Nassir Al-Arifi

Effective elastic thickness of the Arabian plate: Weak shield versus strong platform

The fan wavelet method has been employed to calculate high-resolution maps of variations of the effective elastic thickness (EET) for the Arabian plate and surroundings. As the initial data, we use high-resolution gravity field, topography, and recent models of sedimentary basins. The western part of the plate is generally characterized by low to midvalues of EET (10–30 km) while the eastern one by high values (50 km and more in the core). This finding confirms that the pronounced asymmetry of the plate is rather associated with fundamental structural differences of the lithosphere than with a forced tilt of the plate due to the rifting in the west-southwest and subduction in the northeast. Therefore, the high topography in the western part of the plate is likely supported by relatively hot mantle that is also responsible for the decrease of EET. These results are generally in agreement with recent seismic tomography models.

Structural cause of a missed eruption in the Harrat Lunayyir basaltic field (Saudi Arabia) in 2009

Harrat Lunayyir is one of the most volcanically active recent basaltic fields in western Saudi Arabia. A period of substantial seismic unrest, featuring more than 30,000 local events, occurred in Harrat Lunayyir in April–June 2009. Although this crisis was presumably related to ongoing magma activity, it ended without any surface volcanic activity. We create new tomographic models of P and S velocities (V P and V S ) and use them to explain the causes of this unrest and the reasons the eruption failed. A large seismic anomaly of high V P /V S ratio below 7 km depth coincides with the locations of more than 50 recent cinder cones with ages of older than 100 ka, and is interpreted as a steady-state magma reservoir. We also identify another seismic anomaly at depths below 15 km, which is interpreted as a conduit for fluids and melts from deeper sources. Because the location of this conduit is slightly outside the main reservoir, some of the incoming material was dispersed. As a result, the activation of the crustal reservoir was not sufficiently strong to pierce the rigid basaltic cover and cause an actual eruption dur ing the crisis in 2009.

Hydrogeological vulnerability and pollution risk mapping of the Saq and overlying aquifers using the DRASTIC model and GIS techniques, NW Saudi Arabia

AbstractSaq and overlying aquifers serve as important sources of water supply for agricultural and domestic usage in Saudi Arabia. Due to urbanization and growth in the agricultural sector, groundwater resources are over-exploited and are prone to quality deterioration. The aquifer vulnerability technique helps delineate areas according to the susceptibility to groundwater contamination. Various parameters pertaining to the surface and subsurface environment were synthesized to represent the data variation in the 3D horizon. Estimates of the parameters, such as recharge, soil media, and vadose zone, were obtained based on modified criteria to account for data variability. Statistical analysis indicates that the input parameters are independent and contribute individually to the vulnerability index. For vulnerability assessment, the DRASTIC model was considered due to the large number of data input parameters. Based on the vulnerability index, the study area is classified into low to very high vulnerability classes. To assess the human interaction on the groundwater environment, the land-use pattern was included as an additional input layer. Sensitivity analyses helped to compute the influence of the input layers on the vulnerability index and the model calibration through revised weights. The model validity tests were performed by comparing the NO3, SO4 and Cl concentration with the different vulnerability zones. The aquifer vulnerability maps developed in the present study may serve as an important tool for effective groundwater resource management.

Anisotropic tomography of Hokkaido reveals delamination‐induced flow above a subducting slab

We present a new 3-D anisotropic seismic model for the crust and upper mantle beneath Hokkaido (Japan) based on tomographic inversion of P and S arrival times from a regional seismic database. The P model is parameterized with three parameters at each point that describe the azimuthal anisotropy; the S model is represented isotropically. The isotropic P and S velocity anomalies match nearly perfectly. In the crust, they show a prominent linear anomaly in central Hokkaido along the Kamuikotan and Hidaka Belts, which represents the area of eastward underthrusting of the Japan Block underneath the Kuril fore arc. We interpret the high-velocity anomaly beneath the Hidaka zone as being delaminated mafic crust and entrained mantle lithosphere, which developed due to crustal shortening in the collision zone. One of our vertical sections shows a very unusual configuration for a subduction zone: a low-velocity slab is overlain by a high-velocity body in the mantle wedge. We propose that the high-velocity delaminated material sinking along the slab surface prevents the escape of fluids and melts from the upper part of the slab, where they are generated due to phase transitions. As a result, a large portion of the fluids is entrained downward and lowers the seismic velocities in the slab. The azimuthal anisotropy in the crust clearly corresponds to the major tectonic units and delineates the major suture zones. In the mantle, the anisotropy has a fan-shaped configuration and most likely represents the deviating of flows starting in southern Hokkaido and splitting into three directions. The western and eastern flows proceed toward the two volcanic groups on Hokkaido, and they may carry with them additional material to trigger the characteristic caldera-forming eruptions in these groups.

Isostatic Model and Isostatic Gravity Anomalies of the Arabian Plate and Surroundings

The isostatic modeling represents one of the most useful “geological” reduction methods of the gravity field. With the isostatic correction, it is possible to remove a significant part of the effect of deep density heterogeneity, which dominates in the Bouguer gravity anomalies. Although there exist several isostatic compensation schemes, it is usually supposed that a choice of the model is not an important factor to first order, since the total weight of compensating masses remains the same. We compare two alternative models for the Arabian plate and surrounding area. The Airy model gives very significant regional isostatic anomalies, which cannot be explained by the upper crust structure or disturbances of the isostatic equilibrium. Also, the predicted “isostatic” Moho is very different from existing seismic observations. The second isostatic model includes the Moho, which is based on seismic determinations. Additional compensation is provided by density variations within the lithosphere (chiefly in the upper mantle). According to this model, the upper mantle under the Arabian Shield is less dense than under the Platform. In the Arabian platform, the maximum density coincides with the Rub’ al Khali, one of the richest oil basin in the world. This finding agrees with previous studies, showing that such basins are often underlain by dense mantle, possibly related to an eclogite layer that has caused their subsidence. The mantle density variations might be also a result of variations of the lithosphere thickness. With the combined isostatic model, it is possible to minimize regional anomalies over the Arabian plate. The residual local anomalies correspond well to tectonic structure of the plate. Still very significant anomalies, showing isostatic disturbances of the lithosphere, are associated with the Zagros fold belt, the collision zone of the Arabian and Eurasian plates.

Three-dimensional density model of the upper mantle in the Middle East: Interaction of diverse tectonic processes

We present a three-dimensional density model of the lithosphere and upper mantle for the Middle East and surroundings based on seismic, gravity, and seismic tomography data and analyze the main factors responsible for the density variations. The gravity effect of the crust is calculated and removed from the observed field using the most recent crustal model. The residual gravity anomalies are jointly inverted with the residual topography to image the density distribution within the upper mantle. The inversion is constrained by an initial density model based on seismic tomography. The obtained density variations span in a large range (±60 kg/m3), revealing strong asymmetry in the density structure of the Arabian plate. The uppermost mantle layer in the Arabian Shield is relatively dense. However, below a depth of ~100 km we observe a strong low-density anomaly. In contrast, the mantle density in the Arabian platform increases at the same depths. The most pronounced decrease of the mantle density occurs in the Gulf of Aden, Red Sea, and East African Rift. Underneath the northern Red Sea the low-density anomaly is limited to the depth ~150 km, while in the southern part it is likely linked to a mantle plume. The densest mantle material is found under the South Caspian basin, which is likely associated with an eclogite body in the uppermost mantle. In the collision zones (the Zagros Belt and the Hellenic Arc), the high-density lithosphere shows the location of the subducting plates.

Groundwater Characteristics and Pollution Assessment Using Integrated Hydrochemical Investigations GIS and Multivariate Geostatistical Techniques in Arid Areas

In arid areas, groundwater quality is an important concern for human life and natural ecosystems. Rapid economic development impacts greatly on limited groundwater resources. This study examines the impacts of natural and anthropogenic contaminations on groundwater quality by integrating hydrochemical investigations, GIS and multivariate geostatistical techniques. Major and trace elements in groundwater were analyzed to evaluate the shallow alluvial aquifer connected to the fractured basement aquifer in the Jazan area of Saudi Arabia (KSA). Results show that the groundwater of the aquifer inland is generally under free conditions and is declining with time, while in the coastal areas, there is a disturbance of the groundwater balance resulting in upward leakage of deep saline water and seawater intrusion. Groundwater types are varied from calcium bicarbonate to sodium chloride, reflecting meteoric water recharge to seawater influence. The pollution risk is high, since the water table is shallow and the aquifers are highly permeable. Urbanization has put stress on groundwater quality due to wastewater contamination from pit latrines and leakage from the unrehabilitated sewage system. Chemical equilibrium and saturation indices were calculated, showing that most of the water samples were under-saturated with respect to anhydrate, aragonite, calcite, dolomite, gypsum and halite mineral phases. This affects carbonate and evaporite minerals in the aquifer, causing ground subsidence and sinkholes in the coastal area. Factor and cluster analyses were used to classify and examine the processes affecting groundwater quality. The main factors impacting on the groundwater hydrochemistry were identified as: dissolution of several minerals, evaporation, human impact, mixing between geothermal and other waters as well as rainfall recharge.

Breathing of the Nevado del Ruiz volcano reservoir, Colombia, inferred from repeated seismic tomography

Nevado del Ruiz volcano (NRV), Columbia, is one of the most dangerous volcanoes in the world and caused the death of 25,000 people in 1985. Using a new algorithm for repeated tomography, we have found a prominent seismic anomaly with high values of the Vp/Vs ratio at depths of 2–5 km below the surface, which is associated with a shallow magma reservoir. The amplitude and shape of this anomaly changed during the current phase of unrest which began in 2010. We interpret these changes as due to the ascent of gas bubbles through magma and to degassing of the reservoir. In 2011–2014, most of this gas escaped through permeable roof rocks, feeding surface fumarole activity and leading to a gradual decrease of the Vp/Vs ratio in the reservoir. This trend was reversed in 2015–2016 due to replenishment of the reservoir by a new batch of volatile-rich magma likely to sustain further volcanic activity. It is argued that the recurring “breathing” of the shallow reservoir is the main cause of current eruptions at NRV.

The feeder system of the Toba supervolcano from the slab to the shallow reservoir

The Toba Caldera has been the site of several large explosive eruptions in the recent geological past, including the worlds largest Pleistocene eruption 74,000 years ago. The major cause of this particular behaviour may be the subduction of the fluid-rich Investigator Fracture Zone directly beneath the continental crust of Sumatra and possible tear of the slab. Here we show a new seismic tomography model, which clearly reveals a complex multilevel plumbing system beneath Toba. Large amounts of volatiles originate in the subducting slab at a depth of ∼150 km, migrate upward and cause active melting in the mantle wedge. The volatile-rich basic magmas accumulate at the base of the crust in a ∼50,000 km3 reservoir. The overheated volatiles continue ascending through the crust and cause melting of the upper crust rocks. This leads to the formation of a shallow crustal reservoir that is directly responsible for the supereruptions.

Application of repeated passive source travel time tomography to reveal weak velocity changes related to the 2011 Tohoku‐Oki Mw 9.0 earthquake

Temporal changes of seismic velocities may provide important information on the processes that occur inside the Earth. However, using body wave data with passive sources faces the problem of an uneven distribution of rays, which may cause artifacts with stronger amplitudes than the actual velocity changes in the Earth. We propose an algorithm for the selection of similar data sets in different time periods that minimize the artifacts related to variable data distributions. In this study, we used the data of the Japan Meteorological Agency for several years before and after the Mw 9.0 Tohoku-Oki event that occurred on 11 March 2011. We performed careful testing using different synthetic models, showing that the selected data subsets allow detecting weak velocity changes with amplitudes above 0.2%. The analysis of the experimental data revealed important features associated with the stress and deformation distributions after the megathrust event. In the upper crust, we found a large zone along the coast with significant P velocity increase likely caused by compression of crustal rocks. This zone was cut by several elongated anomalies with local velocity decrease coinciding with the limits of the maximum slip area. These anomalies possibly mark the areas of major ruptures and deformations after the Tohoku-Oki earthquake. In the coupling zone at a depth of 40 km, we observe a velocity decrease in the area of the Mw 7.7 aftershock representing strong fracturing in the focal zone. Beneath the volcanic arc, we observe significant (up to 0.5%) decrease of P velocity but less prominent S velocity changes.