The 1998–1999 seismic series at Deception Island volcano, Antarctica

Abstract

During the 1998–1999 Antarctic summer the pattern of seismic activity at Deception Island volcano changed significantly. The change was characterized by the occurrence of an intense swarm of volcano–tectonic (VT) earthquakes. More than 2000 VT earthquakes with S–P times smaller than 4 s were recorded in the period January–February 1999. Pure volcanic events were also detected; especially long-period (LP) events, volcanic tremor and some hybrid events. Seismic monitoring was performed using two short-period small-aperture arrays, among other instruments. Based on their signal-to-noise ratios we selected 863 VT earthquakes, 350 LP events and tremor episodes, and 9 hybrid events for analysis. We estimated apparent slowness and back-azimuth for all events using the Zero Lag Cross-Correlation array technique. Combining this information with S–P times and other indirect evidence, we identified two different source regions. LP seismicity is located less than 1–1.5 km southwest of the Fumarole array site. These events are likely to have a hydrothermal origin. VT earthquakes and hybrid events are located at depths of 0.3–10 km in an area under the bay of Deception Island. The area extends from the Fumarole array to the northeast with epicentral distances that range from 0.5 to 12 km. Most hypocenters are clustered in a small volume of around 8 km 3. The sources of the LP seismicity and the VT earthquakes are spatially distinct, which indicates that they are not produced by the same mechanisms. Moment magnitude analyses of the VT earthquakes provide an average magnitude of 0.5 and very low average stress drop, around 1 bar. A study of first motion of the P-waves suggests that the events in this small source region should have a variety of source mechanisms. This is supported by the existence of families of events with the same waveforms. The occurrence of repeating fracture processes with low stress drop and small fault dimensions can be explained by the lubrication of pre-existing zones of weakness by pressurized fluids. The most probable hypothesis that explains the generation of this seismic series at Deception Island is: a seismic series caused by the stress generated by the uplift of the source area due to a magmatic injection in depth. We favor this hypothesis since it is compatible with the majority of the characteristics of the seismicity and explains the spatial and temporal behavior of the series.