Normal Faulting during the August 1989 Earthquakes in Central Afar: Sequential Triggering and Propagation of Rupture along the Dôbi Graben
Résumé
In August 1989, an earthquake sequence including ten events with 6.3≥M≥5.5 in the first two days produced widespread ground deformation in the Dôbi graben of central Afar. Numerous surface breaks with complex geometry, including fresh scarplets with vertical throws up to 30 cm high and open fissures up to 30 cm wide, were observed. Coseismic slip incremented the deformation (normal faulting, block tilting, and counterclockwise rotation of basaltic slices) accumulated in the last 2 m.y. in the transfer zone between the Dôbi and Hanle grabens. By combining maps of surface ruptures, relative event relocations with the local Djibouti network, published focal mechanisms, and source sizes, we tentatively relate most of the mainshocks of the sequence to slip on individual faults. The largest shocks at 11h16 on 20 August 1989 (MS 6.2) and at 1h09 on 21 August 1989 (MS 6.3) ruptured southern segments of the southwestern bounding fault of the graben. A dozen other faults also slipped along the edges of, and inside, the graben. On average, triggered seismic faulting propagated about 35 km northwestward along the graben in about 20 hr. Slip on the main faults was coupled with slip on secondary antithetic faults branching from them at depth. Although the Dôbi earthquakes ruptured part of the fault array between the Asal rift (1978 sequence) and the Serdo region (1969 sequence), an approximately 50-km-long gap subsists along the Der'êla half-graben. We infer the patterns of surface faulting in the Dôbi sequence, which coinvolved bookshelf-faulting about both horizontal and vertical axes, to typify the complexity of coseismic stress release in central Afar and in other active zones of distributed extension (e.g., Iceland, Abruzzi, Basin and Range).