The 02 January 2018 ML 5.0 Oued Djer Earthquake in the Seismotectonic Context of the Southwestern Mitidja Basin Area, North-Central Algeria

Abstract

A moderate earthquake (ML 5.0) occurred in the Oued Djer region of the Tell Atlas, Algiers Province, Central Algeria, on 02 January 2018 (20:59:00 UTC), 10 km southwest of Mitidja Basin (36.39° N, 2.54° E), at a depth of 4 km. The earthquake is significant because it occurred immediately south of the epicentral area of the 1988 MS 5.4 Oued Djer Earthquake and a few kilometers west of the 1867 Mouzaia-El Afroun Earthquake, a large historic event (I0 = IX EMS-98). In this study seismological, morphotectonic, and geological field data were collected to identify the likely seismogenic structure and give new insight to better understand the seismotectonic configuration of the study area. The mainshock–aftershock sequence was recorded by the Algerian Digital Seismic Network, and a temporary seismic network installed 1 day after the mainshock. By analyzing 97 relocated aftershocks (1.0 < Md < 4.3) recorded during the first 2 months after the mainshock, we demonstrate that the mainshock occurred on a N80° E-striking, 65° S-dipping reverse fault with significant right-lateral component, this in consistence with the ENE–WSW nodal plane of the mainshock focal mechanism. The vertical distribution of the aftershocks is between 3 and 5 km which is shallower than the seismic sequences happened recently in the Tell Atlas. The main aftershocks present reverse and strike-slip focal mechanisms. Our field investigations yield evidence of N80° E-striking, north-vergent reverse faulting, which affects the northeastern boundary of Boumedfaa Basin, a small Miocene depression situated adjacent to the northwestern edge of the Mitidja Basin, in agreement with the results of the microseismic study. In the epicentral area, we highlight Quaternary faults striking N120° E that cut the alluvial terraces of Bouroumi River and are related to NW–SE-striking dextral strike-slip fault. These results show that active deformation in the southwestern edge of the Mitidja basin is complicated and implies more than a fault segment, the combination of strike slip and reverse tectonics is proven here.