ATTENUATION AND SOURCE PARAMETERS AT DECEPTION ISLAND (SOUTH-SHETLANDISLANDS, ANTARCTICA)

Deception Island is the most active volcano of the South Shetland Isla nds-Antarctic Peninsula group, experiencing eruptions in 1967, 1969 an d 1970. Local attenuation derived from coda analysis and source parame ters derived from Prune's model, for well located seismic events, have been studied in order to complement the available geophysical informa tion. Results show abnormally low Q(0) values and an abnormally high f requency dependence, as well as large dispersion. These factors are st rongly dependent on the path travelled by the seismic wave. Retrieved values of the source parameters (stress drop, seismic moment and sourc e radius), are again abnormally low compared to world-wide average val ues, for example, those obtained for the Oroville, California aftersho ck series between June and September, 1976. These results are consiste nt with some aspects of the geology of Deception Island, such as the v ery high degree of fracturing and faulting, and the existence of a str ong hydrothermal alteration affecting most of the subaerial volcanic r ocks. Moreover, the pattern defined for the lateral variations of Q(0) shows minimum values in the inner bay of the island, close to the mos t recent eruption vents. A large reduction in spectral amplitudes over a particular frequency range occurs in several observations, correspo nding to the path crossing the zone of highest attenuation. This obser vation suggests the existence of a hot magmatic intrusion produced dur ing the most recent eruption, and coincides with the superficial low d ensity mass distribution obtained from the gravimetric model and the l ong wave magnetic field component obtained from magnetic surveys. The width of this intrusion is estimated to be about 200 m, in agreement w ith the previous results obtained analyzing residuals of the location of seismic events.