The improvement of the earthquake and microseismic Horizontal-to-Vertical Spectral Ratio (HVSR) in estimating site effects

The horizontal-to-vertical spectral ratio (HVSR) has been widely used to calculate near-surface site effects including fundamental resonance frequency and soil amplification. In this study, we investigated the features of HVSR in order to assess the reliability of this method. The main premise of Nakamura (1989; Nakamura, Y., 2000. Clear identification of fundamental idea of Nakamura's technique and its applications. In: Proceedings of the 12th World Conference on Earthquake Engineering. Auckland, New Zealand.) HVSR method is to assume there are little to no amplification or de-amplification of motion recorded by the vertical component. In this study, we disregarded this assumption and used different proposed spectral ratios of earthquake accelerograms to examine and evaluate the accuracy of the HVSR technique. In addition, to increase the accuracy of the results, near-surface attenuation parameter Kappa (K), as an important spectral decay parameter which has influence on the site effects, has been incorporated in proposed spectral ratios. The efficiency of proposed spectral ratios was evaluated using 3948 accelerograms (Includes surface and borehole stations) of 496 earthquakes from 18 KiK-net accelerometery stations of the two regions of Japan with moment magnitudes ranging from 3 to 5. The two selected areas have different geological characteristics and different site effects parameters. According to the results, The HVSR method is under-determined at stations where the waves are mainly amplified as they pass through the site. Moreover, in a single station by calculating the amplification value on the vertical component, Site effects measurements obtained from surface and borehole data along with classical HVSR technique have a lower mean absolute error than those obtained by using HVSR technique alone.