THE SEARCH FOR ACTIVE FAULTS AND THE ASSESSMENT OF SEISMIC HAZARD IN THE NORTHEASTERN U.S

Much of what is known about seismic hazards in regions of active seismotectonics comes from identifying active faults and assessing the mean repeat times of strong earthquakes on those faults. While the same ideas should hold true in less seismically active intraplate areas like the Northeastern U.S. (NEUS), the identification of active faults is very difficult due to the relatively low seismicity rates, the dense vegetation cover, and the widespread recent glacial sediments. Nevertheless, analyses of regional seismic network data of recent earthquakes is starting to give indications about where to look for possible active faults in the NEUS. For example, an analysis of the sequence of 38 earthquakes at Bar Harbor, ME in the fall of 2006 has revealed the location of the fault upon which these earthquakes took place. That fault was a thrust fault with a NNW strike and a dip toward the west. Inferences about the geologic structures upon which past strong earthquakes in the NEUS took place can be made from modern seismic monitoring combined with historic data. Earthquake activity at Amesbury, MA in 1999 combined with the locations of liquefaction features from 1727 suggest that the M 5.6 1727 Newburyport earthquake was probably on a NW-striking thrust fault. Modern earthquake activity in central New Hampshire from Lake Winnipesaukee south to Concord, NH may be late aftershocks from a strong earthquake in 1638. The M 4.4 1982 Gaza, NH earthquake in this seismic zone suggests that the 1638 event may have occurred on a NS-striking thrust fault. These studies of recent earthquakes indicate places where focused geological and geophysical studies could look for evidence of active faults and of past strong earthquake activity. New insight into the seismic hazard of the region would be obtained should any indicators of active faulting be found.