EARTHQUAKE-INDUCED DEFORMATION OF ORGANIC-RICH LAKE SEDIMENTS IN THE NORTHEASTERN UNITED STATES – PRELIMINARY RESULTS FROM WALDEN AND SLUICE PONDS

Historic descriptions of earthquakes in New England indicate that eastern Massachusetts experienced groundshaking of intensity VI or VII during the A.D. 1727 Newbury and A.D. 1755 Cape Ann earthquakes and potentially during a less well-documented A.D. 1638 Central New Hampshire earthquake. Lakes that formed in the northeastern United States after the retreat of the Late Pleistocene ice sheet are abundant and preserve a continuous sedimentary record that might show traces of these historic seismic events. Based on the intensity distributions of the historic earthquakes and lake basin morphology, two lakes have been targeted for paleoseismic studies: Walden Pond in Concord, MA, and Sluice Pond in Lynn, MA. Bathymetric and seismic surveys were carried out to understand large-scale basin architecture. Up to 1.6 m long sediment cores were retrieved from the deepest areas of each lake. The lake deposits were interpreted using computerized tomography (CT) scanning, macroscopic sediment core descriptions, smear slide analysis, magnetic susceptibility measurements, loss on ignition (LOI) procedures and laser diffraction particle size analysis. Koster et al. (2005) developed an age model for sediments from the deepest sub-basin of Walden Pond. The age model for Sluice Pond is based on radioisotopes (²¹⁰Pb, ¹³⁷Cs, ¹⁴C), and the well-constrained Ambrosia pollen horizon indicative of European contact in the region (i.e. ~ A.D. 1630). The lake sediments are composed of massive organic-rich mud and the homogeneous nature of the sediment renders the identification of earthquake-induced deformation difficult. During seismically-induced failure, the semi-consolidated, cohesive mud could experience fracturing. In addition, mobilization of slope sediments and differential gravitational settling of resuspended lake deposits can cause the deposition of coarser clastic layers. The well-developed age models for both lakes allow the tight constrain of the 17^th and 18^thcentury horizon and any related earthquake-induced deformation in 15-30 cm depth of the sedimentary record.

Koster D., Pienitz R., Wolfe B.B., Barry S., Foster D.R. and Dixit S.S. 2005, Paleolimnological assessment of human-induced impacts on Walden Pond (Massachusetts, USA) using diatoms and stable isotopes. Aquat. Ecosyst. Health Manage. 8: 117–131.