Seismicity analysis at the Prestige oil-tanker wreck area (Galicia Margin, NW of Iberia)

Abstract

An evaluation of the geological hazard related to the sinking of the Prestige oil-tanker in November 2002 in the Galicia margin, NW of Iberia, was supported by the Spanish research agency. It included a specific seismic study of the local activity and of the regional seismicity in this margin, results of which are presented here. Published global catalogues show that diffuse seismicity of moderate magnitude (events up to 5.6 mb) can be associated with the Northern flank of the Galicia Margin, with a roughly E–W distribution. However, event locations in this area are poorly constrained due to the inappropriate geometry of permanent seismic networks. We collected and analyzed new data from a dense deployment of 10 OBS around the wreck site and from a network of 12 land stations at the NW corner of Iberia. During a 7 months operation period, the land network permitted location of 120 events with magnitudes ranging from 1.6 to 4.8, more than 30% of which were not reported by the Spanish permanent array, confirming the existence of a moderate seismicity in this area. Five new focal solutions have been obtained for events on-land, providing additional information about the active faults in the region. The OBS network ran for 1 month and allowed the location of three earthquakes of magnitude≤2.5 near the sinking site. Four more events have been identified, with just a rough determination of their epicenters based on polarization of the P-wave arrivals. Besides these earthquakes, the OBS data recorded numerous seismic events of different types. They include: i) groups of very short and regularly spaced pulses that can be explained as acoustic calls generated by finback whales; ii) a large number of short-duration seismic events (2–3 s long) identified only at individual OBS; iii) local intense monochromatic signals lasting for up to 4 h that can be described as harmonic tremors. These last two types of seismic events might be related to hydrothermal activity within the uppermost sediments and deserve additional studies to appropriately establish the seismic hazard of this area.