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Site Amplification in Glaciated Terrain: HVSR and MASW Measurements in New England

Session: How Well Can We Assess Site Effects So Far? III

Type: Oral

Date: 4/21/2021

Presentation Time: 03:00 PM Pacific

Description: 

Near surface geology plays an important role in earthquake ground motion estimation and thus is a vital area of study for infrastructure resilience. Soil profiles with lower velocity sediments overlying higher velocity bedrock amplify energy at certain frequencies due to resonance effects and conservation of energy across an impedance boundary. The NEHRP estimates this site amplification using Vs₃₀: the average shear wave velocity of the top 30 meters of soil. Vs₃₀ is a good preliminary step in estimating site amplification, but it does not 1) provide information concerning the depth to the impedance boundary which determines the frequency of shaking and 2) capture effects from soil deeper than 30 meters. In this study, we use two measurements, Multi-channel Analysis of Surface Waves (MASW) (Park 1998, 1999) and the Horizontal to Vertical Spectral Ratio (HVSR) (Nakamura, 1989) to provide greater constraint on site near surface S-wave velocity profile than Vs₃₀ alone. MASW estimates S-wave velocity profiles using surface wave dispersion and HVSR provides the site fundamental frequency, f₀. We can combine these measurements to estimate two parameters that define the site amplification: 1) f₀ and 2) the average shear wave velocity to the impedance contrast. We study three glacially derived surficial geologic units in New England: 1) glacial lakes, 2) river basins and 3) marine clay deposits and within each unit, calculate the fundamental resonance using the HVSR and a dispersion curve using MASW at multiple sites and identify variability within and between units. We find that in the field, the addition of an HVSR measurement is easy and well worth the time to supplement the MASW derived S-wave profile and show that a site can be characterized this way using f₀ and shear wave velocity to the impedance boundary, a similar, but slightly more robust classification than Vs₃₀ alone. Our results will ultimately be used to develop updated soil amplification maps for the New England region.

Presenting Author: Marshall A. Pontrelli

Student Presenter: Yes

Authors