The stations of the National Seismic Network of the SGS National Center for Earthquakes and Volcanoes has monitored a remarkably high number of seismic activities since 23/4/1430 H (19/4/2009 AD). Thirty thousand (30,000) earthquakes had been monitored, most of which were very weak earthquakes, and therefore, were not analyzed. Two hundred seven (207) earthquakes were measured 3 - 5.39 on the Richter scale and were felt by some of the residents of Al-Ais, Hijrat Al-Hadma, Al-Qarasa, Al-Ameed, Al-Farea, Al-Sahla, Al-Maramia, Al-Mashash, Wadi Sinan, and Yanbu. The strongest earthquake was felt in Madina, 210 km from the epicenter.
The Harrat Al-Shaqa is located between Al-Ais and Umluj and extends between the northwest and the southeast areas. It is one of the Kingdom’s small Harrats in terms of surface area, occupying about 65 km by 55 km, between longitudes 37.50o and 38.08oE and latitudes 24.86o and 25.36oN and covering an area of about 3,575 square kilometers. The lavas in this area flowed through Wadi Al-Hail in the west and through Shuaib Al-Owayned down to the coastal plain at a distance of 50 km east of Umluj. This Harrat is divided into smaller Harrats and segments. The most famous cones of Harrat Al-Shaqa include the hills of Al-Miqrah, Zatra, Al-Aqabat, Al-Ghadiaa, Hasina, and Al-Safah. The Harrat is located at a distance of 150 km west of the MMN volcanic zone.
On 24/5/1430 H, an earthquake, measuring 5.4 on the Richter scale, was felt in most regions of Madina. Fissures and cracks of more than 8 km were formed in the affected area in the northern Harrat Al-Shaqa.
The studies conducted by the SGS in collaboration with the US Geological Survey showed that the crustal deformations that were associated with the recent seismic activities resulted in the doming and bulging of about 30 cm, which caused the 8-km fissures with a width ranging between several centimeters to several meters depending upon the type of the rocks that were cut by the fissures. These studies also confirmed that the magma chamber was unstable and was still receiving more and new magmas that triggered the occurrence of greater than 4 magnitude earthquakes Crustal deformation and bulging are still active due to the continued magmatic activities. A new set of longitudinal fissures parallel to the largest fissure in the Harrat were observed and documented. The ensuing earthquakes and ground deformations due to these magmatic activities were caused by tensile forces that trend northeast-southwest, and by a magmatic intrusion with a magma volume of about 0.13 km3 and by a ground uplift with a width of approximately less than 4 km.
The SGS established a monitoring network for the ground deformations in the surface of the Harrat, as well as the monitoring of seismic and thermal activities.
Part of the fissures associated with the May 19, 2009 AD (25/5/1430H) earthquake in the Harrat Al-Shaqa.
A fissure in the Harrat Al-Shaqa that was formed by the recent earthquakes.
Part of the fissures associated with the May 19, 2009 AD (25/5/1430H) earthquake in the Harrat Al-Shaqa. Their widths ranged from several centimeters to several meters, depending on the type of the rocks that were cut by the fissures. The maximum breadth measured in the basement rock hills is about 90 cm.
The lava flows of an old volcano in the Harrat Al-Shaqa