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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.

Harrat Rahat, one of several large, basalt-dominated volcanic fields in western Saudi Arabia, is a prime example of continental, intraplate volcanism. Excellent exposure makes this an outstanding site to investigate changing volcanic flux and composition through time. We present 93 40Ar/39Ar ages and six 36Cl surface-exposure ages for volcanic deposits throughout northern Harrat Rahat that, when integrated with a new geologic map, define 12 eruptive stages. Exposed volcanic deposits in the study area erupted 570 ka. Over the past 570 k.y., the average eruption rate was 0.14 km3/k.y., but volcanism was episodic with periods alternating between low (0.04–0.06 km3/k.y.) and high (0.1–0.3 km3/k.y.) effusion rates. Before 180 ka, eruptions vented from the volcanic field’s dominant eastern vent axis and from a subsidiary, diffuse, western vent axis. After 180 ka, volcanism focused along the eastern vent axis, and the composition of volcanism varied systematically along its length from basalt dominated in the north to trachyte dominated in the south. We hypothesize that these compositional variations

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.

Near-surface seismic measurements, along with relevant geotechnical methods, have become important techniques for delineating the elastic coefficients and geotechnical parameters of shallow foundations. In the north Jeddah expansion zone, shallow seismic measurements at 76 sites have been carried out to estimate both the P- and S-wave velocities for near-surface sediments. These data have been processed where the results revealed a three-layered ground model that reaches to 30 m depth with varying velocities and thicknesses. Then, the elastic coefficients such as Young’s modulus, bulk modulus, shear modulus, ultimate bearing capacity, and allowable bearing capacity have been assessed based on the estimated seismic wave velocities. It can be concluded that bearing capacity is closely related to shear wave velocity and shear modulus. Comparing our results with the published safe ranges of allowable bearing capacities, it is recommended that the first layer in the western zone as well as the second layer is the most suitable areas for constructing safe engineering foundations in the area of interest. Additionally, the best locations of these foundations can be easily identified from contouring maps.

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.

Distribution of earthquakes along the western part of the Arabian Peninsula is related to the Red Sea floor spreading and volcanism. The Arabian Peninsula contains Cenozoic lava fields (harrats) extending from Yemen in the south up to Syria in the north. Harrat Lunayyir that is a very young volcanic area has suffered from two earthquake swarms with maximum magnitude M W 5.7 caused by the dike intrusion. The area was not considered as a seismic source zone when constructing seismic hazard maps used in the current issue of Saudi Building Code (SBC-2007). In this work, probabilistic seismic hazard is estimated for the Harrat Lunayyir area and the town of Alays located a few kilometers to south-east of the Harrat. Two models of seismic source zones describing seismic process in Harrat Lunayyir are considered as alternatives in the logic tree scheme, namely: the areal source model and the fault (characteristic earthquake) source model. The results of probabilistic estimations of seismic hazard show that the area is characterized by significant level of hazard. The expected peak ground amplitudes at rock site and for return period 2475 yrs are larger than 200 cm/s 2 and 140 cm/s 2 for the central part of the area and the town of Alays respectively. The level of uncertainty quantified in terms of “relative uncertainty” is the largest in the central part of the area.

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.

Rabigh city located along the coastal plain of the Red Sea in western Saudi Arabia is economically significant. This pioneering study assesses the site characterization of Rabigh city based on integrated geological, geotechnical, and seismological data. These datasets include geotechnical data from 100 boreholes, 127 microtremor records, and detailed geologic maps of the area of study. The grid interval for microtremor sites is 1 km. The fundamental resonance frequency (fo) and maximum amplification factor (Ao) are identified from the horizontal-to-vertical spectral ratio (HVSR) curves from the recorded data. The distribution map of fo displays three distinct zones, high fundamental frequency (greater than 1 Hz), moderate (0.5 Hz– Hz), and low (less than 0.5 Hz), while the obtained Ao ranges between 2 and 4.2. The horizontal-to-vertical spectral response inversion technique has been conducted for 40 microtremor records in the frequency band 0.2–5 Hz to evaluate the shear-wave velocity for 30 m depth (Vs30). The computed values of Vs30 range from 208.5 m/s to 516 m/s. According to the National Earthquake Hazard Reduction Program (NEHRP) soil site characterization, Rabigh city is differentiated into two soil classes, C and D. The detailed geotechnical investigations for sabkhah sediments are highly recommended for reducing their environmental risks to urban facilities and economic projects in Rabigh area.

Rapid social and industrial development of the Makkah and Jeddah regions emphasizes necessity of reassessment of seismic hazard, results of which are essential for aseismic design, emergency management, and insurance regulations. In this work, the deterministic seismic hazard studies are applied to evaluate level of seismic hazard for the Makkah region using the up-to-date geophysical, geological, and seismological database, and using different techniques. The assessment has been performed in terms of peak ground acceleration (PGA) and presumably active faults are considered as sources of earthquakes. Results of the study show that, in the context of the used models of seismic sources, the level of seismic hazard in the region is controlled by magnitude of earthquakes that may occur at the intersection of NE-SW faults (the Ad Damm fault zone and the Wadi Fatima Shear Zone) and the NNW-oriented faults that run parallel to the Red Sea coast (Red Sea Margin Faults group). It is important to make accurate mapping of faults using up-to-date data and to estimate if the faults are active at present. Particular attention should be given to careful estimation of maximum magnitude of possible earthquakes that can occur along the active fault.

In this paper, we present the first detailed study of earthquake detection capabilities of the Saudi National Seismic Network (SNSN), which is operated since 2006 by National Center of Earthquakes and Volcanoes, Saudi Geological Survey (SGS). Seismic monitoring in Saudi Arabia has a great improvement after the installation of the Saudi National Seismic Network and upgrading the previous analog telemetry networks. The spatial distribution of the seismic stations covers most of Saudi Arabia and concentrated mainly in the Western part. Since the operation of the network, a significant number of local, regional, and teleseismic events are located by manually and automatic processing. Consequently, the data was revised and archived in the databases. The detection-location capability of the Saudi National Seismic Network has been estimated for the period 2016–2017. The obtained maps show that the network in this time period has the ability to detect and locate earthquakes occurred in Saudi Arabia and its surroundings. For the present network configuration and stations characteristics, the technique determines the lowest magnitude of events that the seismic network is able to detect, locate, and estimate errors in both location and origin time for different magnitudes. According to the obtained results in this study, all Harrats (volcanic) regions and recent active areas as Jazan, North Umluj, and the Gulf of Aqaba indicate that the detectability level is less than magnitude (ML)

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.

The 2009 earthquake swarm that occurred in Al-Ays area, Saudi Arabia was believed to be of magmatic origin and was caused by magma rising underneath the area and installment of a subsurface dyke that reached up to 3.5 km below the ground surface. This study uses the results of an integrated geophysical survey conducted across the major surface rupture, which appeared in the area after the earthquake swarm, along with other evidence, to suggest a different scenario. Contradicting focal mechanisms, temporal and spatial distribution of earthquakes, mixed frequency earthquakes, a previous earthquake swarm, recent similar swarms along the Red Sea coast, and the alignment of earthquakes along a major northwest-southeast trending fault are among the evidence presented to refute the one-decade old subsurface dyke intrusion model. Last but most important, the geophysical data we acquired across the surface rupture in the area show clear evidence of shallow negative flower structure in the ruptured zone, implicating the occurrence of a transtensional tectonic movement, that may have caused the rupture and triggered the associated earthquake swarm. The evidence presented in this article prove that the 2009 earthquakes episode in the Al-Ays area was triggered by regional tectonics readjustment in the area, rather than magma upwelling and subsurface dyke intrusion, that if present, could be a secondary effect of the tectonic movement.

Mafic volcanic fields are widespread, but few have erupted in historic times, providing limited observations of the magnitudes, dynamics, and timescales of lava flow emplacement in these settings. To expand our knowledge of effusive mafic eruptions, we must evaluate solidified flows to discern syn-eruptive conditions. The Harrat Rahat volcanic field in western Saudi Arabia offers a good opportunity for this, with a historical eruption in 1256 CE and many well-preserved prehistoric flows. We combine historical observations and rheological and morphological analyses of the youngest flows with analytical models to reconstruct eruptive histories and lava flow emplacement conditions in Harrat Rahat. Petrologic analysis of samples for emplacement temperatures and crystallinities shows cooling trends from vent to toe of ~ 1140 to ~ 1090 °C at rates of 2–7 °C km−1, crystallinities increasing from 0.5 to 60%, and apparent viscosities increasing from 102 to 109 Pa s. High-resolution topographic data facilitates quantitative analysis of morphology and interpolation of pre-eruptive surfaces to measure flow thicknesses, channels, and levees, and enables calculation of eruptive volumes. Analytical models relating flow morphology to emplacement conditions are applied to estimate effusion rates. Within the suite of studied flows, volume estimates range from 0.07 to 0.42 km3 dense rock equivalent, with effusion rates on the order of 10 to 100 s of m3 s−1 and durations from 1 to 15 weeks. These integrated analyses quantify past lava flow emplacement conditions and dynamics in Harrat Rahat, improving our understanding and observations of fundamental parameters and controls of effusive eruptions in mafic volcanic fields.

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.

Ubhur district is the northern extension of Jeddah. A lot of the new investments and mega structures will be constructed in this area; the highest skyscraper in the Arabian Gulf region will be built in Ubhur. The surface geology of the investigated region is complex, where the sabkha deposits spread everywhere. The subsurface structure of the subsurface soil is complex and includes different soil classes. Microtremor data was acquired at 76 sites located in Ubhur district. The horizontal to vertical spectral ratio technique is used to determine the resonance frequency (f0) and the maximum amplification factor (A0) at the selected sites. This technique is considered as the cheapest and most reliable one for determining the resonance frequency. The acquired data was processed using the Geopsy software where the reliability of the H/V curves and peaks was considered through the data processing and analysis. The results indicate that the predominant resonance frequency of Ubhur district ranges between 0.77 and 5.7 Hz, while the maximum amplification factors range from 2 to 6. It is indicated that the central zone of the Ubhur district is considered suitable for the construction of high-rise facilities compared to other parts of the region, which should be taken into consideration during land use planning.

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.

Earthquake ground motion model is an essential part of seismic hazard assessment. The model consists in several empirical ground motion prediction equations (GMPEs) that are considered to be applicable to the given region. When the recorded ground motion data are scarce, numerical modeling of ground motion based on available seismological information is widely used. We describe results of stochastic simulation of ground motion acceleration records for western Saudi Arabia. The simulation was performed using the finite fault model and considering peak ground acceleration and amplitudes of spectral acceleration at natural frequencies 0.2 and 1.0 s. Based on the parameters of the input seismological model that were accepted in similar previous studies, we analyze influence of variations in the source factor (stress drop) and in the local attenuation and amplification factors (kappa value, crustal amplification). These characteristics of the model are considered as the major contributors to the ground motion variability. The results of our work show that distribution of simulated ground motion parameters versus magnitude and distance reveals an agreement with the GMPEs recently used in seismic hazard assessment for the region. Collection of credible information about seismic source, propagation path, and site attenuation parameters using the regional ground motion database would allow constraining the seismological model and developing regional GMPEs. The stochastic simulation based on regional seismological model may be applied for generation of ground motion time histories used for development of analytical fragility curves for typical constructions in the region.

Harrat Lunayyir is one of the smaller lava fields in western Saudi Arabia that is of current interest due to a dike intrusion episode in 2009, an ongoing swarm of earthquakes and the possible hazard that pose. In addition to seismology, other geophysical data have been used to study the structure of the area, and the available aeromagnetic information is shown and discussed here. The reduced to the pole (RTP) magnetic grid and its enhancements have been used to define some of the main anomalies, and these have been correlated with the known geology, including the Red Sea coastal dike system. There are numerous linear features that are possibly related to the dikes or faulting, but within the area of the harrat, details due to the underlying structure are largely obscured by the magnetic surface lavas. Northeasterly trends in the magnetic data may indicate old zones of weakness that intersect one of the main coastal dikes at the location of the recent seismic activity and surface fissure, suggesting that this is a point of weakness resulting in the volcanism and seismicity that also appears to be largely limited to two of the NE trends. The association of the recent seismicity with a known geological and aeromagnetic feature is important in determining the seismic hazard for the region, especially if the location of future activity can be used to reduce overall uncertainty in the analysis by identifying potential fault sources. Here, the seismicity appears to lie on one of the NNW-trending coastal dikes that have been reactivated recently along a section between two NE-trending older faults.