Site dominant frequencies in Lima, Peru, by h/v spectral ratio of seismic records

Abstract

Strong motion records are important for improving seismic design, damage assessment and analysis of earthquake effects. Despite the relevance of having strong motion networks, their implementation in Peru has been performed in the very recent years. For instance, only five strong motion stations in Lima recorded the 2007 Mw 7.9 Pisco earthquake, whereas 55 stations recorded the 2021 Mw 6.0 Mala earthquake. The current number of instrumentations in Lima city provides an opportunity to improve soil characterization. This study aims to determine the dominant frequency (fd) of the seismic stations located in Metropolitan Lima. The procedure we adopted, first considered collecting, for an arbitrary station, all the available events recorded from 2011-2021, including the 2007 Mw 7.9 Pisco earthquake. Then, the pseudo spectral acceleration, using 5% damping, for each component were computed, and the horizontal-to-vertical (H/V) spectral ratios were calculated. Finally, the average H/V spectral ratio was used to adequately characterize the values of fd . In total, we evaluated 51 stations throughout Lima city. Values of fd lower than 1 Hz were observed for the coastal areas in which the underlying soil consist of clayey/sandy deposits and regions with an important impedance contrast in the deeper part of the substructure. These values increment towards the center of the city coinciding with surficial gravel deposits.