Since late 2007, there have been more than thirty thousand earthquakes recorded in the Harrat Al-Shaqa (Lunayyir), a field of basaltic volcanic flows of Holocene age, located in western Saudi Arabia, northeast of Yanbu.
The most recent seismic activity occurred near the intersection of the dikes, largely inactive that extend along the Red Sea coast, and one of the transform faults of the Red Sea. Some of these transform faults also extend into the Arabian Shield and intersect the network of faults in the Precambrian terrane. Although the population is scattered throughout the region and the nearest town to the main seismic activity center is Al-Ais, which is more than 40 kilometers away, a small local network with 12 broadband seismometers was established to cover this region, measure and monitor its seismic activities.
On May 19, 2009 in the Harrat Al-Shaqa (Lunayyir), northeast of Yanbu, 19 earthquakes, measuring more than 4 on the Richter scale were recorded, including a 5.4 magnitude earthquake on the Richter scale, which damaged structures and buildings in Al-Ais City. From May 13 to June 7, 2009, a series of earthquakes, about 950, occurred, whose epicenters were projected as points onto a 36-kilometer Google Earth image. The dark areas on the map show the volcanic flows (Harrats) while the other colors indicate the magnitudes of the recorded earthquakes. The blue color indicates weak tremors of magnitudes less than 2; the turquoise, between magnitudes of 2-3; the green, 3-4; the red, 4-5; and the pink, the major earthquake event that happened in May 19 with a 5.4 magnitude in the Richter scale.
The Harrat Al-Shaqa (Lunayyir) earthquakes from May 13 to June 7, 2009.
The earthquakes were charted on a map using the first vertical derivative of the magnetic field, reduced to the pole to link the seismic activity to the plan structures of the region. It is clear that the area with high spatial frequencies or high-capacity details, covering a large portion of the center of the image, is spatially associated with the surface volcanic flow areas that tend to be more magnetic than the surrounding rocks. The seismic activity zone trends to the north-northwest, roughly in line with the extensions of clear dikes and faults that trend to the northwest of this magnetic image. These regional Cenozoic dikes emerged during the initial rifting phase that occurred during the beginning of the rifting of the Red Sea. The magnetic maps also show some seismic trends to the northeast that traverse the Harrats, which may represent a coastal transform fault system, an extension of the on-land transform fault system. The recent seismicity may be due to the reactivation of the areas of the intersecting fault networks, although in other regions, the coastal dike system appears to be largely inactive.
Comparison of the map of the first vertical magnetic derivative reduced to the pole in Harrat Al-Shaqa and the epicenters of the earthquakes that occurred from May 13 to June 7, 2009.
The 8-km long surface cracks appeared during the event that occurred on May 19, 2009 in the northern part of Harrat Al-Shaqa. The Satellite InSAR initial space imagery showed a disturbance on the surface of the Harrat over a distance of dozens of kilometers in the region, the central graben, and a bulge in two areas on both sides of the recent dike, with a graben between them. The dimensions of these deformations were identified, and they are useful in determining the depth of the magma chamber, which was measured to be less than 5 km. The magma chamber’s width was about 2 m, and has a shallow depth of about 2 km. Also, tension was detected in the earth’s crust, trending to the northeast in this region of the Red Sea margin. The magma did not reach the surface of the earth. Despite the current decline in the levels of seismicity, the SGS continues to monitor the region closely.
The disturbance on the earth’s surface and deformations associated with the earthquake of May 19, 2009 in the Harrat Al-Shaqa (Lunayyir).