A Case Study of Accelerometric Records Analysis of May 21st, 2003, Boumerdes (Algeria) Earthquake

Introduction

Algeria is one of the most active areas in the Mediterranean basin where occur periodically major earthquakes (Algiers, 1716; Oran, 1790; Djidjel, 1856; El Asnam, 1954/1980; Boumerdes, 2003; Ambraseys & Vogt, 1988; Maouche et al., 2008).

On May 21^st, 2003, the Algiers-Boumerdes region in north Algeria was stricken by a major earthquake of 6.8 magnitude on the Richter scale. The main shock has induced damage spread mainly in the region of the Eastern part of Algiers Capital. It was felt until 250 km from the epicenter which was localized at sea. Peak ground accelerations (PGA) in East-West (E-W), North-South (N-S) and Vertical (V) directions of 0.58g, 0.34g and 0.22g, respectively, were recorded at Keddara station at approximately 20 km from the earthquake epicenter. At approximately 150 m of this station, peak ground accelerations of 0.33g, 0.26g and 0.25g in (E-W), (N-S) and (V) direction, respectively, were recorded. It’s important to conduct more exhaustive studies to understand the difference in PGA between the two close stations. For this purpose, the present study is conducted in order to analyze these records through the characteristics of strong ground motions, the effects of different parameters such as damping ratios, soil conditions and epicentral distance on normalized response spectra and the quantification of site effects during this earthquake. We, also, compare the near-field mean response spectra obtained during this earthquake with the Algerian seismic design spectra (RPA99 – 2003 version) and with two other well-known design spectra: Eurocode 8 and UBC97 in order to contribute to the future revision of RPA99. These several records were obtained by the instrumentation network of the National Center of Applied Research in Earthquake engineering (CGS) in Algiers.

The very important variation in observed PGA between the two stations, Keddara 1 and Keddara 2 (Table 1), concerns more particularly accelerations in (E-W) direction; what could be explained by a presence of an important site effect particularly at Keddara 2 station. However, accelerations in (E-W) direction are largely higher than those in (N-S) direction for almost the whole stations which recorded the main shock. This observation is related to the fault directionality (Laouami et al., 2006).

Station/Epicentral Distance (km)	Component	amax (cm/s2)	vmax (cm/s)	dmax (cm)
Keddara 1 / 20	E-W	331.55	17.95	9.78	10.06
	N-S	260.14	12.51	4.06	06.75
	V	246.40	15.79	8.44	08.34
Keddara 2 / 20	E-W	577.40	18.84	9.39	15.27
	N-S	344.74	11.32	3.98	10.71
	V	224.95	15.70	7.70	07.03
Dar El Beida / 29	E-W	537.45	28.23	12.69	08.56
	N-S	501.00	40.78	18.74	05.65
	V	164.14	11.23	5.79	07.54
Hussein Day / 36	E-W	269.50	18.56	6.77	05.30
	N-S	231.60	18.02	7.46	05.32
	V	085.94	08.12	4.21	05.49
Kouba / 41	E-W	311.90	15.75	6.95	08.74
	N-S	148.30	20.30	8.80	03.17
	V	151.00	08.33	4.65	10.12
Tizi Ouzou / 49	E-W	195.20	10.00	3.46	06.75
	N-S	192.00	06.53	2.40	10.81
	V	087.00	06.82	1.94	03.63
Blida / 72	E-W	046.35	03.35	1.02	04.21
	N-S	038.62	03.38	1.12	03.79
	V	028.53	01.45	0.70	09.50
Azazga / 75	E-W	120.00	14.00	4.35	02.66
	N-S	086.00	12.25	3.51	02.01
	V	046.30	08.74	6.11	03.70
El Afroun / 86	E-W	164.00	05.06	0.44	02.82
	N-S	090.93	05.40	0.32	01.00
	V	030.70	01.24	0.11	02.20
Hammam Righa / 110	E-W	105.00	10.40	5.18	05.03
	N-S	073.50	12.51	7.54	03.54
	V	067.23	05.48	2.74	06.13
Meliana / 130	E-W	031.24	01.72	1.35	08.11
	N-S	025.70	02.28	0.63	04.30
	V	016.50	01.94	1.19	06.64
Ain Defla / 151	E-W	033.00	01.57	0.96	12.85
	N-S	025.00	01.36	0.99	13.38
	V	010.00	01.22	1.15	07.73