Microearthquakes and neotectonics of Abu-Dabbab, Eastern Desert of Egypt

Unlike a typical mainshock-aftershock sequence, there is no obvious mainshock in the August 2004 earthquake sequence at Abu-Dabbab in the central eastern desert of Egypt. Therefore, the August 2004 pattern shows the characteristics of swarm behavior. With few exceptions, most earthquake swarms are associated with volcanic regions, and their occurrence has been often related to the movement of magma. The focal mechanisms of swarms are typically normal slip or occasionally strike-slip. The periodic occurrence of earthquake swarms accompanied by knocking sounds at Abu-Dabbab is well-known since the early 1980s. The annual activity rate recorded by the Egyptian National Seismographic Network (ENSN) indicates that the seismic energy was released by both background seismicity and four microearthquake swarms from 2001 to 2005. In the present paper, we have used microearthquake monitoring techniques along the Egyptian Red Sea Hills to investigate the neotectonics of the Abu-Dabbab region. With a temporary dense, digital seismological network we were able to record the complete spectrum of seismic energy released by both the background seismicity and a microearthquake swarm with a total of 866 locatable events that lasted for one week in August 2004. These locatable events are tightly clustered within a circular area with a 3.5-km radius and oriented in an E-W to ENE-WSW direction perpendicular to the Red Sea rift axis. The majority of these microearthquakes are located between 5-14 km deep that likely delineates two seismogenic zones centered at depths of 6 km and 9 km respectively. One way to better understand earthquake swarms and neotectonic processes is to look at focal mechanisms and local stress field. Here we present, for the first time, focal mechanism solutions for 17 microearthquakes. The obtained solutions reflect a highly complex tectonic setting along the investigated area; however, more than half the fault-plane solutions indicate reverse faulting with a strike-slip component. The remainder show normal faulting with a strike-slip component. We investigated the present-day stress field by analyzing the directions of maximum horizontal stress ({sigma}1) as derived from earthquake focal mechanisms. The results indicate that the orientation of P-axes reflect E-W alignment of ({sigma}H) with a vector mean of 85 degrees + or - 2 degrees . Thus the stress directions are consistent with the overall regional direction of maximum horizontal stress in southern Egypt. Moment tensor inversions show clear non-double-couple (volumetric) components that for some events come up to 35%, and high heat flow values (92 mW/m2) are also reported. Therefore we suggest that present-day seismicity and deformation at Abu-Dabbab is originated by local stress and is likely related to magmatic intrusion into the upper crust triggered by regional tectonics.