Source mechanism of Vulcanian degassing at Popocatépetl Volcano, Mexico, determined from waveform inversions of very long period signals

Abstract

[1] The source mechanism of very long period (VLP) signals accompanying volcanic degassing bursts at Popocatépetl is analyzed in the 15-70 s band by minimizing the residual error between data and synthetics calculated for a point source embedded in a homogeneous medium. The waveforms of two eruptions (23 April and 23 May 2000) representative of mild Vulcanian activity are well reproduced by our inversion, which takes into account volcano topography. The source centroid is positioned 1500 m below the western perimeter of the summit crater, and the modeled source is composed of a shallow dipping crack (sill with easterly dip of 10°) intersecting a steeply dipping crack (northeast striking dike dipping 83°northwest), whose surface extension bisects the vent. Both cracks undergo a similar sequence of inflation, deflation, and reinflation, reflecting a cycle of pressurization, depressurization, and repressurization within a time interval of 3-5 min. The largest moment release occurs in the sill, showing a maximum volume change of 500-1000 m 3 , pressure drop of 3-5 MPa, and amplitude of recovered pressure equal to 1.2 times the amplitude of the pressure drop. In contrast, the maximum volume change in the dike is less (200-300 m 3 ), with a corresponding pressure drop of 1-2 MPa and pressure recovery equal to the pressure drop. Accompanying these volumetric sources are single-force components with magnitudes of 10 8 N, consistent with melt advection in response to pressure transients. The source time histories of the volumetric components of the source indicate that significant mass movement starts within the sill and triggers a mass movement response in the dike within a few seconds. Such source behavior is consistent with the opening of a pathway for escape of pent-up gases from slow pressurization of the sill driven by magma crystallization. The opening of this pathway and associated rapid evacuation of volcanic gases induces the pressure drop. Pressure recovery in the magma filling the sill is driven by diffusion of gases from the resulting supersaturated melt into bubbles. Assuming a penny-shaped crack at ambient pressure of 40 MPa, the observed pressure and volume variations can be modeled with the following attributes: crack radius (100 m), crack aperture (5 m), bubble number density (10 10 -10 12 m À3 ), initial bubble radius (10 À6 m), final bubble radius ($10 À5 m), and net decrease of gas concentration in the melt (0.01 wt %).Citation: Chouet, B., P. Dawson, and A. Arciniega-Ceballos (2005), Source mechanism of Vulcanian degassing at Popocatépetl Volcano, Mexico, determined from waveform inversions of very long period signals,

Eruption dynamics at Mount St. Helens imaged from broadband seismic waveforms: Interaction of the shallow magmatic and hydrothermal systems

“…[28] We perform full-waveform inversion of the LP and VLP events in a manner similar to the method employed by Chouet et al [2005]. A modification to perform the inversion in the frequency domain [Auger et al, 2006] greatly reduces computation time and permits a grid search over a large volume.…”

“…This represents more than 1,000 days of computation time if each Green function is computed with finite difference individually. Following Chouet et al [2005], we make use of the reciprocity relation to greatly reduce the number of calculations required to derive the Green functions. The reciprocal relation between a source and receiver is described by…”

“…[9] Where VLP earthquakes have been observed [Kawakatsu et al, 1992;Kawakatsu et al, 1994;Neuberg et al, 1994;Kaneshima et al, 1996;Ohminato et al, 1998;Arciniega-Ceballos et al, 1999;Legrand et al, 2000;Nishimura et al, 2000;Rowe et al, 2000;Kumagai et al, 2001;Almendros et al, 2002;Hidayat et al, 2002;Hill et al, 2002;Aster et al, 2003;Chouet et al, 2003Chouet et al, , 2005 they are typically attributed to fluid-rock interactions, as with LP events. However, instead of resonance in a fluid-filled crack, VLP events may result from longer-term inertial volume changes in fluid conduits.…”

The source of infrasound associated with long‐period events at Mount St. Helens

“…[75] Periodic, rapid discharge of gas in association with seismic LP and VLP activity has been observed at numerous other volcanoes [e.g., Gil Cruz and Chouet, 1997;Chouet et al, 2005], and has also been associated with infrasound radiation when infrasonic sensors were deployed [Yamasato, 1998;Yamasato et al, 2002;Petersen and McNutt, 2007]. In the crater of Galeras volcano, Colombia, Gil Cruz and Chouet [1997] photographed explosive gas emissions along a crack bisecting the dome (120-150 m long and a few millimeters wide) that were correlated with recorded LP events.…”

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.…”