Slowness anomalies from two dense seismic arrays at Deception Island volcano, Antarctica

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 wa s 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 earthquak es 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 m easured at the two array sites. We investigate the causes and implications of these anomalies by first comparing the effectiveness of estimating the h orizontal slowness vector using three different techniques: the multiple si gnal classification (MUSIC) approach, the zero-lag cross correlation (ZLC) method, and plane-wave fitting to P-wave arrival times. While each techniqu e provides the same horizontal slowness vector as the most likely estimates , the plane-wave fitting is associated with the most robust definition of m easurement uncertainties. We then investigate the dispersive properties of Rayleigh waves in the 1-8 Hz frequency band at both arrays and invert the t wo dispersion curves for a shallow velocity structure. The results indicate a marked difference in the seismic velocities for the shallower 200 m bene ath the two sites. This may be reconciled with the observed wave vector ano malies by assuming the existence of a sharp lateral velocity heterogeneity, the effect of which would be to bend downward rays impinging at the northe rnmost array. The reliability of this hypothesis is verified by computing f inite-difference wave fronts in a 2D heterogeneous medium. Based on the mor pho-structural characteristics of the volcano, the inferred velocity discon tinuity maybe associated with the ring-fracture system bordering the collap sed caldera structure that extends over the inner part of the island.