Analysis of dynamics of vulcanian activity of Ubinas volcano, using multicomponent seismic antennas
Introduction
The physical mechanisms behind vulcanian events remain a challenge in our community, with andesitic volcanoes characterized by violent and unpredictable eruptions. It is known that the vulcanian phenomenon is controlled by the physical and chemical properties of the magma. Vulcanian episodes can be generated by magma with intermediate properties between basaltic and rhyolitic, which comes from the deepest parts of the Earth, and which is less dense than the surrounding rock (Sparks, 2000).
Fragmentation of viscous magma by brittle failure is thought to be responsible for explosions in silicic volcanoes (Alidibirov and Dingwell, 2000). Melnik and Sparks (2002) have modeled unsteady conduit flow in explosive eruptions after unloading through dome collapse. These models have been applied to the episodes of explosive activity that occurred shortly after dome collapse at Soufrière Hills volcano. The unloading triggers gas exsolution and magma rising in the conduit, increasing its internal pressure. An explosion is triggered when the internal pressure equals the tensile strength of the magma. Overpressure is responsible for magma fragmentation generating gas-particle dispersion, which propagates to the exit of the conduit to form a volcanic column in the atmosphere (Melnik and Sparks, 2002). Other studies also suggest that fragmentation occurs when a critical overpressure or critical elongation strain rate of magma is reached (Alidibirov and Dingwell, 1996, Papale, 1999, Alidibirov and Dingwell, 2000).
Geophysical measurements have been performed to study this phenomenon for andesitic volcanoes. Iguchi et al. (2008) focused on seismic observations and ground deformation. Thus, a common sequence of phenomena associated with vulcanian eruptions, in particular occurred with inflation and deflation of the edifice, has been compared at three andesitic volcanoes (Sakurajima, Japan; Suwanosejima, Japan; Semeru, Indonesia).
Another approach was analyzed by Yokoo et al. (2009), who used both an infrasound network and video image time-series observations of the vulcanian eruption that occurred at Sakurajima Volcano (Japan) in January 2002. These observations suggested that the volumetric increase of the gas pocket caused a swelling of the surface of the crater bottom, and its subsequent failure. When the expansion velocity exceeded a threshold level, the main impulsive compression phase radiated with a high velocity through the sudden release of the pressurized gases. This volume change indicated that the vertical displacement of the swelling ground was of the order of 1 m, assuming that the radius of the lava plug was approximately 10 m.
Druitt et al. (2002) surveyed the eruption of the Soufriere Hills volcano in Monserrat in 1998, and reporting on 88 vulcanian episodes that revealed the vulcanian mechanism. This mechanism can be broken down into the following scenario. The explosion starts when the pressure in the conduit goes over a threshold trigger of the cap of degassed crystal-rich magma. A fragmentation wave goes down the conduit into a region of pressurized magma with an approximate velocity of 50 m/s, which results in an upward speed of around 140 m/s.
On the other hand, small-aperture seismic arrays have been useful to locate seismic sources in volcanic unrest, as seismic waveforms have a lack of clear body-wave phase arrivals. Emergent onset in long-period and tremor seismic waves makes it extremely difficult to solve a source localization from classical hypocenter methods based on phase picking and calculated travel-times. Therefore, different methods of source localization have been applied to array data recorded from volcanic unrest (Saccorotti and Del Pezzo, 2000, Almendros et al., 2001a, Almendros et al., 2001b, Métaxian et al., 2002, La Rocca et al., 2004, Di Lieto et al., 2007, Inza et al., 2011, O'Brien et al., 2011). Also, high resolution techniques for multicomponent array data have been developed that can be applied to the locating of seismic sources (Miron et al., 2005, Paulus and Mars, 2006).
A field experiment was carried out from May to July 2009 at Ubinas Volcano (Peru) that was carried out by a research team from the French Institut de Recherche pour le Développment, University College Dublin, Ireland (Volume project) and the Instituto Geofísico del Perú, with two small-aperture seismic arrays that were composed of three-component seismometers deployed on two flanks of the Ubinas Volcano.
Inza et al. (2011) presented a source localization method MUSIC-3C, based on the use of 3C seismic arrays. MUSIC-3C provided realistic estimates of the depth of volcanic sources, performed on synthetic data. Also, an explosion earthquake and a LP event recorded during the field experiment were located by using MUSIC-3C. The locations of these events were at an altitude of 4200 m for the explosion and 2240 m for the LP event respectively. The explosion earthquake and the LP event were interpreted as resulting, respectively, from a fragmentation process and shear-fracturing of magma at the conduit walls.
In the following three sections, we focus our study towards the application of seismic arrays to locate and interpret the vulcanian events recorded at Ubinas volcano. In Section 2 it described the data recorded during the field experiment. The source localization analysis in Section 3 will unfold the source localization method MUSIC-3C by using real and synthetic data set. Finally, in Section 4, results will be discussed taking into account the methodology and observation aspects.
Ubinas volcano (16° 22′ S, 70° 54′ W; altitude, 5672 m) began to erupt on March 25, 2006, after nearly 40 years of quiescence. Situated in the Central Volcanic Zone (CVZ, southern Peru), Ubinas Volcano is an active andesitic stratovolcano that is truncated in the upper part by a caldera of 600 m in diameter (De Silva and Francis, 1991). The caldera floor is a flat area that lies at an altitude of approximately 5100 m. The active crater is situated in the southern section; the bottom is 300 m under the caldera floor (Fig. 1). Ubinas is considered to have been the most active Peruvian volcano during the last 500 years, which has threatened 3500 people who live on the edge of the Ubinas Valley (Rivera et al., 2010). Arequipa Airport is situated 60 km east of the Ubinas volcano, and it has had to be closed several times since 2006, due to ash emissions. Under the 6th EU Framework Programme project known as VOLUME, the Instituto Geofísico del Perú with the cooperation of the Institut de Recherche pour le Développment (France) started seismic monitoring of Ubinas volcano to understand the activity associated with this eruptive sequence. A network of four digital 1-Hz stations with a radio telemetry system has been operating there since 2006, with the data transmitted to Arequipa Instituto Geofísico del Perú observatory. At the time of this study, the eruption was characterized by almost permanent ash emissions. Two main types of degasing were observed: 1. Exhalations that rose a few hundred meters above the crater rim; and 2. Plumes that were produced by explosions that could reach 10 km above sea level (Rivera et al., 2010), and which were critical to aircraft safety. This activity is thought to be related to a magmatic plug that is positioned at the bottom of the southern part of the caldera wall (Macedo et al., 2009).
Section snippets
Data collection
From May to July 2009, two small-aperture seismic arrays were deployed at Ubinas Volcano: on the north part (the NUBI antenna) and the west part (the WUBI antenna) (Fig. 1). The NUBI antenna was composed of 10 instruments: eight Guralp-6TD and two Guralp-3ESP seismometers, while the WUBI antenna consisted of six Guralp-6TD seismometers and six Titan-Neomax Agecodagis instruments. Each station included a seismic three-component sensor and a GPS receiver, and was set up to continuously record at
Processing
A primary objective of this study was the location of the sources of the 16 vulcanian events that were identified, as in Table 2, to try to better understand the explosive dynamics at Ubinas. With this aim, we applied the MUSIC-3C method, as proposed by Inza et al. (2011). Given the lack of a velocity model for the Ubinas volcano, the propagation medium was assumed to be homogeneous.
This next section is divided into several parts. We begin by identifying the frequency bands where the energy of
Source location
The main result of this study is the identification of two distinct sources for each explosion, which were located at different depths in the conduit. The seismic signal was composed of an initial low-frequency part [0.5–2.2 Hz] and a second high frequency part [2.2–6 Hz]. This second frequency band is predominant in the coda of the explosions recorded during our experiment. Aki (1981) and Aki and Chouet (1975) have attributed coda waves to the scattering of seismic waves in the crust. The coda
Acknowledgments
We are grateful to Alain Burgisser for their discussions on vulcanian eruptions. The appropriate comments made by an anonymous reviewer greatly contributed to improve the manuscript.
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2020, Journal of Volcanology and Geothermal ResearchCitation Excerpt :The rate of energy release by these explosions is rapid, and the events can be hazardous, due to the generation of PDCs, shock waves, or ballistics. Recently, intense Vulcanian activity has been observed with seismometers at many volcanoes around the world, resulting in a variety of types of analysis and observations, such as Very Long Period events (Arciniega-Ceballos et al., 1999), and radiated seismic energy at Popocatépetl (Arámbula-Mendoza et al., 2016), observation of tilt before an explosion at Stromboli (D'Auria et al., 2006), source location at Ubinas (Inza et al., 2014), source inversion at Sakurajima (Tameguri et al., 2002), seismic characterization at Soufrière Hills (Druitt et al., 2002), to mention a few. To quantify and characterize Vulcanian activity, infrasound studies can be enlightening (Johnson and Aster, 2005; Marchetti et al., 2009; Fee and Matoza, 2013).
Detection of pre-eruptive seismic velocity variations at an andesitic volcano using ambient noise correlation on 3-component stations: Ubinas volcano, Peru, 2014
2019, Journal of Volcanology and Geothermal ResearchCitation Excerpt :For example, the sensitivity kernel at a frequency of 0.3 Hz has large values up to 2 km below the surface and coincides with the distribution in depth of the VT seismic activity (Fig. 13b). The corresponding apparent velocity are thus sensitive to perturbations in the seismogenic zone of Ubinas (Fig. 13c) similar to that observed for events on 2009 (Inza et al., 2014). Fig. 14 displays measurements of seismicity, plume elevation, thermal anomalies and SO2 flux that can be compared with the estimated velocity variations.
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Asymmetrical structure, hydrothermal system and edifice stability: The case of Ubinas volcano, Peru, revealed by geophysical surveys
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