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1 Acta Geophysica

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The 2021-2022 Mw 6.0 Bejaia Bay, NE Algeria, earthquake sequence: tectonic implications at the Algerian margin between lesser and greater Kabylian blocks

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Yelles-Chaouche A. , Abacha I. , Boulahia O. , Beldjoudi H. , Aidi C. , Bendjama H. , Tikhamarine E.-M. , Mohammedi Y. , Chami A. , Chimouni R. , Kherroubi A.

tom Vol. 72, no. 2

529--551

On March 18, 2021, at 00:04 UTC, a strong earthquake (Mw 6.0) hit Bejaia city, 200 km east of Algiers. Its epicenter was 15 km northeast of Cap Carbon in Bejaia Bay, making it the largest earthquake recorded offshore since the devastating earthquake (Mw 6.8) in Boumerdes on May 21, 2003. The earthquake had a maximum intensity of VII (EMS 98), triggering hundreds of aftershocks and damaging 2000 houses and social infrastructures, as well as causing several rock falls along the rocky coastline, but no human casualties were reported. The waveform inversion and spectral analysis of the mainshock and its largest aftershocks indicate an alignment along an E-W thrust fault plane offshore, dipping southward, the mainshock seismic moment of M0 = 9.7e + 17 N.m. corresponding to a magnitude Mw = 6.0. The aftershocks illuminated a surface 22 km long (N-S) and 12 km wide; their statistical parameters were assessed by Guttenberg-Richter relationship, Omori decay, and temporal clustering. The b-value is estimated at 0.83, the p value at 0.95, and the n-value at 0.75 (i.e., 75% triggered events), which follows classical patterns of aftershock sequences and suggests the sequence tectonic genesis. Furthermore, previous studies showed that the epicentral area was positively charged by coseismic static stresses transferred from recent events in the Bejaia-Jijel margin. Exactly one year later, on March 19, 2022, a moderate earthquake struck the same epicentral zone, 3 km NW of the first shock, generating a moment M0 = 8.5e + 16 N.m. corresponding to a magnitude Mw = 5.3; its focal mechanism also revealed an E-W striking reverse fault with a small strike-slip component. The present-day local stress field is characterized by a contractional tectonic regime (R ‘ = 2.99 ± 0.24) and d orientation (N345°E) consistent with the maximum regional compressive stress direction (NNW-SSE). The 2021-2022 Bejaia Bay seismic sequence underlined the active tectonics linking the major E-W offshore thrust fault system and the NW-SE strike-slip Babors Transverse Fault system. This sequence, along with a number of other earthquakes that occurred around the Lesser Kabylia Block (LKB) over the last decade, likely highlights the ongoing incipient subduction process between Africa and Eurasia along the northern Algerian margin.