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Slowness Anomalies from Two Dense Seismic Arrays at Deception Island Volcano, Antarctica

Osservatorio Vesuviano
Via Diocleziano 328, 80124 Napoli, Italy
gilberto{at}ov.ingv.it
(G.S., E.D.P.)

Instituto Andaluz de Geofisica
Universidad de Granada
Campus Universitario de Cartuja
18071 Granada, Spain
ibanez{at}iag.ugr.es; ecarmona{at}iag.ugr.es.
(J.A., E.C., J.M.I.)

In this article, we analyze the data collected by two short-period seismic arrays deployed at Deception volcano, Southern Shetland Islands, Antarctica. The field survey was conducted during the 1998–1999 austral summer and was aimed at a quantitative assessment of the complex wave fields associated with the magmatic and hydrothermal activity of the volcano. The two arrays had apertures of 320 m and 240 m and were separated by a distance of about 3 km. During the experiment, the arrays recorded several regional earthquakes related to the dynamics of the Bransfield Strait and adjoining areas and local volcano–tectonic earthquakes. Seismograms of earthquakes recorded at regional distances reveal a marked difference in the apparent velocities measured at the two array sites. We investigate the causes and implications of these anomalies by first comparing the effectiveness of estimating the horizontal slowness vector using three different techniques: the multiple signal classification (MUSIC) approach, the zero-lag cross correlation (ZLC) method, and plane-wave fitting to P-wave arrival times. While each technique provides the same horizontal slowness vector as the most likely estimates, the plane-wave fitting is associated with the most robust definition of measurement uncertainties. We then investigate the dispersive properties of Rayleigh waves in the 1–8 Hz frequency band at both arrays and invert the two dispersion curves for a shallow velocity structure. The results indicate a marked difference in the seismic velocities for the shallower 200 m beneath the two sites. This may be reconciled with the observed wave vector anomalies by assuming the existence of a sharp lateral velocity heterogeneity, the effect of which would be to bend downward rays impinging at the northernmost array. The reliability of this hypothesis is verified by computing finite-difference wave fronts in a 2D heterogeneous medium. Based on the morpho-structural characteristics of the volcano, the inferred velocity discontinuity maybe associated with the ring-fracture system bordering the collapsed caldera structure that extends over the inner part of the island.

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