THE EXPECTED EFFECT OF LOCAL SURFICIAL GEOLOGY ON THE GROUND MOTIONS OF FUTURE EARTHQUAKES IN BOSTON, MASSACHUSETTS

Seismic hazard and risk assessment in an urban area like Boston, Massachusetts require a knowledge of local surficial geology and how that geology might modify bedrock ground motions in future earthquakes. Because of Boston's setting where a river estuary and harbor area have been extensively modified by manmade land, there are many areas in Boston where the local soils will modify and amplify local earthquake ground shaking. Several observational and theoretical studies have started to delineate those areas in Boston that might be expected to experience the most hazardous ground shaking in future earthquakes. In the Back Bay and Cambridge near the Charles River, ground-motion amplification is likely to be greatest at periods of 0.5 to 1.0 sec. On Beacon Hill and nearby downtown Boston, increased ground shaking at periods of .20 to .40 sec would likely be experienced. South Boston could feel increased ground shaking in the vicinity of .2 sec period. Spatial variations in Boston's surficial geology suggest that earthquake ground motions could fluctuate noticeably across the city.