iPlex Lunch spring-2017

Seismic Site Characterization: Mark, Measure & Cut for VS30

I present a review of the state-of-practice for estimating VS30, the time-averaged shear-wave velocity of the upper 30 m, as well as select developments for advancing measured or proxy-based VS30 methods. VS30 values have traditionally been derived directly from on-site array-based records of seismic travel-times. As a result of cost and/or environmental factors that restrict the mobilization of on-site recording arrays, remotely-derived proxy-based methods have been used to estimate VS30 values. Thus, proxy-based methods—commonly using map information: geology, slope, terrain, or their hybrids—serve as a stopgap solution until on-site measurements are available. Because of the indirect nature of these map-based methods, proxy-based VS30 estimates are known to have substantial uncertainties, whereas inter- and intra-method variability of measured VS30 values is typically 5-10%. To reduce uncertainties, Iwahashi et al. (2016) used a large data set of recently available measured VS30 values and an improved terrain framework to recalibrate their proxy-based VS30 model. For sites where cost isn’t a factor, there are advancements in the analyses of earthquake and microseismic data from seismic monitoring stations that allow estimations of VS30. Herrick et al. (2017) found P-wave-based VS30 estimates from earthquake sources correlate with array-based VS30 measurements in the VS30 range of 500-1500 m/s, while earthquake-based VS30 estimates outside of this range differ by more than 10 percent. Hassani et al. (2017) expanded the Hassani and Atkinson (2016) study, which compared VS30 to earthquake-based estimates of the dominant site frequency (fd), to fd based on microseisms and found the earthquake and microseismic fd estimates scale linearly. Lastly, Yong et al. (2017) found measured VS30 values correlate well with array-based VR40 (Rayleigh-wave phase velocity at the 40-meter wavelength) estimates, thus indicating VR40 is a potential proxy for estimating VS30 values.