CHARACTERIZATION OF LIQUEFACTION SUSCEPTIBILITY IN THE BOSTON, MASSACHUSETTS METROPOLITAN AREA

The highly populated Boston metropolitan area and surrounding communities are underlain by large regions of Quaternary through Holocene sediments and non-engineered artificial fill, all of which are potentially susceptible to liquefaction during coseismic ground motions. The occurrence of several large historic earthquakes of about M6.0 (e.g. 1727 and 1755) demonstrates the need to constrain the susceptibility of these sediments to liquefaction. We use surficial geologic mapping and analyses of geotechnical boring data to analyze the liquefaction susceptibility of these geologic units and to develop regional susceptibility maps for eight USGS 7.5-minute quadrangles that include the downtown Boston area and surrounding communities. Surficial geologic data were compiled using published and unpublished geologic maps, aerial photographic interpretation, and field reconnaissance mapping. Liquefaction triggering threshold ground motions were determined using geotechnical analyses on a digital database of 2963 geotechnical boreholes, primarily from the downtown artificial fill units but also extending into the surrounding communities. An additional 12,000 geotechnical boring logs were collected and used to characterize the subsurface geology and confirm the surficial mapping. Our results show that much of the downtown Boston area is underlain by extensive regions of non-engineered artificial fill that, when saturated, is susceptible to liquefaction during seismic loading. The highly heterogeneous nature of the fill results in large variability in susceptibility to liquefaction. Holocene alluvial and marsh deposits in the region are also moderately to highly susceptible to liquefaction. Much of the outlying area is underlain by Pleistocene and Quaternary glacial and glaciofluvial deposits, which have low to moderate susceptibility to liquefaction. The study provides data needed to assist in characterizing seismic hazards and mitigating risks, and provides information to assist communities in urban planning and emergency response.