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

Abstract

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= ∼2000 m/s).

A Shear-Wave Velocity Model in the City of Oued-Fodda (Northern Algeria) from Rayleigh Wave Ellipticity Inversion

“…The Vs values for the Ante-Neogene basement vary between and 2000 m/s and 2600 m/s. The velocity values are in agreement with the ones obtained by Layadi et al [17].…”

“…This study improves the one made by the WCC Report [7] in Oued-Fodda city and carries on the one made in the city of Chlef [16,17] for contributing to seismic hazard and risk mitigation in the region.…”

“…The Upper Miocene sediments, which underlie the Quaternary deposits, consist mainly of blue marls and limestones. Talaganov et al [38], assigns a Vs value that ranges between 1050 m/s and 1070 m/s using the seismic refraction method, meanwhile Layadi et al [17], provides values between 1000 m/s and 1300 m/s for the upper Miocene deposits by applying the RWE inversion.…”

“…There is not any geotechnical information about the Mesozoic basement in the Oued-Fodda region, even though this formation outcrops in the Temoulga Massif, 1 km east of the city. Layadi et al [17], assigns a Vs value that ranges from 2000 m/s to 2600 m/s in the city of Chlef (about 15 km west of Oued-Fodda). A Vs value of 2800 m/s was assigned at a depth of about 1km in Chlef region by tomographic inversion [40].All the geotechnical information for the study area is summarized in Table 1.…”

“…In this regard, a complete site effect investigation has been recently conducted in the city of Chlef [16,17] in order to improve the study presented in the WCC report [7]. Standard spectral ratio, receiver functions and HVSR methods have been applied for both earthquakes and ambient vibrations.…”