A moderate earthquake (Mw5.6) struck Algiers, Algeria on August 1, 2014. The ground motions of the mainshock were recorded by the permanent strong-motion network and signals from 71 aftershocks were recorded by the temporary network, which was installed just after the mainshock. Using observed data from the temporary network, we took three steps; locations of the aftershocks, estimation of site amplification effects and Q-factor, and simulation of the ground motions. We applied the modified joint hypocenter determination (MJHD) method to locate the aftershocks. Totally 65 events were successfully relocated. The results yield the averaged Vp/Vs ratio of 1.71 and S-wave velocity of 3.4 km/s in the target area. To estimate site effects and Q-factor, waveform data from 47 aftershocks with high signal-to-noise ratio are selected for spectral inversion analysis. Station CGSS is used as a divisor site and station CGSH as a reference site. Three assumptions are made for the reference site; theoretical transfer function calculated from the existing velocity structure model, averaged H/V ratio from S-coda waves of aftershocks, and a constant value (=2) due to the free surface effect. The estimated site effects from different assumptions show similar frequency characteristics at each station and the estimated Q-factor shows the frequency dependence. Finally we simulated ground motions of the mainshock at the temporary stations using the obtained site effects and Q-factor, along with theoretical source spectra and aftershock phase characteristics. The predicted peak ground accelerations at station BKDR shows good agreements with the recorded ones at the collocated permanent station BDRA, suggesting that the averaged H/V ratio assumption at the reference site can provide better estimations.
Keywords: Algiers, MJHD, Spectral Inversion Method, Site Effect, Q-factor, Divisor site. H/V ratio