Site‐Effects Investigation in the City of Chlef (Formerly El‐Asnam), Algeria, Using Earthquake and Ambient Vibration Data

Abstract

Weak‐motion recordings of local earthquakes and ambient vibration data in the 0.2–15 Hz frequency range were used to quantify site effects in downtown Chlef city, Algeria. Nine short‐period stations were installed in the city. A total of 15 local events with 3.1≤ M L≤4.9 were selected. Three experimental techniques were applied: standard spectral ratios, receiver functions, and horizontal‐to‐vertical spectral ratios using ambient vibrations (HVSRN). Theoretical HVSRN curves were also calculated from suggested soil models for each site. Fundamental frequencies obtained from all techniques showed good agreement. Two groups of fundamental frequencies were distinguished in the city. In the northern part of the study area, the frequency peaks varied from 0.3 to 0.4 Hz; in the southern part they varied from 0.6 to 1.6 Hz. The amplification factors of these frequency peaks were 4–8. Earthquake and ambient vibration data were able to identify fundamental frequency peaks successfully; however, the amplitudes of the peaks were slightly lower for ambient vibrations. Moreover, the fundamental frequency peak progressively increased from northwest to southeast, which is consistent with the direction of decreasing thickness of a Miocene layer overlying the basement. We infer, as expected, that the large impedance contrast between Miocene deposits and Mesozoic basement, which can be interpreted as seismic bedrock, is the cause of the fundamental frequency. Furthermore, earthquake data were more appropriate for identifying peaks at frequencies higher than the fundamental one. Indeed, a clear peak was observed at all northwestern sites in the 3–4 Hz range, with an amplification factor of 3–5. At the other sites, particularly in the center of the city, the peak was less evident.