Correlation between seepage activity and micro seismicity on the Vestnesa Ridge, NW- Svalbard continental margin

Abstract:

We study seismicity along the active gas-seepage system of Vestnesa Ridge offshore the west Svalbard margin using 4-component ocean bottom seismometers (OBSs). In addition to recording nearby oceanic ridge earthquakes and sporadic local events, we observe large number of seismic signals that are characterized by wide frequency content and duration shorter than the closest observed earthquakes. The events are picked automatically using standard STA/LTA algorithms and analyzed in terms of their periodicity and intensity. Notably, in one deployment with 3 OBSs around a seabed pockmark the majority of the observed events on two nearby seismometers come from one specific direction, suggesting that active seepage may be the source of the documented seismicity. Frequency analysis of the datasets reveal tidal periodicity and larger time-scale periods with no clear connection to tides. We then cross correlate the hourly counts of events with modelled tidal mean sea level variations for the area. Results from all seismometers indicate a strong correlation between the tidal peaks/lows and increase in the number of events recorded. We observe maximum correlation 1-2 hours before the high tide with corresponding minima at +/- 6 from them. Further analysis shows potential seasonal changes of the patterns. We envision that the documented seismicity may be related to near-surface deformation following the response of the seepage system to hydrostatic pressure changes. Ongoing experiments will hopefully allow to better pinpoint the sources of the signals and help elucidating the physical processes behind it.