Subsoil structure and site effects: A comparison between results from SPAC and HVSR in sites of complex geology |
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| Institution: | 1. Instituto de Ingeniería, Universidad Nacional Autónoma de México, Ciudad Universitaria, Coyoacán, 04510 México, D.F. México;2. Department of Civil Engineering, Aristotle University of Thessaloniki, P.O.B. 424, GR-54124, Thessaloniki, Greece;1. Instituto de Ingeniería, Universidad Nacional Autónoma de México, Ciudad Universitaria, Coyoacán, México, D.F. 04510, Mexico;2. Department of Civil Engineering, Aristotle University of Thessaloniki, P.O.B. 424, GR-541124 Thessaloniki, Greece;1. Department of Chemistry and Physics, University of Almeria, 04120 Almeria, Spain;2. Department of Mining, Geological and Cartographic Engineering, Polytechnic University of Cartagena, 30203 Cartagena, Murcia, Spain;3. Faculty? of Engineering, National Autonomous University of Mexico, Mexico |
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| Abstract: | We present the results of the analysis of array recorded microtremors at 14 sites, close to the edges of the Mygdonian basin in northern Greece (Euroseistest). These measurements were made in order to better constrain the geometry and velocity structure of the basin as the soil layers taper out close to rock outcrop, where geology is complex and we may expect significant changes of the subsoil structure over short distances. The data were analysed using the SPAC method and HVSR. The first interprets the measurements as Rayleigh waves (for the vertical component we analysed) and allows to invert a phase velocity dispersion curve from computed correlation coefficients. The second estimates a local transfer function directly, from ratios of Fourier amplitude spectra. A phase velocity dispersion curve could be derived for 12 of the 14 measurement sites, and at three of the sites no resonant frequency was observed in the HVSR. It is encouraging that we obtained good results at most of our sites, in spite of the lateral heterogeneity expected close to the edges of Euroseistest. Our results allow us to obtain shear wave velocity models at most of the measurement sites (12 out of 14). They are also useful to explore the relation between size of the array and wavelength range for which a dispersion curve may be estimated, which in our case has strong limitations. We identify the frequency of resonance of the sediments as a small loss of coherency in SPAC’s correlation coefficients. Finally, we also consider the applicability of the joint inversion of the resonance frequency determined using HVSR and the phase velocity dispersion curve obtained from SPAC. |
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| Keywords: | SPAC method Microtremor array measurements Correlation coefficients Phase-velocity dispersion curve Vs velocity profile HVSR method Resonant frequency |
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