# Probabilistic Seismic Hazard Assessment of the Southwestern Region of Saudi Arabia
## 3 Results and Discussion  
Within the boundaries of the Red Sea Rift, the computed hazard results (Figure 4) exhibit a continuous distribution with greater values, especially along the southern shore. Because it is located close to the border between the Nubian and the Arabian Plates, this area of study has the highest seismic hazard levels when compared to other areas. The contours maps (see Figure 4) that show PGA, SA at 0.2 s (SA (0.2 s)), and SA at 1.0 s (SA (1.0 s)) highlight the fact that regions close to the Red Sea shore have significantly higher hazard levels than those further inland. In other words, hazard decreases toward the northeastern perpendicularly to the Red Sea Rift. This is consistent with the hypothesis that higher seismic hazard values are obtained in the vicinity of the rift. In this context, 11 significant cities were selected for a detailed computation (Figures 4 and 5), guided primarily by factors such as their geographic distribution across the studied area, socioeconomic significance, population density, and proximity to the Red Sea Rift. Ground motion values up to 0.14, 0.26 and 0.18 g for mean PGA, SA (0.2 s) and SA (1.0 s), respectively, are shown in the southwest part of the studied region under B/C NEHRP site conditions and with a 10% probability of occurrence over 50 years (equivalent to a return period of 475 years) (see Figure 4). In particular, the obtained mean PGA values for the cities of Farasan (located on the Farasan Island) and Jazan are 0.11 and 0.04 g, respectively, obtaining 0.03 g in the cities of Al Qunfundhah, Al Lith and Al Birk (refer to Table 2). Figure 4: Isoacceleration maps depicting hazard values for B/C NEHRP site condition. The studied cities’ locations (ABH: Abha, BAH: Al Bahah, BRK: Al Birk, BSH: Bisha, FAR: Farasan Island, JAZ: Jazan, KHM: Khamis Mushait, LTH: Al Lith, NAJ: Najran, QNF: Al Qunfundhah, TAT: Tathleeth) are marked with solid black circles. Figure 5: Hazard curves, under B/C (continuous lines) and C (dashed lines) site conditions, for the selected cities. Similarly, for these cities and soil conditions, the equivalent SA (0.2 s) values are 0.27, 0.12, 0.10, 0.08 and 0.07 g, respectively. On the other hand, cities farther northeast, like Tathleeth, Bisha and Al Bahah, exhibit even lower hazard levels. For example, the obtained PGA values for a 475-year return period under B/C soil conditions are below 0.02 g. Cities at intermediate distances from the rift zone, such as Khamis Mushait, Najran and Abha, display PGA values under B/C soil conditions for a 475-year return period of 0.03, 0.03 and 0.01 g, respectively, and SA (0.2 s) values of 0.06, 0.06 and 0.01 g, respectively. Table 2 provides additional ground motion values, estimated return periods, and site characteristics for these cities. These data become an important source for more in-depth local studies in the future. Another result derived from the present study is the computation of UHS and hazard curves for the chosen cities. These outcomes are helpful for formulating building codes, insurance policies, and financial risk management approaches focused on minimizing earthquake impacts on communities and infrastructures. They provide crucial information to engineers and architects regarding anticipated ground motions, assisting in the design of resilient structures capable of withstanding seismic events. In this regard, Figure 5 provides insightful seismic hazard findings, particularly the calculated hazard curves for the 11 cities under study and the two selected site conditions. Figure 6 depicts the UHS, including all considered site categories and return periods. This is a key component of seismic hazard analysis.