SEISMICITY REEVALUATION OF THE NORTHERN RED-SEA

The Aqaba subnetwork of five vertical short-period stations of the sei smological observatory of King Saud University was installed in late 1 986 along the eastern side of the Gulf of Aqaba, northern Red Sea. Dur ing the first six years (1986 August to 1992 July) of the subnetwork o peration, 400 microearthquakes were detected. Of these, 93 events were recorded by most of the subnet stations and were located. Their epice ntres lie in the northern part of the Red Sea between latitudes 25.5 d egrees N and 27.5 degrees N and longitudes 33.5 degrees E and 36 degre es E along the axial depression of the Red Sea where the large intrusi ons (deeps) are located. Magnitudes of the locatable events ranged fro m 2.1 to 4.8. Two intensive swarms of about 200 microearthquakes occur red in February and June of 1992. The February swarm is the first inte nsive sequence observed in the surveying area since the establishment of the KSU network. Frequency-magnitude analysis of the recorded event s for the period 1986-1992 yielded 3.543 for a and 0.658 for b. These relatively higher b values (0.658) are a good indication of the crusta l heterogeneity under the spreading zone of the northern Red Sea. USGS and KSU data together show 3.41 for a and 0.49 for b. This study, tog ether with historical data, confirms that the area is very seismically active and that the activity is mainly of swarm type, and may be attr ibuted to the subsurface magmatic activity and spreading centres that are usually associated with strike-slip and normal faulting, respectiv ely.