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Regional Seismic Site Characterization Maps of Massachusetts using a Depth to Bedrock and a Surficial Geology Map

Description: 

Site characterization maps are essential for regional planning and seismic hazard analyses and are most often represented in terms of Vs30-based NEHRP site classifications. Recent trends in engineering seismology have provided evidence that the site fundamental frequency (f₀) may be an additional (or alternative) site characterization parameter for seismic site response. In this work, we present regional seismic site classification maps in terms of both Vs30 and f₀ for Massachusetts using the State 1:24,000-scale surficial geologic map (Stone and others, 2018) and the State 100-m resolution depth to bedrock map (Mabee and others, 2023). In addition to these maps, we also present 1342 HVSR f₀ measurements and 35 Vs profiles in the state. Massachusetts typically has soft glacial sediments overlying hard basement rock as a result of the Wisconsin glaciation. During the Wisconsin glaciation, the Laurentide ice sheet covered the state, clearing most of the existing pre-glacial materials and depositing glacial sediments on the cleared bedrock surface. With this unique high impedance contrast structure – soft, low velocity sediment over hard, high velocity bedrock – we model the overburden using a layer-over-halfspace model composed of an average overburden velocity (Vs_avg), a depth to bedrock (d_s) and a bedrock velocity (V_R). To estimate Vs_avg in the layer-over-halfspace model, we combine the surficial geologic map units into four groups based on their similar mechanical properties, group the Vs profile stations, and then fit a Vs_avg distribution to each grouping. To estimate d_s in the layer-over-halfspace model, we use the state depth to bedrock map of Mabee and others (2023) which includes depth to bedrock uncertainty. These two maps provide estimates of d_s and Vs_avg with uncertainty at each location in the state. Using the layer-over-halfspace model at a 100-m resolution grid, we develop high resolution maps of f₀ and Vs30. We present these regional maps of Vs30 and f₀ and highlight differences within three subregions: Boston, Cape Cod, and the Connecticut River Valley.

Session: From Sensors and Networks to Site Characterization and Site Response: Coming Full Circle

Type: Oral

Date: 4/18/2023

Presentation Time: 02:15 PM (local time)

Presenting Author: Marshall A. Pontrelli

Student Presenter: Yes

Invited Presentation: 

Authors