Seismic source mechanism of degassing bursts at Kilauea Volcano, Hawaii: Results from waveform inversion in the 10–50 s band

Abstract

) summit eruptive activity at Kilauea Volcano is characterized by explosive degassing bursts accompanied by very long period (VLP) seismic signals. We model the source mechanisms of VLP signals in the 10-50 s band using data recorded for 15 bursts with a 10-station broadband network deployed in the summit caldera. 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. The VLP signals associated with the bursts originate in a source region ∼1 km below the eastern perimeter of Halemaumau pit crater. The observed waveforms are well explained by the combination of a volumetric component and a vertical single force component. For the volumetric component, several source geometries are obtained which equally explain the observed waveforms. These geometries include (1) a pipe dipping 64°t o the northeast; (2) two intersecting cracks including an east striking crack (dike) dipping 80°to the north, intersecting a north striking crack (another dike) dipping 65°to the east; (3) a pipe dipping 58°to the northeast, intersecting a crack dipping 48°to the westsouthwest; and (4) a pipe dipping 57°to the northeast, intersecting a pipe dipping 58°to the west-southwest. Using the dual-crack model as reference, the largest volume change obtained among the 15 bursts is ∼24,400 m 3 , and the maximum amplitude (peak to peak) of the force is ∼20 GN. Each burst is marked by a similar sequence of deflation and inflation, trailed by decaying oscillations of the volumetric source. The vertical force is initially upward, synchronous with source deflation, then downward, synchronous with source reinflation, followed by oscillations with polarity opposite to the volumetric oscillations. This combination of force and volume change is attributed to pressure and momentum changes induced during a fluid dynamic source mechanism involving the ascent, expansion, and burst of a large slug of gas within the upper ∼150 m of the magma conduit. As the slug expands upon approach to the surface and more liquid becomes wall supported by viscous shear forces, the pressure below the slug decreases, inducing conduit deflation and an upward force on the Earth. The final rapid slug expansion and burst stimulate VLP and LP oscillations of the conduit system, which slowly decay due to viscous dissipation and elastic radiation. Consideration of the fluid dynamic arguments leads us to prefer the dual-crack VLP source model as it is the only candidate model capable of producing plausible values of length scales and pressure changes. The magnitudes of the vertical forces observed in the 15 bursts appear consistent with slug masses of 10 4 to 10 6 kg.Citation: Chouet, B. A., P. B. Dawson, M. R. James, and S. J. Lane (2010), Seismic source mechanism of degassing bursts at Kilauea Volcano, Hawaii: Results from waveform inversion in the 10-50 s band,

Shallow degassing events as a trigger for very-long-period seismicity at Kīlauea Volcano, Hawai‘i

“…Although our model and that of Chouet et al (2010) are similar in sharing a top-down process behind the VLP seismicity, we do not assume or imply that gas slugs are responsible for the observed degassing events (episodic tremor or explosive events). We suggest that the episodic tremor (gas-piston-like behavior) is due to gas accumulation at the top of the lava column due to a rheological barrier (Patrick et al 2010), and the explosive events may be caused by violent overturning and degassing of nearsurface lava triggered by large rockfalls (which have immediately preceded most if not all of the explosive events) ).…”

“…Based on our video timing and observations, Chouet et al (2010) also propose a top-down model. In their study, the authors use computational fluid dynamic modeling (James et al, 2008) to characterize a gas slug reaching the top of the lava column and bursting as a source process for the explosive events.…”

Viscous plugging can enhance and modulate explosivity of strombolian eruptions

“…Our experiments build on previous work carried out in single-viscosity systems (James et al, 2004(James et al, , 2006(James et al, , 2008(James et al, , 2009Lane et al, 2013), and we adopt a similar analogue methodology. We also use a computational fluid dynamic model (James et al, 2008;Chouet et al, 2010) to conduct numerical simulations of the same scenario at the volcano scale, in order to explore the applicability of our laboratory results to the natural system.…”

Source mechanism of a VLP event immediately before the 2014 eruption of Mt. Ontake, Japan

“…Therefore, analyses of VLP events at a volcano are crucial for understanding fluid states and behaviors beneath the volcano. Some VLP events are associated with phreatic eruptions (e.g., Legrand et al 2000;Jolly et al 2010;Maeda et al 2015) as well as various types of magmatic eruptions (e.g., Chouet et al 2003Chouet et al , 2005Chouet et al , 2010.…”