In this study, seismic sources of the three events mentioned above have been studied for obtaining fault plane solutions and seismic moment (moment magnitude). The best fittings result among the observed and synthetic waveforms for the three events were obtained. In addition, the inversion results including moment tensor components, decompositions (double couple and the compensated vector dipole CLVD ratio), the moment magnitudes (Mw), and shift in origin time (velocity reduction) were also determined. The obtained focal mechanisms results revealed different styles of faulting. For Khamis Mushait event, Fig. 7 (a, b) normal movement with strike slip component is obtained, figure (7b) represented by double couple (DC) ratio (46% and 54% CLVD ratio. The source mechanism showed that there are two fault planes, one trending in an east west (perpendicular to Red Sea main rift axis), and the other almost perpendicular to it. The moment magnitude (Mw) was computed at 3.5 and depth at 16.0 km, figure (7b). The main event and the two before it occurred in an area of intersection valleys along the lake of the Tandah Dam, which may have occurred because of the effect of lake formed behind the Dam, (changing of pure pressure beneath the lake). Talwani (1988) and Talwani and Rajendran (1991), in some seismically active areas in the world studied the nature and reasons of occurrence the intraplate and near plate boundary earthquakes and stated that earthquakes tend to nucleate near the fault intersections at the pre-existing weakness zones. Therefore, we may attribute the occurrence of the events of Khamis Mushait to the presence of the area of weakness behind the lake of the Tandah Dam.
For the second event occurred in Ahad Rafidah area, Fig. 8 (a, b) the obtained focal mechanism delineated normal movement with small strike slip component, figure (8b) and represented by 58% DC and 42% CLVD ratio. The source mechanism showed that there are two fault planes, all almost trending in northwest-southeast, nearly perpendicular to Red Sea main rift axis. The moment magnitude was computed at 3.7 and depth of 14 km, figure (8b). Based on what the visiting team reported, it was no valleys intersection at the site of the event, meaning that no intersection appears on the surface of any faults cut the area. Therefore, this event may have occurred because of the tension forces affecting the Red Sea, which caused the reactivation of one of the pre-existed faults in that area, parallel or perpendicular on the Red Sea main rift axis. Indeed, based on the catalog of the SNSN no seismic activity has ever been detected on this site, as well as the site of the Khamis Mushait earthquake. Thus, locations of these events are considered as new seismic sources that will contribute in updating seismic hazard studies in the south of Saudi Arabia region.
As for the third event, Alshoqiq event, Fig. 9 (a, b) the source mechanism revealed strike slip movement with reverse component, figure (9b), the DC ratio was 70% and 30% CLVD ratio. The source mechanism showed that there are two fault planes, one trending in northwest (nearly perpendicular to Red Sea main rift axis), and the other almost parallel to Red Sea main rift axis. The computed moment magnitude was 3.78 and the depth was computed at 20 km, respectively, figure (9b). This event is located near the Red Sea coast and according to the SNSN, it occurred in an area characterized by occasional seismic activity, in opposite side of a seismically active area inside the Red Sea. Thus, we may attribute the occurrence of this event to the reactivation of one of the faults located on the Red Sea coast, parallel to the main rift axis. Here, we can ask a question: Is it possible for a small earthquake to occur with a reverse movement in areas very close to the original extension zone (Red Sea)? The answer is yes, this can happen, as this reverse movement represents a reactivation of some of the normal faults that already exist in the basement of the study area. Simpson and Lee (1998) studied similar cases in different regions of the world and proved that this reverse movement was originally formed by extension movement and may represent a reactivation of pre-existing normal faults.
The obtained CLVD ratio for the three events is ranging from 30% up to 54% for Khamis Mushait and Ahad Rafidah event, respectively, while reached 70% for Al-Shoqiq event. This high ratio could be artefacts due to inadequate crustal model, but it can rather be a phenomenon of tensile earthquakes being created by combining shear and tensile motions on a fault during the rupture process, Vavryčuk (2011, 2013, and 2015). When considering the noticed higher CLVD ratio in general for all events and especially for Al-Shaqiq event (70%), where this event is located near the coast of the Red Sea, we kept in mind the possibility of volcanic activity under this area that may have caused this event. This led us to test the frequency spectrum of some waveforms for selected stations to see whether the occurrence of these event is related to any volcanic activity or due to the natural tectonic tension forces in the Red Sea.
The determined frequency contents showed that these events occurred within frequencies greater than 10 Hz in the waveforms of stations that used in locating and in inversion for computing the moment tensors of these events, figures (10, 11, and 12). The nature of earthquakes is often classified according to the content of frequencies in the seismic waves caused by the occurrence of these earthquakes (Power et al., 1994; Miller et al., 1998), which most of them are divided into two groups according to the amount of signal power around the frequency 5 Hz. Thus, waveforms of high frequency content are interpreted as the result of a brittle fracture, while those with low frequencies between 0.2 to 5 Hz are due to a movement associated with a crack or a crack or a conduit section (Chouet, 1988; Neuberg et al., 2000). In our case, when considering; (a) the frequency contents of the three earthquakes, higher than 10 Hz. [figure 10 (a, b), Fig. 11 (a, b), and Fig. 12 (a, b)]; (b) non-occurrence of previous earthquake swarms in these areas, even during the occurrence of these recent events; (c) no migration of seismicity has been detected (seismic activities with varying depths); and (d) no previous volcanic record, even historically, this led us to suggest that these events occurred due to natural tectonic movements on pre-existed faults due to regional stresses affect the area, (Re Sea and triple junction of Red Sea, Gulf of Aden, and Afar area).
When considering the importance of this study, it is represented in; (a) providing additional new information related to detecting of new seismic sources in the south of the Kingdom, which contributes to updating studies of seismic risk assessment in this region; (b) there are many previous seismic hazard assessment studies, (Al-Amri 1995, Al-Amri and Alkhalifah 2004, Al-Haddad et al., 1994, Barazangi, 1981, and Thenhaus et al., 1987) conducted in the southern part of the Kingdom, but the locations of these recent seismic sources have not been taken in account in their calculations; (c) the area is in the process of constructing several dams to store water for agricultural purposes and human use; and the region was exposed to more than one earthquake, whether historical or recent, that caused seismic effects in the region; and (d) according to the vision of 2030 in the Kingdom of Saudi Arabia and the beginning of development in the southern region, the region has become promising for the establishment of vital and strategic projects with establishment of new populated residential communities.
All the above-mentioned points indicate the importance of this study in terms of providing new and useful information needed for updating seismic risk assessment studies in the southern region of Saudi Arabia. In addition, this study drew our attention to increase the seismic monitoring stations around the sites of new seismic sources in the Kingdom, and to redistribute of seismic stations in some regions of the Kingdom.