H/V ambient noise for a rapid assessement of an unstable zone geometry : road slide case in Lakhdaria (Algeria)

Following torrential rainfall that struck the area of Lakhdaria during the winter of 2006, a slide road slope carried away half of the road on forty meters length, leaving a five meters high scarp. The remaining part of the road and houses located upstream were threatened by a potential second slip as showed by the cracks and the déformations observed around the scarp. Ambient vibration recordings were carried out 3 days after this event, with the aim of circumscribing the unstable zone around the scarp, characterized by a strongly disturbed soil. The results of this study showed that in the investigated area, H/V curves exhibit a frequency peak around 6 Hz behind the edge of the scarp and another one around 2 Hz, along the road, on both sides of the scarp. The first peak is related to the disturbed and unstable section of the soil, which is about 6m thick (slip surface depth), in good agreement with field observations and confirmed by geotechnical investigations. The second peak is related to a deeper geological interface. Upstream of the road, where there are some houses, H/V curves are flat, indicating the absence of the strongly disturbed soil section. These results allowed us to rapidly evaluate the lateral extension and thickness of the unstable zone. This study shows the reliability of H/V ambient vibration method to characterize the geometry of "fresh" sliding zones or unstable disturbed soil masses

Assessement of flood hazards in eastern Mitidja Plain (Hamiz river and Reghaïa river) Algiers - Algeria

The Eastern Mitidja Plain, highly urbanized in its Northern part, is crossed by two main rivers: the Réghaïa and Hamiz wadis. Along these wadis, anarchical buildings are developing. An exceptional flood may weaken these zones. Flood hazard détermination needs a statistic analysis of daily maximum rainfalls, which adjustments to the Gumbel law allows to determinate the maximum rainfall for different return periods. The calculation of maximum discharges linked to différent return periods allows evaluating the overflowing on cross section along the Hamiz and Réghaïa wadis. Hazard maps, obtained for each wadis, evidence zones that are potentially threatened by flooding. Such maps should be considered and used as the main tools reduce the risk

Assessement of flood hazards in eastern Mitidja Plain (Hamiz river and Reghaïa river) Algiers - Algeria

The Eastern Mitidja Plain, highly urbanized in its Northern part, is crossed by two main rivers: the Réghaïa and Hamiz wadis. Along these wadis, anarchical buildings are developing. An exceptional flood may weaken these zones. Flood hazard détermination needs a statistic analysis of daily maximum rainfalls, which adjustments to the Gumbel law allows to determinate the maximum rainfall for different return periods. The calculation of maximum discharges linked to différent return periods allows evaluating the overflowing on cross section along the Hamiz and Réghaïa wadis. Hazard maps, obtained for each wadis, evidence zones that are potentially threatened by flooding. Such maps should be considered and used as the main tools reduce the risk

Evidence of seismites in coastal Quaternary deposits of western Oranie (northwestern Algeria)

Coastal Quaternary deposits of western Oranie show typical soft-sediment deformations including sedimentary dykes, sand volcanoes, sismoslumps, thixotropic bowls, thixotropic wedges, diapir-like structures, and faults grading. Field observations indicate that these deformations exist at several levels of the studied deposits along the west Oranian coast. This study demonstrated that these structures are earthquakes-related, by analysis of potential trigger sources. Several arguments demonstrate the seismic origin: the depositional environment rich in water that located in an active tectonic region, the thixotropic nature of deformations and their large vertical and horizontal diffusion in the Quaternary series consistent with a seismic recurrence. This allows characterizing these Quaternary soft-sediment deformations as seismites that were triggered by earthquakes

Ambient Vibration Techniques Applied to Explain Heavy Damages Caused in Corso (Algeria) by the 2003 Boumerdes Earthquake: Understanding Seismic Amplification Due to Gentle Slopes

International audienceNo summar

Heavier Damages without Site Effects and Site Effects with Lighter Damages: Boumerdes City (Algeria) after the May 2003 Earthquake

International audienc

THE 20 MARCH 2006 EARTHQUAKE OF LAALAM (KABYLIA, ALGERIA) (Mw=5): GEOLOGICAL AND GEOPHYSICAL PROSPECTIONS (AMBIENT VIBRATION NOISE) AND CORRELATIONS WITH DAMAGES. LE SÉISME (Mw = 5) DE LAALAM (KABYLIE, ALGÉRIE) DU 20 MARS 2006 : PROSPECTIONS GÉOLOGIQUES ET GÉOPHYSIQUES (BRUIT VIBRATOIRE AMBIANT) ET CORRÉLATIONS AVEC LES DOMMAGES

url: http://www.mem-algeria.org/actu/comn/pubt/bulletin_ANGCM/Bulletin_SGN_V20-3.pdfInternational audienceGeological and geophysical (ambient vibrations) prospections conducted after the moderate Laalam earthquake (Mw = 5, USGS), demonstrate that structure destructions following this event are not related to site effects. Nevertheless, damages and human losses (4 dead, 68 injured) may be explained by vulnerable habitations and the epicenter proximity, either directly (ground shaking) or indirectly, by induced effects. Induced effects are constituted of soil and road cracks, rock falls, and a landslide reactivated by the earthquake. However, the surficial cracks are not expressing the fault on the surface, because of the moderate magnitude of the event. This study is comforting the need of moderate earthquake induced effects and raises the important problem of damages caused by low to moderate earthquakes in Algeria and buried fault location and mapping