Sub-vertical low-velocity zone from P and S wave local tomography of the aftershock sequence of the Mw5.9 Chenoua (Algeria) earthquake of October 29, 1989: relict of the Miocene Eurasia–Africa suture zone?

Abstract

Travel time data from the aftershock sequence following the Mw5.9 Chenoua earthquake of October 29, 1989 are inverted to obtain a three-dimensional tomographic image of the region. The data are P and S wave travel times obtained from 394 carefully selected aftershocks recorded at 18 temporary stations between November 7 and 18, 1989. A three-dimensional P wave velocity model down to a depth of 10 km is obtained. At shallow depth, the velocity contrasts clearly outline the Plio-Quaternary sedimentary basins separated by the Chenoua bedrock high. Below 4 km, a sub-vertical east–west low-velocity zone can be seen down to the limit of the resolved model. The fault activated during the Chenoua event, as depicted by the aftershocks distribution is oblique to this structure and north of it. The location and geometry of the low-velocity zone suggest it may correspond to a structure belonging to the suture between Eurasia and Africa, a former north dipping subduction zone, now a block boundary inherited from the Miocene collision phase. This structure may partly constrain the shallower deformation and could be responsible of the localized Sahel anticline north of the stable Mitidja basin.