[1] Using a joint inversion of seismological waveforms and ground displacement observations, we estimate several parameters of the fault geometry and rupture process of the Mw = 6.9 May 21, 2003 Boumerdes-Zemmouri earthquake. The relocated epicenter is considered as a known parameter. Total rupture length, rupture duration, and maximum slip are 55 km (from 3.4°E to 4.0°E), 12 s, and 3 m. The modeled south dipping reverse fault, oriented ENE-WSW outcrops a few km offshore which is consistent with the absence of observed surface rupture inland. Two shallow and relatively localized slip zones are found, on both sides of the hypocenter. To the SW, between Boumerdes and Zemmouri, slip is concentrated between 11 and 2 km depth. To the NE, between Zemmouri and Dellys, slip is concentrated between 6 km depth and the sea floor. Various resolution tests indicate that our model is well constrained by the available data, and help understanding which data constrains each parameter of the model.
On May 21st, 2003, a Mw = 6.8 earthquake struck the central part of northern Algeria causing extensive damage in the Boumerdes area, 40 km east of Algiers. It is among the largest events to occur in the western Mediterranean over the past 25 years. We present GPS measurements of horizontal coseismic displacements that provide new constraints on the rupture geometry. Modeling the data with a uniform dislocation on a rectangular fault in an elastic half‐space, we find that the rupture occurred on a reverse fault dipping 42°S, with its upper edge 6 km offshore and lower edge 4 km inland. The amplitude distribution of the coseismic displacements indicates that the rupture did not reach the surface, at least in its western part, and ended to the west around 3.4°E. Offshore faults like that of the Boumerdes earthquake could account for part of the Africa‐Eurasia relative plate motion in the western Mediterranean and represent a significant seismic threat for Algeria.
S U M M A R YWe investigated post-seismic deformation following the 2003 May 21, M w = 6.9 Boumerdes, Algeria, earthquake using surface displacements from six continuous Global Positioning System sites that operated in the epicentral area for 2.5 yr following the event. We find up to 4 cm of cumulative horizontal displacement during that time period, with a time-dependence well fit by a logarithmic decay. Post-seismic deformation appears to continue at all sites after the 2.5-yr observation period, with rates on the order of 1 cm yr −1 or less. The data is consistent with shallow afterslip (0-5 km) and shows no evidence for afterslip downdip of the coseismic rupture. The data is poorly explained by viscoelastic relaxation in the lower crust or upper mantle, or by poroelastic rebound. The concentration of afterslip adjacent to and updip of the coseismic rupture, at least in the western half of the fault, suggests that afterslip is driven by coseismic stresses. The correlation between the depth of afterslip and that of the sedimentary wedge along the Algerian margin, while coseismic slip occurs in deeper basement rocks, suggests (1) that post-seismic deformation may also involve folding and (2) that spatial variations in frictional properties along the fault correlate with the type of rocks involved.
SUMMARY
We use a combination of seismicity. tectonic features, focal mechanisms, seismic strain and postseismic movement to study the western part of North Algeria, the El Asnam region and its surrounding area in particular. A seismotectonic map of this part of Algeria, delimited by the Mediterranean Sea in the north and the Tellian mountains in the south, was built from available geological and seismological data. An examination of this map shows that the most significant earthquakes are concentrated along tectonic features and quaternary basins elongated in an east‐west direction, suggesting NNW‐SSE compressional movements. During the large El Asnam earthquake of 1980 October 10, Mw= 7.1, vertical movement was measured along a 40 km northeast‐southwest thrust fault. These movements were determined geodetically in 1981 with reference to a basic network previously measured in 1976. In order to control postseismic movement and to ensure the monitoring of the seismic area, a dense geodetic network has been regularly measured since 1986, both in planemetry and altimetry. The results of the altimetric remeasurements show significant vertical movements. The elevation changes of the benchmarks have been deduced from precise levelling measurements: a remarkable uplift (5.1 ± 1.9 mm yr−1) of the northwestern block, during the 1986‐91 period has been observed, whereas the southeastern block is seen to be relatively stable. The Sar El Maǎrouf anticline, situated along the central segment of the El Asnam surface breaks, appears to be growing with a maximum postseismic slip rate of (9.6 ± 1.4 mm yr−1). The mean uplift rates computed for the northwestern block support the view that the 1954 earthquake did not occur on the same reverse fault as the 1980 event.
The REGAT (
“REseau Géodésique de l'ATlas”
) geodetic network is composed of 53 continuously–recording GPS stations distributed in the Algerian Atlas. It spans the whole width of the Algerian coast and reaches 300 km inland, with inter-sites distance of about 100 km. One additional site is located in Tamanrasset in the southernmost part of the country. The network, whose oldest stations started operating in 2007, encompasses the main active tectonic features of the most seismically active segment of the Nubia-Eurasia plate boundary in the Western Mediterranean. Here we describe the network configuration, the data collection and analysis strategy, as well as some preliminary results on horizontal GPS velocities. A detailed analysis of the velocity field in terms of plate boundary kinematics is the topic of a separate publication. The REGAT network fills an important gap in our knowledge of present-day plate boundary deformation in the Western Mediterranean. It will soon be enhanced by an additional 100 sites in order to improve deformation monitoring with a higher spatial resolution for a better assessment of the regional seismic hazard.
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