Zemmouri earthquake rupture zone (Mw 6.8, Algeria) : Aftershocks sequence relocation and 3D velocity model

[1] We analyze the aftershocks sequence of the Zemmouri thrust faulting earthquake (21 May 2003, Mw 6.8) located east of Algiers in the Tell Atlas. The seismic sequence located during ∼2 months following the mainshock is made of more than 1500 earthquakes and extends NE–SW along a ∼60-km fault rupture zone crossing the coastline. The earthquake relocation was performed using handpicked P and S phases located with the tomoDD in a detailed 3D velocity structure of the epicentral area. Contrasts between velocity patches seem to correlate with contacts between granitic–volcanic basement rocks and the sedimentary formation of the eastern Mitidja basin. The aftershock sequence exhibits at least three seismic clouds and a well-defined SE-dipping main fault geometry that reflects the complex rupture. The distribution of seismic events presents a clear contrast between a dense SW zone and a NE zone with scattered aftershocks. We observe that the mainshock locates between the SW and NE seismic zones; it also lies at the NNS–SSE contact that separates a basement block to the east and sedimentary formations to the west. The aftershock distribution also suggests fault bifurcation at the SW end of the fault rupture, with a 20-km-long ∼N 100° trending seismic cluster, with a vertical fault geometry parallel to the coastline juxtaposed. Another aftershock cloud may correspond to 75° SE dipping fault. The fault geometry and related SW branches may illustrate the interference between pre-existing fault structures and the SW rupture propagation. The rupture zone, related kinematics, and velocity contrasts obtained from the aftershocks distribution are in agreement with the coastal uplift and reflect the characteristics of an active zone controlled by convergent movements at a plate boundary.

[1]  C. Coulon,et al.  Post-collisional Neogene magmatism of the Mediterranean Maghreb margin: a consequence of slab breakoff , 2000 .

[2]  G. King,et al.  Active tectonics of the Algerian Atlas Mountains‐evidence from aftershocks of the 1980 El Asnam earthquake , 1989 .

[3]  J. Jacobs.  Strong Algerian earthquake strikes near capital city , 2003 .

[4]  M. Ouyed.  Le tremblement de terre d'El Asnam du 10 octobre 1980 : étude des répliques. Algérie , 1981 .

[5]  Richard G. Gordon,et al.  Closure of the Africa‐Eurasia‐North America Plate motion circuit and tectonics of the Gloria Fault , 1989 .

[6]  Fred W. Klein,et al.  Deep fault plane geometry inferred from multiplet relative relocation beneath the south flank of Kilauea , 1994 .

[7]  M. Meghraoui.  Blind reverse faulting system associated with the Mont Chenoua‐Tipaza earthquake of 29 October 1989 (north‐central Algeria) , 1991 .

[8]  Clifford H. Thurber,et al.  Double-Difference Tomography: The Method and Its Application to the Hayward Fault, California , 2003 .

[9]  F. Bernardi,et al.  The 2003 Boumerdes, Algeria earthquake: Regional moment tensor analysis , 2005 .

[10]  Walter H. F. Smith,et al.  New, improved version of generic mapping tools released , 1998 .

[11]  M. Meghraoui,et al.  Earthquake‐induced flooding and paleoseismicity of the El Asnam, Algeria, fault‐related fold , 1996 .

[12]  T. Kanazawa,et al.  Imaging the source region of the 2004 mid‐Niigata prefecture earthquake and the evolution of a seismogenic thrust‐related fold , 2005 .

[13]  Julien Frechet,et al.  Sismogenese et doublets sismiques , 1985 .

[14]  Clifford H. Thurber,et al.  Earthquake locations and three‐dimensional crustal structure in the Coyote Lake Area, central California , 1983 .

[15]  J. Nocquet,et al.  Geodetic Measurements of Crustal Deformation in the Western Mediterranean and Europe , 2004 .

[16]  H. Philip,et al.  Seismotectonics of the El Asnam earthquake , 1981, Nature.

[17]  Walter Wildi.  La chaîne tello-rifaine (Algérie, Maroc, Tunisie): structure, stratigraphie et évolution du Trias au Miocène , 1983 .

[18]  O. Charade,et al.  Strong Algerian Earthquake Strikes Near Capital City , 2003 .

[19]  Paul G. Richards,et al.  Quantitative Seismology: Theory and Methods , 1980 .

[20]  C. Chiarabba,et al.  Tomographic images of the El Asnam fault zone and the evolution of a seismogenic thrust‐related fold , 1997 .

[21]  C. Thurber,et al.  Detailed fault structure highlighted by finely relocated aftershocks, Arthur's Pass, New Zealand , 2006 .

[22]  F. Waldhauser,et al.  A Double-Difference Earthquake Location Algorithm: Method and Application to the Northern Hayward Fault, California , 2000 .

[23]  L. Rivera,et al.  Stress tensor and fault plane solutions for a population of earthquakes , 1990 .

[24]  Denis Legrand,et al.  Seismic source study of the 1989, October 29, Chenoua (Algeria) earthquake from aftershocks, broad-band and strong ground motion records , 2003 .

[25]  Mustapha Meghraoui,et al.  Géologie des zones sismiques du Nord de l'Algérie : paléosismologie, tectonique active et synthèse sismotectonique , 1988 .

[26]  M. Bezzeghoud,et al.  Seismogenic zone survey by Algerian Telemetered Seismological Network; case-study of Rouina earthquake, 19 January 1992 , 1994 .

[27]  Felix Waldhauser,et al.  hypoDD -- A Program to Compute Double-Difference Hypocenter Locations , 2001 .

[28]  F. Cotton,et al.  Fault location and source process of the Boumerdes, Algeria, earthquake inferred from geodetic and strong motion data , 2005 .

[29]  L. Hood,et al.  Impact demagnetization by phase transition on Mars , 2003 .

[30]  Ziyadin Cakir,et al.  Coastal uplift and thrust faulting associated with the Mw = 6.8 Zemmouri (Algeria) earthquake of 21 May, 2003 , 2004 .

[31]  Nasser Laouami,et al.  The 21 May 2003 Zemmouri (Algeria) earthquake Mw 6.8: Relocation and aftershock sequence analysis , 2004 .

[32]  É. Calais,et al.  Active thrust faulting offshore Boumerdes, Algeria, and its relations to the 2003 Mw 6.9 earthquake , 2005 .

[33]  Pierre Briole,et al.  Slip distribution of the 2003 Boumerdes‐Zemmouri earthquake, Algeria, from teleseismic, GPS, and coastal uplift data , 2004 .

[34]  J. Morales,et al.  Moment tensor solutions for small and moderate earthquakes in the Ibero‐Maghreb region , 2003 .