S-wave velocity structure of Chlef City, Algeria, by inversion of Rayleigh wave ellipticity

摘要

Chlef City, Algeria, which is located in the Lower Cheliff Basin, is vulnerable to seismic hazards. Since there is no constrained velocity model for the Lower Cheliff Basin, particularly at greater depths, we conducted an ambient vibration investigation to map the shear-wave velocity structure beneath the city, with the primary goal of supplementing the existing microzonation studies. Here, we inverted the Rayleigh wave ellipticity measurement curve of ambient vibrations measurements from 120 sites in Chlef City to estimate the shear-wave velocity structure. The study area was subdivided into six zones based on similarities between the observed horizontal-to-vertical spectral ratio of the ambient vibrations curves. Our resultant shear-wave velocity models show that the observed fundamental frequencies (0.3-1.6 Hz) are related to a thick layer (~800 m) of upper Miocene deposits, where Vs = 1000 m/s on average. Generally, the Mesozoic basement shows Vs > 2000 m/s. Moreover, the clear peaks observed at higher frequencies and only in the northwest part of the city are related to a thin layer of Quaternary deposits (Vs = ~250 m/s).