SEISMIC INVESTIGATION OF THE EASTFORD LINEAMENT AND ITS RELATIONSHIP TO RECENT MOODUS CT EARTHQUAKE ACTIVITY AND OLDER FAULTING

Two sets of adjacent shallow seismic profiles were run perpendicular to the Eastford lineament in the vicinity of recent earthquakes that have occurred locally in the Moodus CT area. The data were recorded on a 24-channel Geometrics Strataview digital system using shallow shotgun blank sources located off the ends of each spread, typically with a 3 m geophone spacing. It was found that the lineament is the surface expression of a WNW-dipping normal fault with a few adjacent smaller normal faults on the down-dip side. Besides these shallow seismic results, there are borehole observations of localized intense fracturing at 400 m depth in a nearby well on the northwest side of the lineament and published focal depths and focal mechanisms for recent Moodus earthquakes located somewhat farther WNW that are clustered at approximately 1600 m depth. The focal mechanisms for these earthquakes indicate thrust faulting on approximately a 70 degree WNW dipping fault plane with a strike similar to that of the Eastford lineament; the spatial distribution of the cluster of earthquakes is also aligned with the strike of the nearby Eastford lineament. The upward projection of this 70 degree dipping fault plane passes through the highly fractured zone in the well and coincides with the near-surface normal faulting at the Eastford lineament found seismically. However, these recent earthquakes have thrust mechanisms consistent with the current ENE near-horizontal maximum compressional stress regime in the region. The seismic results showing a net normal fault displacement near the surface at the lineament indicate that although this older normal fault has now been reactivated as a thrust fault, the net cumulative displacement has not yet overcome the previous net normal fault displacement. These results show conclusively that the Eastford lineament is in fact the Eastford fault that extends at least as far SSW as the southernmost seismic lines completed in this study on North Moodus Road.