZ) eruption (Patrick et al, ).…”

“…The shallow summit magma system is known to consist of at least two reservoirs, one beneath the east margin of Halema‘uma‘u Crater at ~1.5 km depth and with a volume on the order of 1 km 3 , and another beneath the south caldera, at 3–5 km depth and with a volume on the order of 10 km 3 (Anderson et al, ; Poland et al, ). The lava lake is linked to the shallower reservoir, as indicated by seismicity as well as the coincidence of reservoir inflation/deflation with changes in lava level (Anderson et al, ; Chouet & Dawson, , ; Dawson & Chouet, ; Patrick et al, ), while the deeper reservoir is connected to the ERZ eruptive vent, which has been the site of ongoing eruptive activity since 1983 (Orr et al, ; Poland et al, ). A hydraulic connection between the summit and ERZ vents via the subsurface magma storage and transport system is demonstrated by the compositional similarity of erupted products at the two sites (Rowe et al, ), gas compositions and emission rates (Elias & Sutton, ), and coincidence of changes in eruptive activity at the summit and ERZ vents (Patrick et al, ).…”

Understanding the SO2 Degassing Budget of Mt Etna's Paroxysms: First Clues From the December 2015 Sequence

“…Magmatic volatiles are known to play a central role in the generation of these basaltic paroxysms (e.g., Aiuppa et al, 2010). Proposed mechanisms include the periodic collapse of gas-rich magmatic "foams" residing at the upper levels of crustal magma storage systems (Allard et al, 2005;Carbone et al, 2015;Chouet and Dawson, 2015) and recharging of relatively degassed shallow magmas with gas-rich magmas from greater depth (e.g., Albert et al, 2016;Pompilio et al, 2017). Improving our understanding of these, or alternative, mechanisms depends on the integration of several methods, and especially real-time observations of gas emissions.…”