THE RECORD OF LARGE HISTORIC EARTHQUAKES IN LAKE SEDIMENTS OF NORTHEASTERN MASSACHUSETTS

We present sediment data from three lakes in northeastern Massachusetts that allow us to determine the fingerprint of two large historic earthquakes, the 1755 Cape Ann earthquake with a magnitude of M=5.9 and the 1638 Southern New Hampshire earthquake with a less well constrained magnitude of M=6.5-7.0. Using a combination of sedimentological, geophysical, geochemical and palynological proxies we were able to identify the signature of the 1755 Cape Ann earthquake in Sluice Pond, Lynn, MA in form of a turbidite mobilized at the steep lake basin slopes during strong groundshaking. We developed a detailed age model using radioisotopes, industrial contaminants and pollen stratigraphy to narrow deposition of the turbidite to the mid-18^th century. The likelihood of mass wasting in 1755 was likely increased by sediment loading of slopes from soil erosion caused by widespread deforestation by the first European settlers arriving in this region in the first half of the 17^th century. While Sluice Pond is located within 40 km of the 1755 epicenter and experienced groundshaking of intensity VII, Walden Pond in Concord, MA experienced shaking of lesser intensity (I=VI), likely not strong enough to cause sediment deformation or mass wasting here. However, in sediments below the European settlement horizon, we were able to identify another deposit with larger particle sizes, probably indicating an older mass wasting event. This coarser deposit can be found in all three basins of Walden Pond providing evidence for a regional trigger such as an earthquake. Preliminary data from Sluice Pond as well as Lake Waban in Wellesley, MA similarly show an unusual deposit likely linked to mass wasting before large scale disturbance of the landscape by European settlers. Here, additional data is necessary to further characterize the deposit and its age. Identifying an earthquake signature for pre-European sediments would allow us to use this fingerprint to extend earthquake histories further back in time potentially through the entire Holocene when New England became ice-free and the lakes formed. The distribution of the three lakes across eastern Massachusetts is well suited to determine thresholds of lake sediment deformation with increasing distance to historically reported epicenters as well as estimates of the size of past earthquakes.