NORTHEAST POCKMARK FIELDS: SHALLOW MARINE LIQUEFACTION LANDFORMS RESULTING FROM SEISMIC EVENTS?

Pockmarks are crater-like seafloor depressions that range up to 30 m deep and 300 m in diameter in Maine estuaries. They commonly occur in tectonically active areas, above oil and gas fields and in temperate, muddy, glaciated estuaries. They tend to occur in fields numbering into the thousands and are usually, though not always associated with subsurface methane accumulations. Natural gas occurs in all muddy embayments in the Northeast, USA, but pockmark fields are well mapped from only a few locations (Penobscot Bay, Somes Sound, Blue Hill Bay (ME) Passamaquoddy Bay (NB). Fields are not recognized in gassy, US Coastal Plain estuaries. Gas and pore water release are assumed to have formed the pockmarks, and tidal current reworking has maintained/reshaped some of them. Although numerous examples of contemporary gas eruptions are cited, where examined with time series multibeam bathymetric and seismic reflection surveys, no demonstrated changes in fields are recognized. It is possible that pockmark initiation is event driven. We suggest that pockmarks may be submarine analogues to terrestrial liquefaction features (sand boils) caused by seismicity. Earthquakes are known to have formed pockmarks and generated gas release in tectonically active areas outside the northeast. Here we present high-resolution swath bathymetry and seismic reflection data. We spatially compare pockmark field clustering with that of seismic events. Pockmarks occur in dense clusters, some of which, but not all, are associated with areas of historic seismic events. We suggest that mapping gas fields and monitoring changes following earthquakes may strengthen the association between pockmarks and seismicity.