Evaluation of Local Site Amplification in Lima, Peru Using Earthquake Ground Motion Records

Abstract eng:
Observed ground motion records were analyzed for estimating site effects in Lima Metropolitan Area, Peru in the frequency range from 1.0 to 20.0 Hz. The spectral inversion method was applied to separate the three effects – path, source and site amplification. This technique is the most appropriate for analyzing site response since amplification is only attributed to the effect of sedimentary soil layers over a basement. The earthquake data used were 55 seismic events observed at 19 seismic recording stations from 2003 to 2013. Most of the earthquakes used in the analysis were small events with a Local Magnitude ML between 4.0 and 5.0. The estimated S-wave quality factor (Qs) of the propagation path is modelled as Qs(f)=95.6f0.66 for the crust and mantle. This study characterizes the site response for the predominant subsurface soil formations over Lima such gravels, sands and clays. Stations installed on alluvial gravel deposits or locally known as Lima conglomerate tend to amplify at frequencies higher than 3 Hz. This material has S-wave velocities increasing gradually with depth from ~400 to ~1500 m/s. The site response of stations installed on sand and clay deposits control the amplification in a wide frequency range. Large amplification factors in the frequency range over 5 Hz is attributed to the effects of the top layers – sands and clays – with Vs ranging between ~100 to ~500 m/s, while amplification factors at frequencies lower than 5 Hz correspond to the high velocity contrast between the shallow and deep layers (Vs > ~800 m/s).

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