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

Sarmadi Mohammad Ali, Heidari Reza, Mirzaei Noorbakhsh, Siahkoohi Hamid Reza

Acta Geophysica

2021

Vol. 69, no. 4

1177--1188

The horizontal-to-vertical spectral ratio (HVSR) has been widely used to calculate near-surface site efects including funda mental resonance frequency and soil amplifcation. 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 identifcation of fundamental idea of Nakamura’s technique and its applications. In: Proceedings of the 12th World Conference on Earthquake Engineer ing. Auckland, New Zealand.) HVSR method is to assume there are little to no amplifcation or de-amplifcation of motion recorded by the vertical component. In this study, we disregarded this assumption and used diferent 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 infuence on the site efects, has been incorporated in proposed spectral ratios. The efciency 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 diferent geological characteristics and diferent site efects parameters. According to the results, The HVSR method is under-determined at stations where the waves are mainly amplifed as they pass through the site. Moreover, in a single sta tion by calculating the amplifcation value on the vertical component, Site efects 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.

Characterization of Site Conditions for Selected Seismic Stations in Eastern Part of Romania

Grecu B., Zaharia B., Diaconescu M., Bala A., Nastase E., Constantinescu E., Tataru D.

Acta Geophysica

2018

Vol. 66, no. 2

153--165

Strong motion data are essential for seismic hazard assessment. To correctly understand and use this kind of data is necessary to have a good knowledge of local site conditions. Romania has one of the largest strong motion networks in Europe with 134 real-time stations. In this work, we aim to do a comprehensive site characterization for eight of these stations located in the eastern part of Romania. We make use of a various seismological dataset and we perform ambient noise and earthquake-based investigations to estimate the background noise level, the resonance frequencies and amplification of each site. We also derive the Vs30 parameter from the surface shear-wave velocity profiles obtained through the inversion of the Rayleigh waves recorded in active seismic measurements. Our analyses indicate similar results for seven stations: high noise levels for frequencies larger than 1 Hz, well defined fundamental resonance at low frequencies (0.15-0.29 Hz), moderate amplification levels (up to 4 units) for frequencies between 0.15 and 5-7 Hz and same soil class (type C) according to the estimated Vs30 and Eurocode 8. In contrast, the eighth station for which the soil class is evaluated of type B exhibits a very good noise level for a wide range of frequencies (0.0120 Hz), a broader fundamental resonance at high frequencies (~ 8 Hz) and a flat amplification curve between 0.1 and 3-4 Hz.