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

Abstract

[1] We analyze the aftershocks sequence of the Zemmouri thrust faulting earthquake (21 May 2003, M w 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.

Searching for the seafloor signature of the 21 May 2003 Boumerdès earthquake offshore central Algeria

“…Offshore Algeria, the presence and position of faults visible at or near the shallow subsurface are debated in the case of the Boumerdès 2003 earthquake (see, for example, Ayadi et al, 2008;Déverchère et al, 2010). A remarkable observation is the presence of several elongated seafloor scarps of likely tectonic origin, first described by Déverchère et al (2005).…”

“…Because of the lack of direct evidence, the trajectory of the rupture towards the surface is more controversial. Ayadi et al (2008) propose that the rupture reached the surface at approximately the shelf edge, i.e. very close to the coastline, whereas Déverchère et al (2005Déverchère et al ( , 2010 and Mahsas et al (2008) favour a flat-ramp geometry of the fault system, leading them to postulate a cumulative fault scarp at ca.…”

“…5-10 km from the coastline. The available focal mechanisms indicate an almost pure reverse faulting and a NNW-SSE-directed regional maximum shortening axis (Stich et al, 2006;Ayadi et al, 2008). The thrusting was responsible for a mean coastal uplift of about 0.5 m, reaching locally almost 0.8 m , as well as for significant coand post-seismic (over 2.5 yr) displacements Mahsas et al, 2008).…”

The large Algerian earthquakes (267 A.D.-2017)

“…It benefits from well instrumental coverage by seismological, geological, geodetic and marine methodologies. The epicenter of this earthquake of magnitude of Mw: 6.8 was located at 36,83°N, 3,65°E near the coastal village of Zemmouri (Bounif et al, 2004, Ayadi et al, 2008. It caused the death of 2360 persons and caused important damages.…”

DIRDOP: a directivity approach to determining the seismic rupture velocity vector

“…This M w = 6.8 event was located offshore at 37.02 • N and 3.77 • E and a focal depth of 7 km in a zone characterized by relatively moderate and diffuse seismicity (Ayadi et al 2003). The main shock has been relocated using HypoDD (Ayadi et al 2008) at Zemmouri el Bahri village, close to the continent, at 36.83 • N and 3.65 • E. This earthquake is the second largest to occur in Algeria since the 1980 El Asnam M s = 7.3 earthquake. In the epicentral area, the main shock severely affected the towns and villages, particularly the coastal towns of Zemmouri and Boumerdes.…”