Abdelkrim Aoudia

Coastal uplift and thrust faulting associated with the Mw = 6.8 Zemmouri (Algeria) earthquake of 21 May, 2003

[1] A shoreline uplift marked by a continuous white band visible at rocky headlands occurred during the 21 May 2003 earthquake (Mw 6.8) in northern Algeria. We measured the amount of coastal uplift on a white band (emerged algae) and harbors quays between Boumerdes and Dellys. Most of measured points were collected using tape and differential GPS on rocky headlands with σ ± 0.15 m error bar (tidal prism). Leveling lines running parallel and orthogonal to the coast also provide the precise amount of uplift in the epicentral area. The uplift distribution shows an average 0.55 m along the shoreline with a maximum 0.75 m east of Boumerdes and a minimum close to 0 near Cap Djinet. The active deformation related to a thrust fault is modeled along the ∼55 km coastline. The dislocation model predicts surface slip on a N 54°E trending reverse fault, dipping 50° SE in agreement with CMT solution and coastal uplift. The faulting characteristics imply a fault geometry with possible sea bottom ruptures between 5 to 10 km offshore.

Seismogenic potential and earthquake hazard assessment in the Tell Atlas of Algeria

Seismotectonic zonation studies in the Tell Atlas of Algeria, a branch of the Africa-Eurasia plate boundary, provide a valuable input for deterministic seismic hazard calculations. We delineate a number of seismogenic zones from causal relationships established between geological structures and earthquakes and compile a working seismic catalogue mainly from readily available sources. To this catalogue, for a most rational and best-justified hazard analysis, we add estimates of earthquake size translated from active faulting characteristics. We assess the regional seismic hazard using a deterministic procedure based on the computation of complete synthetic seismograms (up to 1 Hz) by the modal summation technique. As a result, we generate seismic hazard maps of maximum velocity, maximum displacement, and design ground acceleration that blend information from geology, historical seismicity and observational seismology, leading to better estimates of the earthquake hazard throughout northern Algeria. Our analysis and the resulting maps illustrate how different the estimate of seismic hazard is based primarily on combined geologic and seismological data with respect to the one for which only information from earthquake catalogues has been used.

The 1976 Friuli (NE Italy) thrust faulting earthquake: A reappraisal 23 years later

We revisit the 1976 Friuli earthquake sequence by combining hypocenters relocation, long period surface wave inversion, field geology and strong motion modelling. We show that fault-related folding is the main active deformation by which the seismic energy was released during the main shock (Ms=6.5) and that some of the surface effects reported in 1976 correspond to widespread bedding planes displacements induced by flexural-slip folding. The fault evolved from blind to semi-blind along strike showing the control of the inherited structural geology on the fault surface break and rupture arrest. Our fault model produces waveforms that fit the accelerograms recorded in the area.

The 1998 Bovec-Krn mountain (Slovenia) earthquake sequence

We study the 1998 Bovec-Krn mountain (Slovenia) earthquake sequence by combining hypocenters relocation, strong motion inversion, digital elevation modelling and field geology. The main shock (Ms=5.7), a 12 km right lateral strike-slip event on the Dinaric fault system, occurred on a sub-vertical fault plane. The rupture, confined between 3 and 9 km depth, with no evidence of surface faulting, propagated bilaterally within two structural barriers. The northwestern barrier is at the junction between Dinaric and Alpine structures where there is a sharp change in the geometry of faulting. The southeastern barrier is within the Dinaric system and its surface expression corresponds to the Tolminka-spring perched basin, a 1 km restraining step-over. At this site, the Bovec-Krn earthquake-fault overlaps with a 30 km strike-slip fault segment that is free of aftershocks and could be undergoing an increase of stress. This fault system represents the northern branch of the Idrija right-lateral fault.

A splash in Earth gravity from the 2004 Sumatra Earthquake

The 26 December 2004 Sumatra earthquake displaced a large amount of material in the radial direction with respect to the Earths center, and the earthquake is thus expected to have permanently changed the gravity in a broad region surrounding its causative fault. This article examines gravity changes, which are here quantified by geoid anomaly patterns depicting the displacement of the sea surfaces hydrostatic component after the earthquake. The article focuses on the feasibility of the detection of these patterns by gravity space missions. Models of changes in the rotation and oblateness of the Earth due to global gravitational effects of the earthquake have already been reported [Chao and Gross, 2005].

An Improved Formula of Fundamental Resonance Frequency of a Layered Half-Space Model Used in H/V Ratio Technique

The resonance frequency of the transmission response in layered half-space model is important in the study of site effect because it is the frequency where the shake-ability of the ground is enhanced significantly. In practice, it is often determined by the H/V ratio technique in which the peak frequency of recorded H/V spectral ratio is interpreted as the resonance frequency. Despite of its importance, there has not been any formula of the resonance frequency of the layered half-space structure. In this paper, a simple approximate formula of the fundamental resonance frequency is presented after an exact formula in explicit form of the response function of vertically SH incident wave is obtained. The formula is in similar form with the one used in H/V ratio technique but it reflects several major effects of the model to the resonance frequency such as the arrangement of layers, the impedance contrast between layers and the half-space. Therefore, it could be considered as an improved formula used in H/V ratio technique. The formula also reflects the consistency between two approaches of the H/V ratio technique based on SH body waves or Rayleigh surface waves on the peak frequency under high impedance contrast condition. This formula is in explicit form and, therefore, may be used in the direct and inverse problem efficiently. A numerical illustration of the improved formula for an actual layered half-space model already investigated by H/V ratio technique is presented to demonstrate its new features and its improvement to the currently used formula.

Approximate Formula of Peak Frequency of H/V Ratio Curve in Multilayered Model and Its Use in H/V Ratio Technique

The main peak frequency of the Horizontal-to-Vertical (H/V) ratio curve is the key factor used in the H/V ratio technique since the resonance frequency of the transmission response of the site is estimated from this frequency. However, there has not been explicit formula of the main peak frequency of the H/V ratio curve in multilayered models. In the present study, an approximate explicit equation of the peak frequency of H/V ratio is derived for the multilayered models of high impedance contrast between the half-space and surface layers. This approximate equation is then generalized for model of an functionally graded material (FGM) layer over half-space. Then, the approximate equation is used to obtain an explicit approximate formula of the main peak frequency of H/V ratio curve. The principle formula of H/V ratio technique is used along with the obtained approximate formula of the main peak frequency to formulate a new average formula of the shear-wave velocity of a composite layer composed of an arbitrary number of horizontal, homogeneous layers. The new average formula is shown to be more suitable in the use of H/V ratio technique than the currently used ones in the sense that it takes into account the effect of the mass density and the position of sublayers. Finally, some numerical calculation to illustrate the application of the peak formula and the new average formula of shear-wave velocity is presented.