INVESTIGATING THE EARTHQUAKE HISTORY OF NEW ENGLAND IN LACUSTRINE ENVIRONMENTS

Lake sediments in northeastern Massachusetts have the potential to record large historic and pre-historic earthquakes of New England. Studying earthquake activity in an intraplate setting like New England can be difficult as seismogenic faults cannot be identified with certainty. In addition, relatively low strain rates cause the recurrence intervals of large earthquakes to be longer than the historical record. Therefore, the study of earthquake-induced deformation preserved in the geologic record is helpful to gaining a better understanding of the frequency and magnitude of earthquakes in New England. Evidence of one earthquake in particular may be preserved in lake sediments: the M5.9 1755AD Cape Ann earthquake with a predicted epicenter approximately 40 kilometers northeast of Cape Ann, Massachusetts.

Here we investigate potential traces of paleoseismic activity in sediment cores from Sluice Pond, Lynn, Massachusetts. Evidence of ground shaking would include mass movement deposits and small-scale deformation structures such as faults and liquefaction features. The pond itself is a deep, 55-acre natural pond that has a maximum depth of 20 meters. The postulated epicenter for the 1755AD earthquake was approximately 60 km from Sluice Pond, and it was affected by ground shaking of intensity VII. Two cores of 200 and 470 cm length were taken, split, photographed, and described sedimentologically. Radiocarbon dating indicates continuous sedimentation in the deepest lake basin since ~12,000 cal y BP. The majority of the sediment is a dark gyttja with slightly coarser material in the lower 50 cm of the sedimentary record. Macroscopically there were no visible traces of earthquake deformation. Bulk density and organic matter content were obtained through the loss on ignition procedure. Additional studies include detailed grain size analysis in order to determine mass movement deposits as well as CT scans to investigate brecciated intervals and deformation structures that are not visible to the naked eye.

Techniques from this study can be implemented to find deformation related to earthquakes that are not documented in the historic record. This information will aid in the understanding of the paleoseismic history of New England which in turn can help to better prepare for future events.