The evolution of a thrust fault system: processes of rupture initiation, propagation and termination in the 1980 El Asnam (Algeria) earthquake

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Summary We discuss work completed to date on the 1980 Algerian earthquake, and introduce a description of the earthquake in terms of rupture propagation using the nomenclature of fracture mechanics. Field and teleseismic studies have shown the earthquake fault to be divided into three major thrust segments: rupture began at the SW end of the system and propagated along successive segments. Geological sections reveal that different fault segments have suffered different amounts of displacement in the past. The amount of geological displacement increases systematically from almost zero at the SW end of the fault to several hundred metres further NE; faulting at the SW end is very young and appears to be extending the thrust system along strike. The aftershock data of Ouyed et al. reveal that strike-slip aftershocks occur beyond the SW end of the thrust fault. These are accommodating compression by conjugate strike-slip faulting on north- and west-trending planes. This network of conjugate faults plays a crucial role in the initiation of major eathquakes on the thrust system, and may be regarded as a ‘plastic’ zone beyond the end of the thrust fault. Increasing horizontal stress causes the strike-slip system to evolve into a zig-zag network of thrusts which is partly controlled by the pre-existing strike-slip faults. Finally, a through-going thrust can develop and leads to catastrophic rupture along the main fault plane. To describe the propagation of the main shock, we extend Aki's concept of geometric barriers and divide these into two types. In conservative barriers the slip vector is common to both fault planes, whereas in non-conservative barriers the slip vectors are different on the two fault planes that meet at the barrier. Although both types of barrier arrest dynamic rupture, only nonconservative barriers need produce aftershocks, as off-fault deformation is required to accommodate the difference in slip vectors. By considering the aftershock zones as areas of distributed deformation, we show that fracture toughness in the Earth is not a material constant but is related to fault geometry, and therefore changes as fault systems evolve. The current south-westwards growth of the El Asnam thrust system can be seen as the latest phase in an evolution consisting of periods of along-strike extension alternating with periods of increasing net displacement during which no fault growth occurs: these two types of behaviour allow a segmented fault system to be produced.