EARTHQUAKE SWARM NEAR ALBANY, NY, AT THE TIME OF THE MINERAL, VA, SEISMIC EVENT

A swarm of twenty four seismic events, ranging in magnitude from M=1.5 to 2.9, occurred about the time of the Mineral, Virginia, seismic event in eastern New York State between Altamount and Knox (southwest of Albany), about 600 km NNE of the Mineral, Virginia, event. Four events preceded the Mineral main event, from about 1 day to 9 hours before the Mineral event, with a range of M=1.7 to 2.2 , and twenty occurred up to about 110 hours after the main Mineral event, and ranged from M= 1.5 to 2.9. The nominal depths were on the order of 20 km. The events occurred within 4 km of an extrapolated trace of the NNE-striking Saratoga McGregor fault. Other earthquake swarms in the past 5 years (2007, 2009, 2010) have occurred from about 5 to 14 km SSW of the 2011 swarm. These swarms are on strike with the general trend of the Saratoga-Macgregor fault, but the swarms also may have internal WNW-trends, consistent with WNW-trending cross faults similar to those in the Mohawk Valley or fault segment stepovers on the McGregor fault system.

The Saratoga-McGregor fault has been recognized and mapped to the north, where it brings Precambrian basement against Ordovician units, and has been presumed to be primarily a Taconic aged fault with down drop to the east, with a possible strike slip component. The amount of throw at the Ordovician horizons is on the order of 150 m. Other “Mohawk Valley faults”, for which we have seismic reflection data, were active in Iapetan opening time, and were reactivated in the Taconic, Salinic, (neo) Acadian/Alleghanian, and some Mohawk Valley faults presently experience limited seismicity. Outcrops near the 2011 earthquake swarm exhibit NNE-striking fracture intensification domains in the Ordovician Utica shale that have minor offsets. We propose that this earthquake swarm indicates that the Saratoga McGregor fault sustained a stress release in response to the same stress release that faults at Mineral experienced. It might be that the Saratoga McGregor fault was active during Triassic/Jurassic rifting—we have no geological evidence for or against, but the structural location of the Saratoga MacGregor fault indicates the fault could well have experienced minor activity during Triassic/Jurassic rifting, in which case the Saratoga MacGregor fault is in a somewhat similar structural setting as the activated fault(s) at Mineral VA.