INVESTIGATING EARTHQUAKE-INDUCED DEFORMATION FEATURES IN LAKE SEDIMENTS OF NORTHEASTERN MASSACHUSETTS, USA

Based on the historic descriptions and the reconstructed intensity maps for earthquakes in New England, it is clear that large parts of northeastern Massachusetts experienced groundshaking of intensity VI or VII during the A.D. 1727 Newbury and A.D. 1755 Cape Ann earthquakes. Lakes that formed in this area 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 three lakes have been targeted for paleoseismic studies: Pleasant Pond in Wenham, MA, 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 stratigraphy of lake deposits was interpreted using macroscopic sediment core descriptions, smear slide analysis, computerized tomography (CT) scanning and magnetic susceptibility measurements. A detailed age model was developed for Sluice Pond sediments 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). This well-developed age model allows the tight constrain of the A.D. 1755 horizon and any related earthquake-induced deformation in 20-30 cm core depth. The deposits of the three lakes are mainly composed of massive organic-rich mud and the homogeneous nature of the sediment renders the identification of earthquake-induced deformation difficult. The semi-consolidated, cohesive mud most likely experiences fracturing during seismically-induced failure, which we hope to identify in high-resolution CT scans. In addition, mass movements along steep lake sides during earthquake shaking could introduce coarser clastic material into the lake. Detailed laser diffraction particle size analysis should identify such deposits.