Abstract

The northern North Sea is one of the most seismically active regions of Northwest Europe, but offshore seismicity is difficult to study in detail because the severe environment places limits on options for deployment of near-field ocean bottom seismometers (OBS). The design of an OBS system deployed on the Statfjord oil field is presented, together with some results.

Amplitude density spectra of ambient noise from two OBS installations are on average more than a factor of ten higher than the average of three onshore Western Norway Network (WNN) seismic stations. A theoretical detection threshold curve, based on background noise levels, is in close agreement with practical experience of detections at the Statfjord OBS and explains why only about 7 per cent of the events detected in the northern North Sea by the WNN are also detected on the Statfjord OBS. Comparison of earthquake source spectra, recorded on and off shore, reveal no evidence for either high attenuation or amplification in the Viking Graben. Earthquake focal depths in the central northern North Sea seem to be around 20 km.

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