Site Characterization of Strong-Motion Recording Stations of Delhi Using Joint Inversion of Phase Velocity Dispersion and H/V Curve

Site characterization is one of the most important aspects of any strong-motion instrumentation. Nowadays it has become common practice to provide the characterization details up to bedrock level. Without proper site characterization, strong-motion records of any station cannot be fully utilized. In India, strong-motion instrumentation sites were classified in three categories as per V-S30 values. These V-S30 values were estimated using the Borcherdt (1994) methodology, in which physical properties of visible soil layer are used for V-S30 estimation. Because this methodology does not use any field testing, the probability of getting erroneous results is very high. Hence, to get accurate assessment of site characteristics, we conducted field testing at 19 strong-motion instrumentation sites in Delhi, India. The site characteristics assessed here are determined using joint inversion of multichannel analysis of surface waves and horizontal-to-vertical spectral ratio (HVSR) from ambient noise results simultaneously to estimate shear-wave velocity profiles. The benefit of using this method is that it provides site characteristics assessed through shear-wave velocity profiles up to much deeper soil strata. The results obtained from this analysis are further validated using ground response analysis from recorded ground motions. Further, the profiles obtained are studied for uncertainties using the computer program STRATA. Transfer functions obtained from STRATA are then compared with HVSR of ambient vibration records as well as HVSR from weak-motion earthquake records available for the sites. These curves are found to well matching with each other in this study. Site characterization carried out here will be very useful for studies related to seismic-hazard assessment of the Delhi region and studies related to attenuation models.