Inter- and Intra-method Variability of VS Profiles and VS30 at ARRA-funded Sites

Abstract:

The 2009 American Recovery and Reinvestment Act (ARRA) funded geophysical site characterizations at 191 seismographic stations in California and in the central and eastern United States. Shallow boreholes were considered cost- and environmentally-prohibitive, thus non-invasive methods (passive and active surface- and body-wave techniques) were used at these stations. The drawback, however, is that these techniques measure seismic properties indirectly and introduce more uncertainty than borehole methods. The principal methods applied were Array Microtremor (AM), Multi-channel Analysis of Surface Waves (MASW; Rayleigh and Love waves), Spectral Analysis of Surface Waves (SASW), Refraction Microtremor (ReMi), and P- and S-wave refraction tomography. Depending on the apparent geologic or seismic complexity of the site, field crews applied one or a combination of these methods to estimate the shear-wave velocity (V_S) profile and calculate V_S30, the time-averaged V_S to a depth of 30 meters. We study the inter- and intra-method variability of V_S and V_S30 at each seismographic station where combinations of techniques were applied. For each site, we find both types of variability in V_S30 remain insignificant (5–10% difference) despite substantial variability observed in the V_S profiles. We also find that reliable V_S profiles are best developed using a combination of techniques, e.g., surface-wave V_S profiles correlated against P-wave tomography to constrain variables (Poisson’s ratio and density) that are key depth-dependent parameters used in modeling V_S profiles. The most reliable results are based on surface- or body-wave profiles correlated against independent observations such as material properties inferred from outcropping geology nearby. For example, mapped geology describes station CI.LJR as a hard rock site (V_S30 > 760 m/s). However, decomposed rock outcrops were found nearby and support the estimated V_S30 of 303 m/s derived from the MASW (Love wave) profile.