TYPE: Oral

SEISMIC MICROZONATION OF THE BOSTON AREA

BRITTON, JAMES M., EBEL, JOHN E., and URZUA, ALFREDO, Dept. of Geology & Geophysics, Boston College, Weston Observatory, Weston, MA., brittonj@bc.edu, ebel@bc.edu, and urzua@bc.edu

Site amplification is a concern in Boston because it is a city in an area of moderate seismic activity, and it is important to study the site effects of the area because of the large amount of sediment and landfill (e.g., the Back Bay), which may greatly amplify seismic waves. Historic and recent records indicate that motion from nearby and larger, more distant earthquakes has been felt widely in the Boston area over the past few hundred years of recorded historical and network seismicity. Intensities up to MMI VI have been reported in Boston, with the higher intensities concentrating in the Back Bay and Beacon Hill areas. The objective of this research is to produce seismic microzonation maps for the Metropolitan Boston area showing the spatial variability of expected ground motion for a given input rock motion. The necessary inputs for the modeling of the site response in Boston are soil profiles, groundwater levels, and geotechnical parameters such as shear-wave velocity, plasticity index (PI), and unit weight. Fortunately, much new information is available on the geotechnical parameters from borings and crosshole measurements performed for the Central Artery/Tunnel project. This information, along with boring log information from the large collection compiled by the Boston Society of Civil Engineers, is being assembled into a queriable ArcView Geographic Information Systems (GIS) database for use in the microzonation maps. The computer program SHAKE uses the geotechnical parameters from the soil columns to compute the response. The response is being determined at evenly spaced intervals throughout the study area. Microzonation maps for the modification of earthquake ground accelerations at different periods (e.g., 0.1, 0.2, 0.5, and 1 seconds) and for peak ground acceleration (PGA) are being produced. Intensities from felt reports of past earthquakes and amplification factors of soil response from a previous microtremor study will be used to verify the amplification factors in the microzonation maps.

This is a student paper