[1] Deception Island (62°59 0 S, 60°41 0 W) is an active volcano located in the Bransfield Strait between the Antarctic Peninsula and the South Shetland Islands. The island is composed of rocks that date from <0.75 Ma to historical eruptions (1842, 1967, 1969, and 1970), and nowadays most of its activity is represented by vigorous hydrothermal circulation, slight resurgence of the inner bay floor, and intense seismicity, with frequent volcano-tectonic and long-period events. In January 2005 an extensive seismic survey took place in and around the island to collect high-quality data for a high-resolution P wave velocity tomography study. A total of 95 land and 14 ocean bottom seismometers were deployed, and more than 6600 air gun shots were fired. As a result of this experiment, more than 70,000 travel time data were used to obtain the velocity model, which resolves strong P wave velocity contrasts down to 5 km depth. The joint interpretation of the Vp distribution together with the results of geological, geochemical, and other geophysical (magnetic and gravimetric) measurements allows us to map and interpret several volcanic features of the island and surroundings. The most striking feature is the low P wave velocity beneath the caldera floor which represents the seismic image of an extensive region of magma beneath a sediment-filled basin. Another lowvelocity zone to the east of Deception Island corresponds to seafloor sedimentary deposits, while high velocities to the northwest are interpreted as the crystalline basement of the South Shetland Islands platform. In general, in the tomographic image we observe NE-SW and NW-SE distributions of velocity contrasts that are compatible with the regional tectonic directions and suggest that the volcanic evolution of Deception Island is strongly conditioned by the Bransfield Basin geodynamics.

The zero‐lag cross‐correlation technique, used for array analysis in the hypothesis of plane waves, has been modified to allow the wave front to be circular. Synthetic tests have been performed to check the capability of the method, which returns the input test data when the source–array distances are not greater than two or three times the array aperture. For this distance range the method furnishes an estimate of the apparent velocity and the epicentral coordinates of the source. For more distant sources the method becomes equivalent to that based on the planar‐wave approximation, which gives an estimate of the backazimuth to the source and the apparent velocity. The method has been applied to seismic data recorded at the active volcano located at Deception Island, Antarctica. 35 volcanic long‐period events occurring in a small swarm were selected. Results show that the epicentres are close to the array (between 0.4 and 2 km) and aligned in a SW direction, in agreement with one of the main directions of the fracture system of Deception volcano.

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Abstract. The seismovolcanic signals associated with the volcanic activity of Deception Island (Antarctica), recorded during three Antarctic summers (1994-1995, 1995-1996 and 1996-1997), are analyzed using a dense small-aperture (500 m) seismic array. The visual and spectral classification of the seismic events shows the existence of long-period and hybrid isolated seismic events, and of low-frequency, quasi-monochromatic and spasmodic continuous tremors. All spectra have the highest amplitudes in the frequency band between 1 and 4 Hz, while hybrids and spasmodic tremors have also significant amplitudes in the high-frequency band (4-10 Hz). The array analysis indicates that almost all the well-correlated low-frequency signals share similar array parameters (slowness and back azimuth) and have the same source area, close to the array site. The polarization analysis shows that phases at high-frequency are mostly composed of P waves, and those phases dominated by low frequencies can be interpreted as surface waves. No clear shear waves are evidenced. From the energy evaluation, we have found that the reduced displacement values for surface and body waves are confined in a narrow interval. Volcanotectonic seismicity is located close to the array, at a depth shallower than 1 km. The wave-field properties of the seismovolcanic signals allow us to assume a unique source model, a shallow resonating fluid-filled crack system at a depth of some hundreds of meters. All of the seismic activity is interpreted as the response of a reasonably stable stationary geothermal process. The differences observed in the back azimuth between low and high frequencies are a near-field effect. A few episodes of the degassification process in an open conduit were observed and modeled with a simple organ pipe.

Abstract. We present a probabilistic method to locate the source of seismic events using seismic antennas. The method is based on a comparison of the event azimuths and slownesses derived from frequency-slowness analyses of array data, with a slowness vector model. Several slowness vector models are considered including both homogeneous and horizontally layered half-spaces and also a more complex medium representing the actual topography and three-dimensional velocity structure of the region under study. In this latter model the slowness vector is obtained from frequency-slowness analyses of synthetic signals. These signals are generated using the finite difference method and include the effects of topography and velocity structure to reproduce as closely as possible the behavior of the observed wave fields. A comparison of these results with those obtained with a homogeneous half-space demonstrates the importance of structural and topographic effects, which, if ignored, lead to a bias in the source location. We use synthetic seismograms to test the accuracy and stability of the method and to investigate the effect of our choice of probability distributions. We conclude that this location method can provide the source position of shallow events within a complex volcanic structure such as Kilauea Volcano with an error of _+200 m.

Abstract. We have found experimental evidence which shows that the volcanic tremor recorded at Deception Island (South Shetland Islands, Antarctica) is a superposition in time of overlapping hybrid events. We studied data from a small aperture seismic array. Data analysis for tremor and hybrids included: (1) spectral analysis; (2) apparent slowness and back-azimuth determination by using the zero-lag cross-correlation method; and (3) polarization analysis. Both types of events share these common features: (a) two dominant spectral bands at frequencies 1-3 Hz (the most energetic) and 4-8 Hz; (b) several coherent phases with the same back-azimuth to the source and apparent slowness along the whole signal; (c) in the high frequency band, the apparent slowness is very low (around 0.17 s/km), indicating the propagation of body waves; (d) in the low frequency band, the apparent slowness is high (around 1.6 s/km), consistent with the presence of surface waves; and (e) clear P-wave onset followed by a complex pattern of Rayleigh waves. Therefore, both types of events are strongly related because they share the same source region, the same wave-propagation properties, and the same wave composition. Moreover, several arrivals, that resemble a single hybrid event, have been found along the tremor signals. Due to these reasons, we hypothesize that volcanic tremor of Deception Island is a superposition of hybrid type events. The source of both types could be the interaction between thaw water and hot materials in a shallow aquifer.

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