NEOTECTONICS AND THE ACCRETED TERRANES OF NEW ENGLAND

Throughout both historic and recent times, the accreted terranes of eastern and southern New England, a major research focus of Chris Hepburn, have experienced a steady rate of earthquake activity. Most of these earthquakes have been small, but a few of them have been damaging. The earthquake epicenters have been spread throughout the accreted terranes with no strong concentrations of earthquake activity in any one area. The earthquakes tend to have very shallow focal depths (many less than 4 km), and only rarely has an earthquake taken place in these terranes where the evidence supports a focal depth below 10 km. The focal mechanisms of earthquakes in this area generally are thrust or oblique slip with a strong thrust component, and the nodal planes for the earthquakes are usually oriented somewhere between N-S and NW-SE. The focal mechanism data indicate that it is minor cross faults, rather than the major through-going NE-SW faults, that are active today, probably because these smaller faults are favorably oriented to the modern plate tectonic stress field while the major NE-SW faults are not. Some of the largest historical earthquakes in the northeastern U.S. (1727, 1755, 1884, 1904) have taken place in the accreted terranes of eastern New England or New York. The preponderance of the earthquake data suggest that older geologic structures that were active during Triassic time and later tend to be the ones that are seismically active today. Although the research of Chris Hepburn, his students, and many colleagues has concentrated on deciphering the Paleozoic history of the accreted terranes of eastern and southern New England, his work also has provided much data for understanding the modern seismotectonics and seismic hazard in this highly populated part of the U.S.