Preliminary Observations of Geotechnical Failures During the 21 May 2003 M 6.8 Boumerdes, Earthquake, Algeria

At 18:44 UTC (19:44 local time) on May 21, 2003, a strong, shallow earthquake of Moment Magnitude (M) 6.8 shook northern Algeria and caused damage in five provinces in the north-central section of the country. Damage was reported over an area about 100 km long and 35 km wide, centered on the city of Boumerdes. The hardest hit areas were in the coastal province of Boumerdes, mainly in the cities of Boumerdes, Zemmouri and Thenia. The earthquake appears to have been generated by an offshore south-dipping thrust fault oriented N54°E extending for about 35 km from Dellys to Corso. The fault rupture was bilateral, with a greater asymmetry to the southwest. The ground motions recorded from the mainshock were significantly higher than median values predicted by standard attenuation relationships. Liquefaction and lateral spreading occurred near the Isser River and areas with extensive beach sand deposits. In the port of Algiers, nine piers suffered damage where liquefaction, loss of ground support, and lateral displacement (seaward movements) of the bulkheads were pervasive. Cracks developed on the Keddara and Beni Amrane Dams and the water line running from the Keddara Dam to the Boudouaou water treatment plant suffered damage at two locations. * Printed with permission of the Earthquake Engineering Research Institute. Originally published as a chapter in the EERI Boumerdes, Algeria Reconnaissance Report. INTRODUCTION At 18:44 UTC (19:44 local time) on May 21, 2003, a strong, shallow earthquake of Moment Magnitude (M) 6.8 shook northern Algeria and caused damage in five provinces in the north-central section of the country. The epicenter was located offshore at 36.91°N 3.58°E (Center of Research in Astrophysics, Astronomy and Geophysics [CRAAG]), 7 km north of Zemmouri in the province of Boumerdes, and about 50 km east of the capital city of Algiers (Fig. 1). Damage was reported over an area about 100 km long and 35 km wide, centered on the city of Boumerdes. The hardest hit areas were in the coastal province of Boumerdes, mainly in the cities of Boumerdes, Zemmouri and Thenia (Fig. 1), as well as the eastern districts of Algiers. Most of the construction in the damaged areas was built in the last 30 years; however, several large buildings dating from the colonial era (early 20 century) were heavily damaged in the districts of Belcourt, Bab-El-Oued and El-Casbah in Algiers. The affected area is heavily developed and urban. The total estimated death toll was over 5,000 and about 11,000 people were injured. Damage was estimated at US$5 billion. About 182,000 apartments and private houses were damaged, of which 19,000 suffered complete collapse. The earthquake also generated a tsunami observed as far away as the Balearic Islands and the southern coast of Spain. However, there was little or no tsunami damage along the Algerian coast. In this paper, we document some preliminary observations concerning the source of the earthquake, the earthquake ground motions recorded in northern Algeria, and geotechnical ground failures resulting from strong ground shaking. SEISMOTECTONIC SETTING Northern Algeria is located along the convergent plate boundary between Eurasia and Africa (Fig. 2). This is one of the most seismically-active regions in the western Mediterranean, having experienced several large, damaging earthquakes during historic time, including the 1716 Algiers (Epicentral Intensity, Io IX), 1790 Oran (Io XI), 1889 Mascara (Io IX) earthquakes, and more recently the 1980 surface wave magnitude (Ms) 7.3 El Asnam, 1985 Ms 6.0 Constantine, 1989 Ms 6.0 Tipasa, 1994 Ms 6.0 Mascara, 1996 Ms 5.7 Algiers, and 1999 Ain Temouchent Ms 5.8 earthquakes.