Surface Wave Methods in Inversely Dispersive and Non-1D Sites

Session: Advances in Upper Crustal Geophysical Characterization [Poster]
Type: Poster
Date: 4/29/2020
Time: 08:00 AM
Room: Ballroom
Description: 

Earthquake site classification based on average shear wave velocity (Vs) of the upper 30 meters (Vs30) requires measurement of the Vs depth profile at a site. Surface wave array methods generate dispersion estimates which can be jointly inverted with horizontal-to-vertical spectral ratios (HVSR) to retrieve layered Vs estimates for earthquake site classification. Non-invasive methods for earthquake site characterization are applied as part of a seismic microzonation mapping project for Vancouver, British Columbia. Dispersion curves are obtained from active-seismic multichannel analysis of surface waves (MASW) and passive-seismic ambient vibration array (AVA) field surveys for sites with lateral variations or inversely dispersive media. Velocity inversions can induce higher modes in dispersion estimates, and incorrect mode identification can affect the accuracy and reliability of velocity profiles obtained from inversion. Dispersion curves from passive-seismic survey data are calculated separately for horizontal and vertical components. Estimates with poor correlation over the same frequencies for different components may indicate the presence of higher modes (Tokimatsu et al. 1992). A slant f-k transform approach is used to identify frequencies with modal superposition from active-seismic data (Serdyukov et al. 2019). Incorrect mode identification can be evaluated in terms of effect on site classification from Vs profile inversions. Geotechnical data (e.g. cone penetration testing) is converted to 1D Vs profiles and an inverse distance weighting is applied to generate reference Vs profiles at surface array sites. HVSR amplification functions are obtained from each site to test for lateral variability. The amplification functions and reference Vs depth model are used to constrain model parametrization prior to inversion. This study will help develop a methodology to evaluate suitability and reliability of surface wave inversion methods in sites with complex geology.

Presenting Author: Chris Boucher

Authors