Papers by Aomar IBEN BRAHIM
Reference : FP6 PROHITECH project: Volume 6: Seismic protection of historical buildings: applicat... more Reference : FP6 PROHITECH project: Volume 6: Seismic protection of historical buildings: application... ... Books : Book published as author, translator, etc. ... Jaspart, Jean-Pierre mailto [Université de Liège - ULg > Département Argenco : Secteur MS2F > Adéquat. struct. aux exig. de fonct.& perfor. techn.-écon. >] ... There is no file associated with this reference. ... All documents in ORBi are protected by a user license.
Teleseismic data recorded by the NEAREST (Integrated observation from NEAR shore sourcES of Tsuna... more Teleseismic data recorded by the NEAREST (Integrated observation from NEAR shore sourcES of Tsunamis: towards an early warning system) temporary array are inverted for the 3-D upper mantle P-wave velocity structure beneath the SW margin of Iberia including the Gulf of Cadiz. In this area, the convergence between the European and African plates plays a crucial role in the evolution
This study has the aim of assessing the seismic vulnerability of constructions in urban areas in ... more This study has the aim of assessing the seismic vulnerability of constructions in urban areas in Morocco. For this purpose, we use statistics of a survey on the buildings stock in Morocco that was implemented in the year 2000 by the Housing Observatory of the Real Estate Promotion Direction of the State Secretariat of Housing (HOREPD, enquête-2000). These statistics helped define the different types of housing, and their structures. This information, along with past-earthquakes observed damages allowed us to rank the different buildings into vulnerability classes. For the different types of constructions, we find four vulnerability classes, from class A to D. We then use a semi-empirical method for the computation of the buildings vulnerability index and fragility curves for the different vulnerability classes. We use these fragility curves to simulate the consequences of a seismic intensity IX on the different vulnerability classes. It turns out that the consequences would be catastrophic for class A and would have very little effect on class D buildings.
In this paper, we introduce a new approach for the assessment of buildings vulnerability towards ... more In this paper, we introduce a new approach for the assessment of buildings vulnerability towards tsunamis. This vulnerability is sought based on factors that may provoke major damages to a building after the passing of a tsunami. In this formulation, the influencing factors are taken into account through a specifically defined combination. The vulnerability thus used, is a combination of seven coefficients, four of which are related to the considered building site, while the other three coefficients concern the intrinsic properties of the structure itself. Weights are subsequently introduced based on the importance of the different coefficients for the building resilience. This approach is then applied to a Class B building and our results are compared to those found for a real case of a similar structure in Banda Aceh, Indonesia after it was struck by the famous December 24, 2004 tsunami. Our results are quite similar and can thus, be used for the assessment of urban tissues vulnerability with regards to tsunamis.
Teleseismic data recorded by the NEAREST (Integrated observation from NEAR shore sourcES of Tsuna... more Teleseismic data recorded by the NEAREST (Integrated observation from NEAR shore sourcES of Tsunamis: towards an early warning system) temporary array are inverted for the 3-D upper mantle P-wave velocity structure beneath the SW margin of Iberia including the Gulf of Cadiz. In this area, the convergence between the European and African plates plays a crucial role in the evolution of tectonic process such as lithospheric subduction. During the experiment, the GEOSTAR multi-parameter deep-sea observatory and 24 seafloor stations were deployed in the Gulf of Cadiz for 11 months (September 2007-August 2008). Two broadband stations were also installed on land near the coast of Portugal. All seafloor stations were equipped with a 3-component broadband seismometer and an hydrophone (all sampled at 100 Hz with a 24 bit digitizer). For the tomography, we collected waveforms from (over 100) teleseismic events (M>=5.5) recorded by both the NEAREST marine stations and the land stations belonging to the Portuguese, Spanish, and Morocco national networks. The P-wave velocity model is obtained by using an iterative non-linear inversion scheme which includes a grid based wavefront tracking to solve the forward problem. In this study we present the data quality analysis, the traveltime residual pattern, and preliminary inversion results. This 3-D model is the first for the area coming from integration of seafloor and land data.
The gulf of Cadiz is a region considered as a complex seismic area, where strong earthquakes occu... more The gulf of Cadiz is a region considered as a complex seismic area, where strong earthquakes occur and where the plate boundary between the African and Eurasian plates is not exactly known. In this paper, we use high resolution seismic data recorded by a network of OBS stations deployed for one year in the Gulf of Cadiz as well as eight permanent Portugal land seismic stations. The OBS network was deployed within an experiment of the NEAREST project. Nearly 600 seismic events are extracted from the recorded data set and their analysis revealed that most of them occur at 20 to 80 km depths, with clusters of seismicity that occur mainly at the Gorringe Bank, within the SW segment of the Horseshoe fault and the Marques de Pombal Plateau and the S. Vicente Fault. A new NW-SE trend of seismicity has been revealed with depths that extend from 35 to 80 km. This seismicity trend is close and nearly parallel to the SWIM (South West Iberian Margin) faults lineament.
NEAREST (Integrated observation from NEAR shore sourcES of Tsunamis: towards an early warning sys... more NEAREST (Integrated observation from NEAR shore sourcES of Tsunamis: towards an early warning system) is an EC funded project (GOCE, contract n. 037110), which aims at the identification and characterization of potential near-shore sources of tsunamis in the Gulf of Cadiz. This area is well known from the catastrophic earthquake and tsunami occurred November 1st, 1755. One of the project's work packages deals with monitoring of recent seismic activity in the Gulf of Cadiz area. For this purpose 24 broadband ocean-bottom seismometers (OBS) from the German DEPAS instrument pool were deployed for 11 months in addition to the GEOSTAR multi-parameter deep-sea observatory. The GEOSTAR observatory and the 24 OBS were deployed and recovered during two expeditions with RV Urania in 2007 and 2008. Broadband seismic signals were recorded with 100 Hz sample rate in water depths between 2000 and 5000 m. We recorded signals from very long periods (tides), oceanic and atmospheric signals, teleseismic, regional and local earthquakes, active seismic surveys, up to low-frequency vocalizations of most probably fin and blue whales. A proper way to identify different sources of seismic signals is the use of spectrograms. An important question is, if whale callings on seismometer components were not previously misinterpreted as harmonic tremors (related to methane release etc.) and other "interesting" seismic phenomena. We want to stress the importance to identify and characterize biological sources of seismo-acoustic signals before interpreting "unknown" types of seismic events in ocean seismic recordings.
The geophysical and geological investigations conducted so far in the Gulf of Cadiz allow us toda... more The geophysical and geological investigations conducted so far in the Gulf of Cadiz allow us today to have an idea of the largest active faults that can generate destructive earthquakes and tsunamis comparable to the Nov 1st, 1755 Lisbon event. However, their kinematics and seismic activity are poorly known because the seismic networks based on land do not allow a precise hypocenter location and estimation of focal mechanisms for the smaller events. Therefore the EC project NEAREST (Integrated observation from NEAR shore sourcES of Tsunamis: towards an early warning system) was initiated (GOCE, contract n. 037110). One of the main objectives of the project is the characterisation of the tsunamigenic sources in the Gulf of Cadiz through seismological monitoring of natural seismicity by means of 24 BB seismometers deployed for 11 months in addition to the GEOSTAR multi-parameter deep-sea observatory. Together with the dense onshore seismic networks the temporary OBS network will allow the location and characterization of small seismic events more precisely than it can be done with onshore stations only. Spectrograms are used to identify previously unknown earthquakes. One of the major questions is the maximum depth of seismic activity beneath the Gulf of Cadiz. In the recording period from September 2007 to August 2008 about 300 events were located within the OBS network using the Portuguese onshore seismic stations. Magnitudes range from 1 to 4.7 (ML from Institute of Meteorology Lisbon, Portugal). Using the OBS network many events could be detected which are not located by the onshore stations. First results show that the events occur to approximately 50 km depth, often deeper than the locations by land stations, and confirming the results available from regional and teleseismic waveform modelling. Focal mechanisms show strike-slip and thrust-slip events.
Journal of Geophysics and Engineering, 2006
This study is based on a site survey carried out in the capital city of Morocco, Rabat. For this ... more This study is based on a site survey carried out in the capital city of Morocco, Rabat. For this aim, we used a technique based on the recording and processing of seismic ambient noise to characterize local geological conditions in terms of the dynamic response of soil during earthquakes. This technique consists of an assessment of the spectral ratio of the horizontal to vertical components of microtremors recorded at the surface during a few minutes, and allows determination of the fundamental period and the maximum amplification factor at the site of measurement. We applied this technique in the city of Rabat to obtain distribution maps of dominant periods and amplification factors assessed in more than 250 sites. In order to discuss the results of this study, we used the information available in this zone about the lithology and topography to correlate the obtained values of dominant periods and amplification factors with the ground conditions. Finally, we established a microzoning map of the city of Rabat based on the contours of dominant periods. The microzoning map established in this study can be used by engineers and decision makers for urban and land use planning and also as a guide in reduction of the seismic vulnerability of buildings.
Journal of Earthquake Engineering, 2004
Measurements of background noise at Al Hoceima city and its extension zones allowed us to elabora... more Measurements of background noise at Al Hoceima city and its extension zones allowed us to elaborate a seismic microzonation map. The analysis of this map shows existing relationships between the observed dominant periods and the local geological conditions. In particular, a strong correlation between the dominant periods and the topography is apparent; indeed, we find larger dominant periods in areas with higher topography such as in the Ras El Abed sector to the north of the city and in the area of Cala Bonita, south of the city. Furthermore, the field work conducted in the region revealed that the karstic phenomenon is well developed in the Ras El Abed sector, and affects the dominant periods and a hence the site response.
Bulletin of Engineering Geology and The Environment
The paper describes the tectonic evolution of the Central High Atlas and the Ouarzazate basin in ... more The paper describes the tectonic evolution of the Central High Atlas and the Ouarzazate basin in Morocco, noting that many of the tectonic phenomena follow the previous structural lines. Although the area is one of relatively low seismicity, its location in an intra plate part of the African Plate and its high morphology, the effects of an earthquake could be considerable. This study combines seismic records from 1900 to 2007 with the integration of existing tectonic and structural information to better define the seismogenic zones. A newly identified cluster of epicenters in a NNW-SSE linear zone may be related to the Imilchil fault, where the earthquakes with magnitudes of 4–5 at depths of 5–20 km have been identified. Recent work in the Ouarzazate basin indicated that the Amekchoud fault ramp could generate seismic events with magnitudes as large as 6.4 Mw. L’article décrit l’évolution tectonique du Haut Atlas central et du bassin de Ouarzazate au Maroc, observant que de nombreux phénomènes tectoniques suivent des traits structuraux préexistants. Bien que la région présente une faible sismicité, les effets d’un séisme pourraient être considérables du fait de sa position au sein d’une intra-plaque de la plaque africaine et du fait de son relief. Cette étude combine des enregistrements sismiques de 1900 à 2007 tout en prenant en compte des informations disponibles d’ordre tectonique et structural afin de mieux définir les zones sismogènes. Un ensemble d’épicentres dans une zone linéaire NNW-SSE, peut être relatif à la faille d’Imilchil, a été nouvellement identifié, avec des séismes de magnitude 4 à 5 et des foyers à des profondeurs de 5 à 20 km. Des travaux récents dans le bassin de Ouarzazate ont indiqué que la faille d’Amekchoud pourrait générer des événements sismiques avec des valeurs de magnitude Mw aussi importantes que 6,4.
Journal of Seismology, 2008
The 1975 May 26 earthquake, of magnitude Ms = 7.9, occurred in the North Atlantic close to the Az... more The 1975 May 26 earthquake, of magnitude Ms = 7.9, occurred in the North Atlantic close to the Azores Archipelago. Its epicentre, as given by US Geological Survey, was 17.5° W, 35.9° N, 200 km south of the Gloria Fault. Several authors determined the focal mechanism as a dextral strike-slip event with no significant dip-slip component, compatible with the relative motion between Eurasia and Nubia plates but away from the presumed plate boundary. The 1975 earthquake generated a tsunami of small amplitude, recorded at the Portuguese tide-gauge network, in Spain and Northern Africa. The peculiar location of the earthquake and tsunami source and the generation of a noticeable tsunami were already discussed by several authors, but up to now, no direct modelling of the tsunami generation and propagation was made to judge the set of source solutions obtained by seismological analysis. In this paper, we present tsunami simulations, backward ray tracing and forward non-linear shallow water simulations using data from Iberia and Azores and Northern Africa. We show that a good fit between observed data and synthetic waveforms can be obtained with a focal mechanism with no significant dip-slip component, favouring its interpretation as almost pure dextral strike-slip event located in an old fracture zone south of Gloria Fault.
Natural Hazards and Earth System Sciences, 2009
... coast, and to another in 1522, observed in Granada and Almeria, close to the Morocco Mediterr... more ... coast, and to another in 1522, observed in Granada and Almeria, close to the Morocco Mediterranean coast (Mendonça, 1758), where violent sea agitation in Bades Ghomera, 36◦16 N, 4◦13 E on the mediterranean coast, was observed during this earthquake (El Mrabet, 1991). ...
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Papers by Aomar IBEN BRAHIM