The updated seismotectonic map (Yelles et al. 2017) shows and confirms that northern Algeria and particularly the city of Algiers are exposed to high seismic risk. This demonstrates the urgency of establishing a short, medium, and long-term prevention plan, allowing the protection of human lives and leading to the preservation of buildings and/or heritage structures.
Several risk assessment studies take into account the analysis of the dynamic behavior of soils and existing buildings. The challenge of this approach is characterized by the difficulty that an urban area is often made up of heterogeneous buildings (structure, material, height, age, maintenance, etc.); and that the ground conditions (depth, layer width, physical and dynamic characteristics, etc) cannot be uniform on the scale of the urban area.
The physical parameter controlling soil fundamental frequency is the shear rate Vs the impedance contrast between the soil layers. The implementation of geotechnical and/or geophysical methods is generally destructive and not easy in existing urban areas, due to its destructive nature and its unpractical tools in an urban environment. Taking advantage of the available soil investigations carried out during the construction of the Algiers metro in routes crossing the study area, the results of the SPT tests were collected and exploited. The ambient background noise of micro-tremors –HVSR, was acknowledged during the last decades as a reliable technique to identify the fundamental frequency of soil deposits near the surface. Being non-destructive, this technique is suitable for existing urban fabrics. But choosing measurement points remains a challenge because of the presence of tunnels housing vital networks having generally unknown routes. A careful interpretation of measured results is, therefore necessary to account for these uncertainties. The HVSR measurements are judiciously chosen (on the land-garden zone). A total of seventy (70) frequency values are identified across the entire study area and used in the analysis. The frequency values obtained using the two techniques on the same points were perfectly correlated.
The ground-structure resonance phenomenon has been identified by comparing frequencies of ground foundations and those of supported buildings. 50 concrete buildings and 177 masonry buildings spread over the entire study area were tested.
Buildings and ground frequency values shown in figures (11), b allowed us to break down the sample of buildings tested into three categories: (1) Buildings with frequency values lower than the ground frequency where resonance phenomenon is totally excluded; (2) Buildings with a frequency value higher than the ground frequency, which in the long term may be exposed to the risk of the resonance due to a decrease in buildings frequency; (3) The third category consists of buildings having their fundamental frequencies close to those of supporting ground, and therefore potentially exposed to a resonance phenomenon.
Tested buildings have close frequencies respectively in the range of [1.3 – 5] Hz for concrete buildings and [2.2 – 3.5] Hz for masonry buildings. The following figures indicate a risk of resonance phenomenon in the investigated buildings, having their fundamental frequency close to [1.95-2.22] Hz for the reinforced concrete structures (Fig. 11a) and [2.22-2.74] Hz for masonry structures (Fig. 11b). We can also notice that the vast majority of buildings within the tested sample are not exposed to the risk of resonance phenomenon.
According to the results obtained from the investigated area, the spatial representation of the ground frequencies (Fig. 12) shows that ground frequency values are essentially close to two interval values [4-6] Hz and [6-9] Hz, higher than building frequencies values. It can therefore be concluded that buildings in the study area, are not exposed to a ground-structure resonance phenomenon. Ground frequency was found lower than those of buildings frequency, 2Hz in a very restricted zone.
Several moderate and major seismic events shook the city of Algiers. Those of the El Asnam earthquakes on 10/10/1980 (Mw = 7.3), and Boumerdes on 05/21/2003 (Mw = 6.8) were among the most documented in the world, because of the lessons generated by their severity. Horizontal acceleration values estimated from recordings of strong movements in the epicenter zone are between 0.25 g and 0.70 g, (Tebbal 1986). Damages were recorded as far as 250 km radius from the epicenter of the first event.
In the case of a second event, a maximum acceleration of 0.58 g was recorded at Keddara at about 20 km, and at 0.34 g in Algiers at about 60 km from the epicenter respectively.
In terms of distance from the Algiers city, the event of 10/10/1980 can be considered distant (36°143N; 1.413E) and the 05/21/2003 as a near event (36°83N; 3°65E). However, the intensity observed in Algiers was the same (VI) for both events.
The post-seismic investigation of constructions in Algiers was carried out in the case of the 2003 event but not in the case of 1980. The percentages in terms of the level of damage reported in relation to all the constructions are shown in Figure (13). This shows that the ratio of constructions classified “red” according to the EMS98 scale is respectively 1.3%; 0.1% and 2.4% for the three districts considered by this study. These values include the pathology of the oldest and that caused by the earthquake event.
If we consider the buildings tested as a representative sample of the study area, the results found in this research, and findings from post-seismic investigations that followed real seismic events, we can conclude that the buildings in Algiers are not exposed to a resonance phenomenon between soil and buildings. The isoseistes plotted two major and two moderates (Bou Medfaa, 07/11/1959; Tipaza, 29/11/1989; El Asnam, 10/10/ 1980; Boumerdes, 21/05/2003) (Fig. 14), having shaken the city of Algiers generated an intensity between V and VI.
Microtremor signals registered at singles stations and spanning the old center of Algiers are analyzed to characterize the site response in terms of resonant frequency, amplification and to consider the possibility of a future site effect occurrence. This study is the initial one dedicated to this region, which has been poorly studied from a site effect point of view. The HV soil and building analysis as well as the different factors such as construction type and geotechnical parameters proved to be very useful in this kind of investigation.