Rapid Post-Earthquake Microtremor Measurements for Site Amplification and Shear Wave Velocity Profiling in Kathmandu, Nepal

This paper presents application of microtremor (ambient vibration) and surface wave field techniques for post-earthquake geotechnical reconnaissance purposes in Kathmandu, Nepal. Horizontal-to-vertical spectral ratios (HVSR) are computed from microtremor recordings at 16 individual measurement locations to obtain an estimate of fundamental frequency (site period) of the subsurface soils. A combination of active-and passive-source surface wave array testing was accomplished at five key sites including Kathmandu's Durbar Square and Airport. Joint inversion of each site's higher frequency dispersion and lower frequency HVSR data sets provides an estimate of subsurface material stiffness [i.e., shear wave velocity (V-S) depth profiles]. Direct comparison of our V-S profiling at Kathmandu Durbar Square and that accomplished by downhole V-S and/or standard penetration testing (SPT) profiling yield similar results. Classification of the five sites based on average V-S, site period, and/or basin depth is presented. There is little differentiation in these site classification designations amongst the five sites, which does not capture significant differences in observed earthquake damage.