# Probabilistic Seismic Hazard Assessment of the Southwestern Region of Saudi Arabia
## 2 Methodology
### Seismic Source Model
Earthquake activity has occasionally been detected in the Red Sea and its coastal districts, as well as in inland areas, from the Yemen seismic source, designated as S42. The first known catastrophic occurrence in this source is the historic Ma'rib Dam earthquake, which happened in 460 AD [44]. The 1982 Dhamar earthquake (Mw 6.2), which occurred more recently, caused extensive damage, fatalities, and ground deformation. This event was followed by a significant number of aftershocks that lasted for around a month. Modeling the seismicity of the volcanic trap series is the goal considered in this seismic zone. Its  
\begin{table}
\begin{tabular}{c c c c} \hline
**Source** & **\(\lambda\) (\(\geq\)Mw 4.0)** & **\(b\)-Value** & **Mmax** \\ \hline
**S02** & 1.21 \(\pm\) 0.27 & 0.78 \(\pm\) 0.05 & 7.20 \(\pm\) 0.18 \\ \hline
**S03** &  
borders to the west and south align with the lines dividing Yemen's continental crust from the oceanic crusts of the Gulf of Aden and the Red Sea, respectively. The Afar triple junction (zones S02 and S06), which is host to the Southern Red Sea source, S43, is distinguished by a relatively high seismic activity. The spatial distribution of the epicenters shows that there is a relationship between the locations of earthquakes and the axial trough and NE-trending faults; it also includes scattered seismic activity in the entire zone. A prominent transform fault-oriented northeast defines the northern border of this seismic source. The distribution of epicenters further highlights the alignment of NE-trending transform faults and the axial trough faults with earthquake locations, indicating activity within these fault systems. An axial trough that is well-developed and resulting from seafloor spreading over the preceding 5 million years is a defining feature of the Central Red Sea source, here named S44 [60]. The axial trough is where the majority of the earthquake activity in this zone happens. The beginning of a transition zone dividing the continental crust to the north from the oceanic crust to the south coincides with the source's northwest boundary. Interestingly, a large earthquake happened in this area in 1967 (mb 6.7). Tectonically, a fractured axial trough made up of several shallow inter-trough areas and deeps is what identifies the Transition Red Sea source, named S45 [60]. The majority of the faults in this zone trend almost exactly north-south. Significantly, seismic activity has been reported to diminish north of latitude 21\({}^{\circ}\) N.  
In contrast with the S45 zone, the Northern Transition Red Sea source, named S46, displays a different fault trend, with the majority of faults aligned northwest-southeast as opposed to north-south. The transform faults heading northeast, and the associated seismic activity are in line with the northern boundary, which is roughly located at the latitude 25\({}^{\circ}\) North. Compared to the central and southern zones, this zone has very little, scattered seismicity. The Sheba Ridge spreading center, which reaches into Djibouti and approaches the Afar triple junction, is a defining feature of the Western Gulf of Aden source, named S49. The Alua Fartak Fault, which shifts the Sheba Ridge by around 160 km, is the most notable of the linear northeast-trending transform faults defining the Sheba Ridge. The Gulf of Aden's central axis experiences the majority of its seismic activity, while there are also infrequent earthquakes occurring northeast of the Gulf. Block changes near the border of the Arabian and Nubian Plates may be the cause of these phases [47]. In 2006, the largest earthquake ever recorded in the Gulf of Aden occurred here (Mw 6.6). Tensile stress mechanism is the primary cause of earthquakes across the Gulf of Aden. The Arabian Plate's areas with comparatively low seismic activity are included in the seismicity background zone, named S57, as defined in [34]. The Arabian Peninsula's regions not covered by other clearly defined seismic sources are mainly covered by it.