Abstract

Permanent and temporary seismic stations that were deployed at Lunayyir area before and shortly after the occurrence of the 19 May, 2009 earthquake swarm (M_w 5.7) have recorded a number great of events. The main objective is to determine the tectonic framework of the area and prepare more representative tectonic model for the area. Through the detailed analysis of aftershock sequence it is clarified that; 1) the major part of the cumulative seismic moment has been released after the occurrence of the largest aftershock (m_b 4.8) during the first hours after the main shock; 2) except the first day (20 May, 2009), no event with local magnitude above 4.0 was recorded during the observation period; 3) their distribution oriented NE and NW; 4) it is clustered at two depths; from 5 to 10 and 15 to 25 km beneath Lunayyir area; 5) it is characterized by successive periods of maxima and minima; and 6) the number of aftershocks decayed rapidly after the occurrence of the mainshock following the relation of n(t)=37.28 t^-0.6 within the first two weeks and changed later for the other two weeks. Fault plane solutions for eighty-four events indicated normal faulting mechanism for the majority of events while strike-slip components accompanied some of the events. Aeromagnetic maps for Lunayyir area confirmed the presence of shallow and deep-seated faults oriented NE and NW. NE (transform fault) trend runs across the Red Sea into a Shield area and could be interpreted as a channel of magma connecting the Red Sea and Lunayyir area. Whereas, NW (Najd system) faults are predominant through the Shield area and intersected with NE fault trend underlying Lunayyir area. Upwelling magmatic intrusions was initiated at the intersected points causing earthquake swarm. Then, Lunayyir area is highly affected by the present- day Red Sea active tectonics.

Key words: Tectonic framework, Lunayyir area, earthquakes, faults.