EARTHQUAKE SWARMS ON THE SARATOGA-MCGREGOR FAULT SYSTEM NEAR ALBANY, NY, INCLUDING ONE SWARM AT THE TIME OF THE 2011 MINERAL, VA, SEISMIC EVENT

A swarm of twenty four seismic events, ranging in magnitude from M=1.5 to 2.9, occurred near Albany, NY, at the time of the 2011 M=5.8 Mineral, VA, seismic event; the swarm was about 600 km NNE of the Mineral, VA, 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 110 hours after the main Mineral event, and ranged from M=1.5 to 2.9. The events occurred within 4 km of the extrapolated trace of the NNE-striking Saratoga McGregor (S-M) fault. Outcrops near the 2011 swarm exhibit NNE-striking fracture intensification domains with minor offset in the Ordovician Utica shale. Other swarms in 2007, 2009, and 2010 occurred about 5 to 14 km SSW of the 2011 swarm on strike with the S-M fault. The 2007 swarm is also located on a WNW-trending lineament and has internal WNW-trends, consistent with WNW-trending cross faults similar to those in the Mohawk Valley or fault segment stepovers. Possible focal depths range from 8 to 18.5 km. These depths make correlation with specific surface structures tenuous, although seismic reflection data and fault traces through the Precambrian of the Adirondack Dome suggest that the faults might be nearly vertical well down into the Precambrian.

The Saratoga-McGregor fault has been mapped to the north, where it brings Precambrian against Ordovician units, and has been presumed to be primarily a Taconic aged fault with down drop to the east on the order of 150 m for Ordovician horizons, with a possible strike slip component. 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 presently experience limited seismicity. If the 2011 earthquake swarm is related to the Mineral VA seismic event, then the 2011 earthquake swarm indicates that the S-M fault sustained a stress release in response to the same stress (and release) that faults at Mineral experienced. It might be that the S-M fault was active during Triassic/Jurassic rifting—the structural location of the S-M fault indicates the fault could well have experienced minor activity during Triassic/Jurassic rifting, in which case the S-M fault is in a somewhat similar structural setting to the activated fault(s) at Mineral VA.