ABSTRACT

In 1638, an earthquake produced severe shaking in eastern Massachusetts and Trois‐Rivieres, Quebec. Previous studies have suggested that the 1638 earthquake had a magnitude MLg6.5±0.5 and was centered in central New Hampshire. In this study, relative earthquake location analysis, computation of focal mechanisms, and computation of focal depths based on fundamental‐mode Rayleigh waves were used to analyze recent, digitally recorded seismic data to more clearly define the possible active fault for this earthquake and determine its seismotectonic framework. The combined results of the analyses are consistent with a thrust fault trending north‐northwest–south‐southeast and dipping eastward in the postulated 1638 epicentral zone. Modern earthquakes in this zone occur at focal depths of about 3–10 km, with most of the events occurring at depths 5  km, suggesting that this is the depth range of the 1638 rupture. Depending on the depth of the pre‐Silurian basement of the Central Maine terrane, the active structure in the 1638 earthquake may have been a basement‐involved thrust fault or a reactivated east‐dipping thrust fault located between the nappes of the overlying Silurian–Devonian aged metasedimentary rocks. The postulated fault plane projects to the surface along portions of the Pemigewasset and Merrimack Rivers. A field survey was undertaken to search for earthquake‐induced liquefaction features along the Pemigewasset, Merrimack, and Winnipesaukee Rivers as well as of the Suncook River Avulsion site. Although several small strata‐bound soft‐sediment deformation structures were found during the survey, none are unequivocally seismically induced.

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