EVIDENCE FOR A TSUNAMI ALONG THE NORTHERN NEW ENGLAND COAST ~2,000 YEARS AGO

TUTTLE, M.P., and MOSELEY, C., M. Tuttle & Associates, 128 Tibbetts Lane, Georgetown, ME 04548, mptuttle@earthlink.net; WITKOWSKI, A., and DANISZEWSKA, G., Department of Paleooceanology, University of Szczecin,71-415 Szczecin, Poland; EFROS, J., and EBEL, J., Department of Geology and Geophysics, Boston College, Chestnut Hill, MA 02167, mptuttle@earthlink.net.

An unusual sand layer found in marshes along 35 km, and possibly 85 km, of the coast north of Cape Ann, Massachusetts, appears to be the signature of a tsunami that struck the region about 2,000 years B.P. The 1-10 cm thick deposit is composed of one to two subunits of massive and fining upward coarse to silty, very fine sand containing angular lithic fragments. The deposit occurs 1.5 to 5 km inland from present-day barrier beaches and thins as it rises in elevation toward the landward margins of marshes. An age estimate of 2,000 years B.P. is based on radiocarbon dating of the deposit in three different marshes. Diatom analysis supports a tsunami origin for the deposit and suggests that subsidence may have accompanied tsunami inundation at least in southeastern New Hampshire. Although their age constraint is poor, earthquake-induced liquefaction features in northeastern Massachusetts, and southeastern New Hampshire, could have formed at the same time as the tsunami deposit. Contemporaneous coastal subsidence and liquefaction would indicate that the tsunami was the result of a near-field coseismic tectonic event. Possible local sources include two major fault systems, the Gulf of Maine and the Norumbega, that traverse the study area and unmapped northwest-oriented faults that are suspected from modern instrumental seismicity. Additional study is needed to define the runup elevation and coastwise extent of the tsunami, to determine the amount and extent of subsidence, to better constrain the age estimate of prehistoric liquefaction, and to model the locations, dimensions, and orientations of possible local earthquake sources. The findings of this study have significant implications for the New England coast in particular and the Atlantic Seaboard in general, which are heavily developed and populated, low-lying in many places, and utterly unprepared for tsunamis.