Interpretation of very long period tremors at Ambrym volcano, Vanuatu, as quasi‐static displacement field related to two distinct magmatic sources

Abstract

A 3‐component broadband seismometer (STS‐2) has been deployed on the SW slope of Ambrym, Vanuatu, a large‐caldera volcano with two persistent lava lakes from July to November 2000. Well‐polarized Very Long Period Tremors (18–22s; VLPTs) occurred quite continuously, whereas other seismic signal with periods of 9s, 6–7s, 5s, 2.3–2.7s, 1.95s and 1s were intermittent. VLPTs signals show two clearly distinct directions pointing towards the caldera. Polarization orientations suggest two sources: the first one, and the most active, fits with the zone from which silicic lava flows erupted in 1986, in the central‐to‐eastern part of the caldera. The second one corresponds to the active zone formed by the complex Marum volcanic center and the Benbow cone, in the western part of the depression structure. The depths are estimated between 2.7–2.9 km with respect to sea level. VLPTs are interpreted as the quasi static‐displacement field of two nondestructive sources. They indicate the persistent continuing magmatic activity beneath the eastern floor of the caldera, although no volcanic activity is observed in this area, and the more obvious, but less active, corresponding to the lava lakes. These data also corroborate the complex internal structure of the Ambrym caldera, suggested by geological data and bimodal magmatism (basaltic, ∼50% SiO2, and silicic, ∼60% SiO2).

Prodigious emission rates and magma degassing budget of major, trace and radioactive volatile species from Ambrym basaltic volcano, Vanuatu island Arc

“…11), which corresponds to a lithostatic depth range of~5.4 to 3.8 km below the vents (or 4.6 to 3.0 km below sea level) where magma ponding may favor olivine crystallization and melt entrapment. This observation fits fairly well with seismic inference of the roof of a magma reservoir at~2.9-2.7 km depth bsl beneath Benbow-Marum craters (Legrand et al, 2005;Fig. 11).…”

“…1c), and the outpouring of an evolved (60% SiO 2 ) lava flow in 1986 (Robin et al, 1993). A preliminary broadband seismic survey in 2000 suggests the presence of two distinct magma bodies beneath the western and eastern parts of the caldera, both at depths of~2.9-2.7 km below sea level (Legrand et al, 2005).…”

“…See discussion in Section 5.2. *Inferred seismic depth of the magma reservoir beneath Benbow-Marum craters (Legrand et al, 2005).…”

“…DER: D'Entrecasteaux Ridge; (b) Ambrym island with its extinct northern centers and its 12 km wide summit caldera cut by the 20°NE-SW rift zone; (c) detailed map of Ambrym caldera showing the two main active cones of Benbow and Marum, the ultimate lava flows, and the site locations of our gas measurements (stars) and rock sampling (numbers) (redrawn from Beaumais, 2013). be investigated for its structure and eruptive history (MacCall et al, 1970;Chase and Seekins, 1988;Robin et al, 1993;Nemeth et al, 2009;Nemeth and Cronin, 2011), its eruptive products (Robin et al, 1991(Robin et al, , 1993Picard et al, 1995;Peate et al, 1997;Beaumais, 2013) and, more recently, its seismic activity (Legrand et al, 2005). Paradoxically, no attention was paid to its voluminous gas emissions, despite their heavy impact upon the local environment and even the health of inhabitants in the downwind part of the island (Cronin and Sharp, 2002;Bani et al, 2009aBani et al, , 2009bAllibone et al, 2012).…”

Surge in sulphur and halogen degassing from Ambrym volcano, Vanuatu

“…The origin for both halogens and sulphur can be assumed to be magma degassing through the lava lakes, which appear to be connected to subjacent magma sources located at a depth of <3 km below sea level (Legrand et al 2005). The D'Entrecasteaux Zone collision with the Vanuatu arc (Fig.…”