(Algiers) reclaimed area by ambient vibration

Arab J Geosci (2017) 10:292 DOI 10.1007/s12517-017-3074-1

ARABGU2016

Imagery of the metamorphic bedrock roof of the Sahel active fault in the Sablettes (Algiers) reclaimed area by ambient vibration HVSR Mohamed Yacine Tebbouche 1,2 & Djamel Machane 2 & Souhila Chabane 1 & El-Hadi Oubaiche 2 & Aghiles Abdelghani Meziani 2 & Dalila Ait Benamar 2 & Hakim Moulouel 2 & Ghani Cheikh Lounis 1 & Rabah Bensalem 2 & Abderrahmane Bendaoud 1

Received: 22 December 2016 / Accepted: 20 June 2017 # Saudi Society for Geosciences 2017

Abstract The Sablettes (Algiers) coastal reclaimed fringe region, located on the hanging wall of the Sahel active fault, is subject to different types of geological hazard such as flood and tsunami, coastal uplift, earthquake, liquefaction, landThis article is part of the Topical Collection on Current Advances in Geology of North Africa * Mohamed Yacine Tebbouche [email protected] Djamel Machane [email protected] Souhila Chabane [email protected] El-Hadi Oubaiche [email protected]

slide, and site effects. In this present work, we used ambient vibration HVSR for imaging the bedrock. The thickness of the sedimentary column under the backfill layer is unknown, and the coastal reclaimed areas are prone to strong amplification of seismic waves. The determination of the depth of the metamorphic base allowed us to establish a mapping of the bedrock roof surface. The 3D representation of this surface enabled us to present models of tectonic structures in this basement (i.e., fault, fold). This analysis will make it possible to make better evaluation of the amplification after having determined the depth of the metamorphic basement exceeding 240 m, which is supposed to have velocities close to those of the seismological basement, as well as the thicknesses of the different layers surmounting it. Keywords Bedrock imaging . Ambient vibrations . HVSR . The Sablettes (Algiers) reclaimed area

Aghiles Abdelghani Meziani [email protected] Dalila Ait Benamar [email protected] Hakim Moulouel [email protected] Ghani Cheikh Lounis [email protected] Rabah Bensalem [email protected] Abderrahmane Bendaoud [email protected] 1

FSTGAT-USTHB, BP 32 El Alia, Bab Ezzouar, 16111 Algiers, Algeria

2

CGS, Rue Kaddour Rahim, Hussein-Dey, Algiers, Algeria

Introduction Given its location in a convergence zone between the African and Eurasian plates, the north of Algeria is a seismically active region. Although earthquakes represent the most important hazard, this region is nevertheless subject to other geological hazard types such us flood (Machane et al. 2004, 2008; Mimouni et al. 2009; Cheikh Lounis et al. 2013), particularly strong in the Sablettes area (Hussein-Dey, Algiers, Fig. 1) that was recently reclaimed, tsunami (Maouche et al. 2009), morphotectonic, which might be expressed by coastal uplift (Meghraoui et al. 2004), as well as landslide (Guemache et al. 2010, 2011; Bougdal et al. 2013). Northern Algeria was affected by destructive earthquakes, which involved in most

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Arab J Geosci (2017) 10:292

Fig. 1 Location of the Sablettes (Hussein-Dey, Algiers) study site (a) in the framework of the Mediterranean Sea (c) and descriptive geological map showing the three faults of the Algiers region and acquisition points of ambient seismic noise in b eastern side and d in the western side: the red points correspond to ambient noise acquisition, and the yellow line corresponds to the electrical resistivity imaging profile location

cases a rupture on a WSW-ENE thrust fault system (Philip and Meghraoui 1983; Meghraoui 1988, 1991; Yielding et al. 1989; Bounif et al. 2004; Yelles et al. 2004; Meghraoui et al. 2004; Déverchère et al. 2005; Yelles-Chaouche et al. 2006). The distribution, the variation, and the attenuation of the damage resulting from earthquakes could predict the presence of site effects, which are often related to the geological characteristics in the vicinity of the considered site. These characteristics may disrupt the behavior of the normal wave field, inducing amplification or damping of the seismic vibrations or even trapping of the waves (Bonilla et al. 1997; Bard 1999). One of the key parameters to evaluate the site effect at a given

location is the thickness of the sedimentary column that overlays the seismic and/or seismological substratum, and the other parameters are the difference of Vs velocity at the interface rock/sediment and the fundamental frequencies of the soil. This information can be easily known geologically welldocumented areas; otherwise, the sediment thickness can be efficiently approached and with few constraints by the HVSR ambient vibration method as it has been done in several cases around the world (SESAME 2005; Guillier et al. 2005; Bonnefoy-Claudet et al. 2006; Chatelain et al. 2008; Pilz et al. 2009; Gallipoli and Mucciarelli 2009; Mainsant et al. 2012; Vella et al. 2013; Matsushima et al. 2014; Rincona et al.


2016) and in Algeria (Machane et al. 2008; Bensalem et al. 2010; Hellel et al. 2012, 2013; Oubaiche et al. 2012; Layadi et al. 2016; Meziani et al. 2016). In this work, we used the HVSR ambient vibration method in the Sablettes coastal fringe. The Sablettes area is the reclaimed coastal area of Algiers and corresponds to an artificial expanse of land acquired on the sea by backfilling (Fig. 1). Before the 1980s, the urbanization of the Algerian coastline was limited to the extension of existing coastal cities. Afterwards, coastal areas, which were initially occupied by a few summer or secondary residences, have hosted numerous real estate and tourism projects. Hence, the use of these artificial areas in recent years has been diversified into new urban neighborhoods such as the Sablettes area. The development of the Sablettes at Hussein-Dey in Algiers is recent (less than a decade). The Hussein-Dey coastal reclaimed area can now be used to develop touristic resorts. Backfill thicknesses are well-known in Sablettes, thanks to geotechnical studies, including cored boreholes. However, the thickness of the sedimentary column under the backfill layer is unknown, and coastal reclaimed areas are known for their strong amplification of seismic vibrations and their liquefaction potential (Saita et al. 2004; Islam et al. 2010). For example, the works of Saita et al. (2004) on coastal reclaimed areas determined amplification values of 3 to 8 for the 1 Hz frequency in the Manila subway site (Philippines). In addition, the walls of an active fault are to be taken into account in the analysis of the site effects, knowing that the acceleration obtained on the hanging wall is always higher (Yu and Gao 2001; Shabestari and Yamazaki 2003) and could reach a value of 50% higher than that obtained on the foot wall (Abrahamson and Somerville 1996). The coastal zone of the Sablettes is partly located on the hanging wall of the Sahel active fault studied by Meghraoui (1988). This fault is considered as a potential source of strong earthquakes (Harbi et al. 2007; Maouche et al. 2011; Heddar et al. 2013), with a length of over 80 km and a dipping towards the north estimated at 50–55° (see Sibson and Xie 1998) characteristic of Algerian reverse faults. The trace of the Sahel fault on the surface is distant only about 1–2 km from the Sablettes, and the expected results may provide us with interesting details on the acceleration values in the study area, which is devoted to host touristic resorts. The present study was carried out in order to map the roof of the metamorphic basement, previously studied by Saadallah (1981), at the Sablettes coastal reclaimed area. The prerequisite to achieve this goal is the knowledge of the thickness of the sedimentary column. The metamorphic basement outcrops in different zones in Bab El Oued (district of Algiers City, see Fig. 1). But at the Ferhani stadium (at the coastline of Bab El Oued), which is located in the same district and where a downhole was performed and the shear waves velocity (Vs) was measured, the metamorphic basement was

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intercepted at a depth of 5 m, with a shear wave velocity Vs reaching 1200 m/s, sometimes topped by a marl, this last formation sometimes outcropping (JICA-CGS 2006; Machane et al. 2008). We think that the 1200 m/s Vs value at 5 m is certainly much greater in the Sablettes where the metamorphic rock is strongly deeper according to the results obtained from an ambient vibration array (GEOTER-MATTA 2015) that definitely gave a Vs value very high. This value is defined for the seismological substratum, determined for example with KiK-net stations for a material that is very close to the seismological basement (Tsuda et al. 2006, 2010). The soil resonance frequency variation is commonly used to estimate the geometry interface between soft sediment and the harder underlying bedrock (Yamanaka et al. 1994; Delgado et al. 2002; Oliveto et al. 2004; Hellel et al. 2012; Oubaiche et al. 2012). In this study, we used the lower frequencies related to the 1D sediments resonance frequency between deeper bedrock (metamorphic basement) and free surface. These frequencies correspond to the HVSR curves peak frequency ranging between 0.6 to 1 Hz. We obtained an isofrequency map and used the relation between the depth-averaged shear wave velocity and the resonance frequency (Haskell 1960) in order to estimate the sediments thickness and the geometry interface between sediments and the metamorphic basement.

Geological setting The geology of the Algiers region is represented by metamorphic rocks (Saadallah 1981) surrounded by Mio-PlioQuaternary sedimentary deposits, limited in their south parts by the Mitidja Quaternary basin (Fig. 1). Our study area is located in the Algiers bay (Fig. 1) on the left bank of El Harrach River. The analysis of the 1/50,000 Algiers geological map (Aymé 1964), along with several cored boreholes carried out in the Sablettes by the central laboratory of the public works (LCTP 2015), allowed us identifying the different lithological units and then correlating and interpreting the data collected by means of geophysical methods particularly ambient vibration recordings. The lithological section (Fig. 2) built up from the core drillings carried out in study area reveals the geometry of the shallow deposits and the local geological information of the soil of the Sablettes. The analysis of the results of the geotechnical tests (LCTP 2015) highlights three geotechnical units: &

Unit I consisting in heterogeneous backfill formed by deposits of anthropic origin that are associated with alluvial deposits of El Harrach River and characterized by a low shear wave velocity (Vs) and a low density, with thickness varying from 3 to 13 m

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Fig. 2 3D representation of the study site lithology in the eastern part (see Fig. 1b), based on boreholes (see location at the bottom of the figure) showing from top to bottom the backfills, fine sand, clay, gravelly sand, and marl

& &

Unit II is an alluvial formation with a thickness reaching 50 m; it is composed of fine to coarse sands with gravelly levels Unit III represents the layer of basement on which the above formations are formed by firm gray marls with high density

From the seismotectonic point of view, the study area is surrounded by three main active faults (Fig. 1c), including the Sahel active reverse fault, which hanging wall serves as the foundation for the entire area in Algiers bay supposedly reaching the left bank of El Harrach River. The other two faults are the Thenia fault (Boudiaf 1996; Moulouel et al. 2016) and South-Mitidja fault. The same is similar for the reverse Sahel fault dipping north (Meghraoui 1988), thus providing a movement from north to south, which gives us a roof of the overlapping compartment located north of the fault and constitute hanging wall. Our geological observations argue in favor of this movement by reverse fault with north dipping already reported by several authors (e.g., Maouche et al. 2011). This was also confirmed by the work of Boudiaf (1996). In addition, Maouche et al. (2011) analyzed the succession of marine terraces and the asymmetric folded structure and determined the geometry of the Sahel fault by assigning it a reverse movement and depth dip values of 40° to 45° NW. This fault borders the Mitidja basin to the north and whose formations, located to the north

are overlapped by the formations of the Sahel which extends as far as the sea. The overlapping compartment, that forms the footwall, is in this case under the Sahel formations and below the Sahel fault plane.

Methods used and data acquisition H/V method Methods based on ambient vibrations, in particular the horizontal-to-vertical spectral ratio method (H/V), use the properties of seismic noise to estimate the soil fundamental frequencies (1D site effect). This technique was proposed for the first time in Japan by Nogoshi and Igarashi (1970, 1971) and then widespread by Nakamura (1989, 2000). Since then, this method is frequently used for site effect studies. Several studies have been conducted in order to obtain a physical explanation of this method (e.g., Lermo and ChavezGarcia 1994; Lachet and Bard 1994; Kudo 1995; Delgado et al. 2000; Luzon et al. 2001; Rodriguez and Midorikawa 2003; Al Yuncha et al. 2004). A fairly general consensus emerged from the seismologist community about the reliability of this ratio, particularly for the estimation of the natural frequency of the soil (SESAME 2005).


The studies on the understanding of the H/V ratio are few compared to those dedicated to the various applications of the Nakamura method and especially those concerning seismic microzoning (e.g., in the city of Thessaloniki of Northern Greece (Theodulidis and Bard 1995); in Lisbon, Portugal (Teves-Costa et al. 1996); in the city of Basel, Switzerland (Fäh et al. 1997); in Caracas, Venezuela (Duval et al. 2001); in Dinar, Turkey (Ansal et al. 2002); in Baetic Cordillera, Spain (Delgado et al. 2002); in Colfiorito, Italy (Cara et al. 2003); in Benevento, Italy (Maresca et al. 2003); in Osaka basin, Japan (Uebayashi 2003); in Algiers and Boumerdes, Algeria (Laouami and Slimani 2013); and in Chlef, Algeria (Layadi et al. 2016). Regarding the geological investigation, some authors (e.g., Haskell 1960; Ibs-von Seht and Wohlenberg 1999; Delgado et al. 2000, 2002; Parolai et al. 2002) developed empirical relationships between soil resonance frequency and sediments thickness. The HVSR ambient seismic noise method consists of recording ambient vibrations at the site, and the peak of the HVSR curve gives the resonance frequency of the soil F0. The experimental validity of this method is very well initiated. For 1D site effects, the resonance frequency F0 of a 1D layered structure is proportional to the shear wave velocity Vs and inversely proportional to the thickness H of this layer, thus F0 = Vs/4H (1) (Haskell 1960). For data acquisition and processing, and the criteria for identifying H/V peaks, we followed the recommendations of SESAME ( 2005) (http://sesame-fp5.obs.ujf-grenoble.fr, European project) and Chatelain et al. (2008). Numerous investigations have been acquired over the 10 years (2000– 2010) in Europe in order to explore the actual capabilities of noise based techniques in view of deriving quantitative information on site amplification (Bard et al. 2010). The SESAME recommendations are the result of a consensus reached by the participants of the European research project SESAME and are based on comprehensive and detailed research work conducted during 3 years. The use of ambient vibrations in understanding local site effects has been studied in detail. The recommended use of the H/V method is to combine several geophysical and geotechnical approaches. In the present study, we also used the electrical resistivity tomography to confirm our HVSR results. More than 100 recordings (Fig. 1 b and d) of ambient vibration were implemented in the Sablettes by using a CityShark II station, which is described by Chatelain et al. (2000, 2012). This station is connected to a three-component 5-s Lennartz seismometer. Recordings were made with a sampling of 200 sps during a period of 15 min. Electrical resistivity tomography method The electrical prospecting is a method to measure the electrical resistivity (Edwards 1977) of geological formations. The

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electric resistivity tomography makes it possible to image in 2D or in 3D the variations of electrical resistivity of the ground. The latter is deduced from the measurement of the potential difference between the electrodes. This method is often used for the search for cavities and cracks, the study of aquifers, as well as the characterization of the superficial terrains. The resistivity model can then be interpreted geologically (Loke 2012). In order to map the roof of the metamorphic base at depth while using HVSR, electrical prospection was used to confirm the contrasts obtained by ambient noise, and while measuring the resistivity of the various geological formations, it increases certainty and reliability of the results of the ambient noise. Nevertheless, this confirmation could only be made for the superficial layers since the depth of investigation by electrical tomography did not exceed the depth of 40 m. This survey was carried out in the western part of the Sablettes of Algiers. It was fulfilled out concomitantly with the acquisition of ambient noise at this location.

Results and interpretations The H/V processing was done with Geopsy (www.geopsy. org) using the default parameters such as window length of 25 s and a value of 40 smoothing constant and a smoothing type from Konno and Ohmachi. The obtained HVSR curves were analyzed and presented in the 0.4–20 Hz frequency range. The curves are relatively coherent (globally homogeneous shape) and have an acceptable standard deviation (for example; the frequencies >1.5 Hz is ±0. 03 Hz) on both the amplitude and frequency of the spectral ratio. From the ambient vibration HVSR curves, we were able to identify three frequency peaks that characterize the eastern part of the studied site: 0.6, 1.7, and 3 Hz (Fig. 3a), as well as other high frequencies that we do not use in our interpretations. For the interpretation of the different peak frequencies, we used the shear wave velocities (Vs) measured by downhole test (LCTP 2015, unpublished report) carried out in the Sablettes (Table 1). Using formula 1, with the depth-averaged shear wave velocity VD, as given by Madera (1970), VD ¼ ∑ ni ¼ 1 hi Vsi =∑ ni ¼ 1 hi

ð2Þ

As the downhole test does not exceed 68 m in depth, beyond this depth an average velocity Vs = 630 m/s is considered as previously used by JICA-CGS (2006) for the Plaisancian marl formation in the Algiers region. This velocity has also been confirmed thanks to the ambient vibration array

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Fig. 3 HVSR curves representative of eastern study area showing frequencies at a1 0.6, a2 1.7, and a3 3 Hz. Dashed curves correspond to standard deviation and continuous curve corresponds to average H/V, and b corresponds to H/V summary; red mark indicates low frequencies around 0.6 Hz

performed in this zone and which gave similar values 645 m/s up to 100 m depth (GEOTER-MATTA 2015). The calculation method used for the determination of the depth corresponding to the 0.6 Hz frequency peak gives h6 ¼ 163:5 m The results obtained for the various peaks are presented in Table 2. According to these results, the peak frequency of 0.6 Hz corresponds to an interface at a depth of about 230 m and could be related to the interface between the metamorphic basement and the overlying sedimentary formation column. The 1.7-Hz peak corresponds to an interface located at 40 m depth. According to the lithology, this depth corresponds to the interface between fine sand and the underlying gravelly sand. According to the downhole results, this interface shows the strongest velocity contrast of the entire sedimentary column. The 3-Hz peak corresponds to an interface at 20 m depth, which could correspond to the presence of clay lenses included in the fine sand formation. In the eastern study area, the majority (Fig. 3b) of the HVSR curves show the presence of a peak in the interval Table 1 Shear wave (Vs) and compression (Vp) velocity and density obtained by downhole for the first five layers (in LCTP 2015, unpublished report), the sixth layer (JICA and CGS 2006), and the last layer (Harbi et al. 2007) Depth (m)

Vp (m/s)

Vs (m/s)

Density (g/cm3)

0–6 6–20 20–39.40 39.40–60 60–68 68–231.5 231.5