Seismovolcanic signals at Deception Island volcano, Antarctica: Wave fieldanalysis and source modeling

The seismovolcanic signals associated with the volcanic activity of Decepti on 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 eve nts 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 an d 4 Hz, while hybrids and spasmodic tremors have also significant amplitude s in the high-frequency band (4-10 Hz). The array analysis indicates that a lmost all the well-correlated low-frequency signals share similar array par ameters (slowness and back azimuth) and have the same source area, close to the array site. The polarization analysis shows that phases at high-freque ncy are mostly composed of P waves, and those phases dominated by low frequ encies can be interpreted as surface waves. No clear shear waves are eviden ced. From the energy evaluation, we have found that the reduced displacemen t values for surface and body waves are confined in a narrow interval. Volc ano-tectonic seismicity is located close to the array, at a depth shallower than 1 km. The wave-field properties of the seismovolcanic signals allow u s 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 i s 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 pi pe.