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Two regional ground-motion models are available at present for western Saudi Arabia, a region that is characterized by considerable seismic activity due to the spreading in the Red Sea and continental volcanism. The first model was created by Sokolov and Zahran (Arab J Geosci 11:38, 2018) using the stochastic simulation technique (Boore in Pure Appl Geoph 160(3):635–676, 2003) and it was reported in tabular form for moment magnitudes (M) from 5 to 7. The second model was developed by Kiuchi et al. (Bull Seismol Soc Am 109:2722–2737, 2019) as mathematical expression (empirical ground motion prediction equation, GMPE) on the basis of records from regional earthquakes with local magnitudes from ML 3 to 5.4. In this work, we first construct a GMPE using stochastic simulation data. This stochastic GMPE considers magnitude, distance and stress drop as input parameters. Second, we compare these two independently developed regional ground-motion models and employ a scheme for the modification of the empirical GMPE up to M 7.5 using the stochastic GMPE as the reference model. The revised GMPEs presented here provide regional ground motion models (PGA and spectral amplitudes at natural periods 0.2 s and 1.0 s) that may be used in logic tree scheme for assessment of seismic hazard in western Saudi Arabia.

SUMMARY Although the Dead Sea Transform (DST) fault system has been extensively studied in the past, little has been known about the present-day kinematics of its southernmost portion that is offshore in the Gulf of Aqaba. Here, we present a new GPS velocity field based on three surveys conducted between 2015 and 2019 at 30 campaign sites, complemented by 11 permanent stations operating near the gulf coast. Interseismic models of strain accumulation indicate a slip rate of $4.9^{+0.9}_{-0.6}~\mathrm{ mm}\,\mathrm{ yr}^{-1}$ and a locking depth of $6.8^{+3.5}_{-3.1}~\mathrm{ km}$ in the gulf’s northern region. Our results further indicate an apparent reduction of the locking depth from the inland portion of the DST towards its southern junction with the Red Sea rift. Our modelling results reveal a small systematic left-lateral residual motion that we postulate is caused by, at least in part, late post-seismic transient motion from the 1995 MW 7.2 Nuweiba earthquake. Estimates of the moment accumulation rate on the main faults in the gulf, other than the one that ruptured in 1995, suggest that they might be near the end of their current interseismic period, implying elevated seismic hazard in the gulf area.

New gravity data reveal a prominent negative anomaly along the main vent axis of the northern Harrat Rahat volcanic field in Saudi Arabia. The low continues north of the volcanic field onto exposures of Proterozoic rocks, indicating that the low is caused not only by the volcanic field (and possibly also underlying Cenozoic sediments), but also the underlying Proterozoic basement. An inversion of the gravity field guided by analysis of aeromagnetic data indicates (1) a broad depression of the basement surface that is deeper along the main vent axis in the eastern part of the harrat and in the southwest part of the volcanic field and (2) less dense basement beneath the vent axis. Low densities within the basement most likely arise from lithologic variations in the basement, predating Cenozoic volcanism, although our analysis does not rule out small volumes of partial melt and higher temperatures or extensive fracturing at depth.

We use earthquake geothermometry, measured heat flow, and structural constraints from P-wave receiver functions to model the thermal evolution of the lithosphere beneath Harrat Lunayyir. We suggest that the lithosphere thinned to its present 60-km thickness in a second stage of lithospheric thinning at 15–12 Ma following initial Red Sea extension at ∼27 Ma. Harrat Lunayyir is an active volcanic field located in the Arabian Shield >150 km east of the Red Sea rift axis. In the lithospheric mantle beneath Harrat Lunayyir we locate 64 high-frequency earthquakes at depths of 42–48 km, all with m L ≤ 2.5 . These brittle-failure earthquakes must have nucleated at relatively low temperatures, based upon global maximum nucleation depths and temperature-dependent-deformation experimental results. Therefore, the mantle earthquakes show that the upper mantle lithosphere is not in thermal equilibrium with the shallow (60 km) underlying asthenosphere. Our thermal modeling indicates that the lithosphere beneath Harrat Lunayyir thinned to its current 60-km thickness at 12 ± 2 Ma, as constrained by thermal modeling of: (1) surface heat-flow; (2) the depth to the mid-crustal brittle–ductile transition, and (3) the depth to the upper-mantle brittle–ductile transition.

We investigate the crustal and upper-mantle shear-velocity structure of Saudi Arabia by fundamental-mode Rayleigh-wave group-velocity tomography and shear-wave velocity inversion. The seismic dataset is compiled using ∼140 stations of the Saudi National Seismic Network (SNSN) operated by the Saudi Geological Survey (SGS). We measure Rayleigh-wave group-velocities at periods of 8–40 s from regional earthquakes. After obtaining 1-D shear-wave velocity models by inverting group-velocities at each grid node, we construct a 3-D shear-velocity model for Saudi Arabia and adjacent regions by interpolating the 1-D models. Our 3-D model indicates significant lateral variations in crustal and lithospheric thickness, as well as in the shear-wave velocity over the study region. In particular, we identify zones of reduced shear-wave speed at crustal levels beneath the Cenozoic volcanic fields in the Arabian Shield. The inferred reductions of 2–5% in shear-wave speed may be interpreted as possibly indicating the presence of partial melts. However, their precise origin we can only speculate about. Our study also reveals an upper-mantle low velocity zone (LVZ) below the Arabian Shield, supporting the model of lateral mantle flow from the Afar plume. Further geophysical experiments are needed to confirm (or refute) the hypothesis that partial melts may exist below the Cenozoic volcanism in western Saudi Arabia, and to build a comprehensive geodynamic–geological model for the evolution and present state of the lithosphere of the Arabian Plate and the Red Sea.

We measured phase velocities at 13 periods from 20 s to 143 s using Rayleigh-wave data recorded at recently-installed, dense (135) broadband seismic stations in the Arabian shield and determined the shear-wave velocity structure. Our results clearly reveal a 300-km wide upper mantle seismic low-velocity zone (LVZ) beneath the western Arabian shield at a depth of 60 km and with a thickness of 130 km. The LVZ has a north-south trend and follows the late-Cenozoic volcanic areas. The lithosphere beneath the western Arabian shield is remarkably thin (60-90 km). The 130-km thick mantle LVZ does not appear beneath the western Red Sea and the spreading axis. Thus, the Red Sea at 20°- 26° N is an asymmetric rift, with thin lithosphere located east of the Red Sea axis, as predicted by the low-angle detachment model for rift development. Passive rifting at the Red Sea and extensional stresses in the shield are probably driven by slab pull from the Zagros subduction zone. The low shear-wave velocity (4.0-4.2 km/s) and the geometry of LVZ beneath the western shield indicate northward flow of hot asthenosphere from the Afar hot spot. The upwelling of basaltic melt in fractures or zones of localized lithospheric thinning has produced extensive late Cenozoic volcanism on the western edge of the shield, and the buoyant LVZ has caused pronounced topography uplift there. Thus, the evolution of the Red Sea and the Arabian shield is driven by subduction of the Arabian plate along its northeastern boundary.

We analyze the results of a Probabilistic Seismic Hazard Assessment obtained for Saudi Arabia using a spatially smoothed seismicity model. The composite up-to-date earthquake catalog is used to model seismicity and to determine earthquake recurrence characteristics. Different techniques that are frequently used for the analysis of input data are applied in the study. The alternative techniques include the declustering procedures for catalog processing, statistical techniques for estimation the magnitude–frequency relationship (MFR), and the seismicity smoothing parameter. The scheme represents epistemic uncertainty that results from an incomplete knowledge of earthquake process and application of alternative mathematical models for a description of the process. Our calculations that are based on the Monte Carlo technique include also a consideration of the aleatory uncertainty related to the dimensions and depths of earthquake sources, parameters of MFR, and scatter of ground-motion parameter. The hazard maps show that the rock-site peak ground acceleration (PGA) is the highest for the seismically active areas in the north-western (Gulf of Aqaba, PGA > 300 cm/s2 for return period 2475 years) and south-western (PGA > 100 cm/s2 for return period 2475 years) parts of the country. We show that the procedures for catalog declustering have the highest influence on the results of hazard estimations, especially for high levels of hazard. The choice of smoothing parameter is a very important decision that requires proper caution. The relative uncertainty that is ratio between the 85th and 15th percentiles may reach 50–60% for the areas located near the zones of high-level seismic activity.

We investigate the lithospheric shear-wave velocity structure of Saudi Arabia by conducting H-κ stacking analysis and jointly inverting teleseismic P-receiver functions and fundamental-mode Rayleigh wave group velocities at 56 broadband stations deployed by the Saudi Geological Survey (SGS). The study region, the Arabian plate, is traditionally divided into the western Arabian shield and the eastern Arabian platform: The Arabian shield itself is a complicated melange of crustal material, composed of several Proterozoic terrains separated by ophiolite-bearing suture zones and dotted by outcropping Cenozoic volcanic rocks (locally known as harrats). The Arabian platform is primarily covered by 8 to 10 km of Paleozoic, Mesozoic and Cenozoic sedimentary rocks. Our results reveal high Vp/Vs ratios in the region of Harrat Lunayyir, which are interpreted as solidified magma intrusions from old magmatic episodes in the shield. Our results also indicate slow velocities and large upper mantle lid temperatures below the southern and northern tips of the Arabian shield, when compared with the values obtained for the central shield. We argue that our inferred patterns of lid velocity and temperature are due to heating by thermal conduction from the Afar plume (and, possibly, the Jordan plume), and that volcanism in western Arabia may result from small-scale adiabatic ascent of magma diapirs.

A fundamental goal of igneous petrology is to quantify the duration of time required to produce evolved magmas following influx of basalt into the crust. However, in many cases, complex field relations and/or the presence of a long-lived magmatic system make it difficult to assess how basaltic inputs relate to more evolved magmas, therefore, precluding calculation of meaningful timescales. Here, we present field relations, geochemistry, 40Ar/39Ar ages, and 36Cl ages for volcanic rocks from the Harrat Rahat volcanic field, Saudi Arabia. These data document a systematic and repeated temporal progression from alkali basalt to trachyte for the youngest eruptives. From ~ 150 to ~ 17 ka the following eruptive sequence occurred four times: (1) alkali basalt, (2) hawaiite, mugearite, or benmoreite, and (3) trachyte. We interpret each eruptive sequence to result from injection of basalt into the crust, and its subsequent differentiation and eruption of progressively evolved magmas. We use the interval time between successive eruptions within a given sequence to calculate the duration of time required to produce trachyte from alkali basalt. Differentiation from alkali basalt to intermediate compositions (hawaiite, mugearite, benmoreite) took ≤ 2 kyr on average. Differentiation from intermediate compositions to trachyte took a maximum of 6.6 ± 3.5 to 22.5 ± 1.6 kyr. Thus, the total duration of differentiation was ~ 9 to ~ 25 kyr. Timescales presented here are insensitive to processes evoked to drive differentiation because they are based solely on the ages and compositions of eruptive products from a system characterized by a simple, repeated differentiation sequence.

A new seismic source model has been developed for the western part of the Arabian Peninsula, which has experienced considerable earthquake activity in the historical past and in recent times. The data used for the model include an up-to-date seismic catalog, results of recent studies of Cenozoic faulting in the area, aeromagnetic anomaly and gravity maps, geological maps, and miscellaneous information on volcanic activity. The model includes 18 zones ranging along the Red Sea and the Arabian Peninsula from the Gulf of Aqaba and the Dead Sea in the north to the Gulf of Aden in the south. The seismic source model developed in this study may be considered as one of the basic branches in a logic tree approach for seismic hazard assessment in Saudi Arabia and adjacent territories.

Lava flow hazard modelling requires detailed geological mapping, and a good understanding of emplacement settings and the processes involved in the formation of lava flows. Harrat Rahat, Kingdom of Saudi Arabia, is a large volcanic field, comprising about 1000 predominantly small-volume volcanoes most of which have emitted lava flows of various lengths. A few eruptions took place in this area during the Holocene, and they were located in the northern extreme of the Harrat Rahat, a close proximity to critical infrastructure and population living in Al-Madinah City. In the present study, we combined field work, high resolution digital topography and morphometric analysis to infer the emplacement history of the last historical event in the region represented by the 1256 AD Al-Madinah lava flow field. These data were also used to simulate 1256 AD-type lava flows in the Harrat Rahat by the MAGFLOW lava flow emplacement model, which is able to relate the flow evolution to eruption conditions. The 1256 AD lava flow field extent was mapped at a scale of 1:1000 from a high resolution (0.5 m) Light Detection And Ranging (LiDAR) Digital Terrain Model (DTM), aerial photos with field support. The bulk volume of the lava flow field was estimated at 0.4 km3, while the source volume represented by seven scoria cone was estimated at 0.023 km3. The lava flow covered an area of 60 km2 and reached a maximum length of 23.4 km. The lava flow field comprises about 20.9% of pāhoehoe, 73.8% of 'a'ā, and 5.3% of late-stage outbreaks. Our field observation, also suggests that the lava flows of the Harrat Rahat region are mainly core-dominated and that they formed large lava flow fields by amalgamation of many single channels. These channels mitigated downslope by topography-lava flow and channel–channel interactions, highlighting this typical process that needs to be considered in the volcanic hazard assessment in the region. A series of numerical lava flow simulations was carried out using a range of water content (0.1–1 wt.%), solidification temperature (800–600 °C) and effusion curves (simple and complex curves). These simulations revealed that the MAGFLOW code is sensitive to the changes of water content of the erupting lava magma, while it is less sensitive to solidification temperature and the changes of the shape of effusion curve. The advance rate of the simulated lava flows changed from 0.01 to 0.22 km/h. Using data and observations from the youngest volcanic event of the Harrat Rahat as input parameters to MAGFLOW code, it is possible to provide quantitative limits on this type of hazard.

In 1999 the Saudi Geological Survey (SGS) was formed. One of the responsibilities eventually assigned to the SGS was the Saudi National Seismic Network. The SGS installed 13 broadband VSAT (very small aperture terminal) telemetry seismic stations in 2004 and is in the process of installing 14 additional stations. By declaration of the Council of Ministers of the Kingdom of Saudi Arabia, SGS also was assigned the task of upgrading 62 seismic stations operated by the King Abdulaziz City for Science and Technology (KACST) and King Saud University (KSU). The goal is to have a single agency responsible for seismic network management and reporting of earthquakes in and around the Kingdom of Saudi Arabia. Seismic data will be shared with other institutions to provide data for seismic research by Saudi scientists. In the longer term the goal is to share data with the international seismological community.