Abdullah Al-Amri

Lithospheric seismic velocity discontinuities beneath the Arabian Shield

We determined crustal and lithospheric mantle velocity structure beneath the Arabian Shield through the modeling of receiver function stacks obtained from teleseismic P waves recorded by the 9 station temporary broadband array in western Saudi Arabia. The receiver function deconvolution technique was used to isolate the receiver-side PS mode conversions. A grid search method, which should yield an unbiased global minimum, was used to solve for a shear wave velocity model that is optimal and has the minimum number of layers needed to fit the receiver function waveform. Results from this analysis show that the crustal thickness in the shield area varies from 35 to 40 km in the west, adjacent to the Red Sea, to 45 km in central Arabia. Stability tests of each solution indicate that the models are relatively well constrained. We have also observed evidence for a large positive velocity contrast at sub-Moho depths at four stations at depths of 80 to 100 km. This discontinuity may represent a change in rheology in the lower part of the lithosphere or remnant structure from the formation of the Arabian Shield.

S wave velocity structure of the Arabian Shield upper mantle from Rayleigh wave tomography

[1] The shear wave velocity structure of the shallow upper mantle beneath the Arabian Shield was modeled by inverting Rayleigh wave phase velocity measurements between 45 and 140 s together with previously published Rayleigh wave group velocity measurements between 10 and 45 s. For measuring phase velocities, we applied a modified array method to data from several regional networks that minimizes the distortion of raypaths caused by lateral heterogeneity. The new shear wave velocity model shows a broad low-velocity region to depths of � 150 km in the mantle across the Shield and a narrower low-velocity region at depths � 150 km localized along the Red Sea coast and Makkah-Madinah-Nafud (MMN) volcanic line. The velocity reduction in the upper mantle corresponds to a temperature anomaly of � 250– 330 K. These findings, in particular the region of continuous low velocities along the Red Sea and MMN volcanic line, do not support interpretations for the origin of the Cenozoic plateau uplift and volcanism on the Shield invoking two separate plumes. When combined with images of the 410 and 660 km discontinuities, body wave tomographic models, a S wave polarization analysis, and SKS splitting results for the Arabian Peninsula, the anomalous upper mantle structure in our new velocity model can be attributed to an upwelling of warm mantle rock originating in the lower mantle under Africa that crosses through the mantle transition zone beneath Ethiopia and moves to the north and northwest under the eastern margin of the Red Sea and the Arabian Shield. In this interpretation, the difference in mean elevation between the Arabian Platform and Shield can be attributed to isostatic uplift caused by heating of the lithospheric mantle under the Shield, with the significantly higher elevations along the Red Sea coast possibly resulting also from lithospheric thinning and dynamic uplift.

Shear‐wave splitting across western Saudi Arabia: The pattern of upper mantle anisotropy at a Proterozoic Shield

We constrain upper mantle anisotropy across the Arabian Shield from shear-wave splitting analyses of SKS phases at eight temporary broadband stations that operated in Saudi Arabia. The direction of fast polarization is consistently aligned north-south and the delay time between fast and slow shear waves is generally 1.0 to 1.5 s, indicating that the mantle anisotropy is relatively homogeneous and coherent. We cannot distinguish between two possible models for the origin of this signal. The observed splitting may reflect fossil upper mantle anisotropy associated with the dominantly east-west accretion of oceanic terranes and formation of the Proterozoic Arabian lithosphere. Our results may also be compatible with present-day asthenospheric anisotropy caused by the northward absolute plate motion of the Arabian plate or northward asthenospheric flow from an Ethiopian mantle plume.

Upper mantle structure beneath the Arabian Peninsula and northern Red Sea from teleseismic body wave tomography: Implications for the origin of Cenozoic uplift and volcanism in the Arabian Shield

Upper mantle structure between 150 and 400 km depth is imaged beneath the Arabian Shield and northern Red Sea by modeling P and S traveltime residuals from teleseismic events recorded on the Saudi Arabia National Digital Seismic Network, the 1995–1997 Saudi Arabian PASSCAL experiment, and three permanent stations (RAYN, EIL, and MRNI). Relative traveltime residuals were obtained using a multichannel cross-correlation method and inverted for upper mantle structure using VanDecars inversion method. The resulting images reveal a low-velocity region (∼1.5% for the P model and ∼3% for the S model) trending NW–SE along the western side of the Arabian Shield and broadening to the northeast beneath the Makkah-Madinah-Nafud volcanic line. We attribute the low velocities to a mantle thermal anomaly that could be as large as 330 K and that is associated with the Cenozoic uplift of and volcanic centers on the Shield. Our tomographic images are not consistent with models invoking separate mantle upwellings beneath the northern and southern regions of the Shield and instead favor single plume or superplume models. We also find little evidence for low velocities beneath the northern Red Sea, suggesting that there might not be a geodynamic link between rifting in the Red Sea and plateau uplift and volcanism in the Shield.

Biocidal polymers: synthesis and antimicrobial properties of benzaldehyde derivatives immobilized onto amine-terminated polyacrylonitrile

BackgroundThe design and applications of antimicrobial polymers is a growing field. Antimicrobial polymers can help to solve the problems associated with the use of conventional antimicrobial agents. Polymers with active functional groups can act as a carrier system for antimicrobial agents. In our study, we aim to prepare and develop some antimicrobial polymers for biomedical applications and water treatment.ResultsThe antimicrobial polymers based on polyacrylonitrile (PAN) were prepared. Functional groups were created onto polyacrylonitrile via amination using different types of diamines such as ethylenediamine (EDA) and hexamethylenediamine (HMDA) to yield amine-terminated polymers. Antimicrobial polymers were obtained by immobilization of benzaldehyde and its derivatives which include, 4-hydroxybenzaldehyde and 2,4-dihydroxybenzaldehyde onto amine-terminated polymers. The antimicrobial activity of the prepared polymers against different types of microorganisms including Gram-positive bacteria (Staphylococcus aureus), Gram-negative bacteria (Pseudomonas aeruginosa; Escherichia coli; and Salmonella typhi) as well as fungi (Aspergillus flavus, Aspergillus niger, Candida albicans, Cryptpcoccus neoformans) were explored by the cut plug method and viable cell counting methods.ConclusionsAmine-terminated polyacrylonitrile were used as a novel polymeric carrier for benzaldehyde derivatives as antimicrobial agents. The prepared polymers can inhibit the growth of the microorganisms. The activity was varied according to the tested microorganism as well as the polymer microstructure. It was found that the activity increased with increasing the number phenolic hydroxyl group of the bioactive group. Finally, it is anticipated that the prepared antimicrobial polymers would be of great help in the field of biomedical applications and biological water treatment.

Seismicity and aeromagnetic features of the Gulf of Aqaba (Elat) Region

Recent seismicity in the Gulf of Aqaba region has been examined in relation to pull-apart tectonics and structures indicated by bathymetry, surface geology, and new analyses of aeromagnetic data. Forty earthquakes occurring in the region from 1985 to 1989 were located using data from a network of five telemetered seismographs in the Midyan region of Saudi Arabia east of the Gulf of Aqaba and from a network north of the gulf in Israel. Interpretation of aeromagnetic data from the Gulf of Aqaba and the adjacent Midyan region confirms previous estimates of offsets across the zone of transform faulting in the gulf. Aeromagnetic anomaly patterns, obtained by reduction-to-the-pole techniques, and earthquake locations provide evidence for continuation of the faulting regime from the gulf northeastward into the Midyan region. The vicinity of the Elat Deep (primarily the southwest flank) probably is undergoing active normal faulting. Other seismicity and first-motion polarity data confirm the regional pattern of NNE trending sinistral strike-slip faulting.

Heterozygous FOXC1 Mutation (M161K) Associated with Congenital Glaucoma and Aniridia in an Infant and a Milder Phenotype in Her Mother

Purpose: To report the genetic basis for congenital glaucoma with clinical aniridia in an infant and a milder phenotype in her mother. Methods: Prospective case series. Results: An infant girl with almost complete lack of iris tissue was referred and treated for congenital glaucoma. Although the presumed clinical diagnosis was aniridia (On-line Mendelian Inheritance in Man [OMIM] AN2, # 106210), PAX6 sequencing was normal. Examination of the infants mother was significant for Axenfeld-Rieger malformation (ARM): prominent Schwabe line, subtle iris hypoplasia, iris stands bridging the angle, increased intraocular pressure, and glaucomatous optic nerve cupping. Both parents and the infant underwent diagnostic FOXC1 DNA sequencing. A heterozygous M161K FOXC1 mutation was found in the infant and her mother but not in the father, who had a normal ocular examination. Discussion: The spectrum of intrafamilial phenotypic variation associated with heterozygous FOXC1 mutation can be wide. FOXC1 mutation can be a cause of congenital glaucoma with clinical aniridia. Although such infants resemble the AN2 phenotype, the glaucoma of AN2 due to PAX6 mutation is typically secondary with onset several years after birth.

Regional waveform propagation in the Arabian Peninsula

Regional waveform propagation is characterized in the Arabian Peninsula using data from a temporary network of broadband seismometers. Between November 1995 and March 1997, 332 regional (delta<15°) events were recorded from nine stations deployed across the Arabian Shield. Regional phase propagation was analyzed in two ways: by individual inspection of the waveforms and by stacking of waveforms. Inspection of the waveforms revealed consistent variations in individual seismograms according to the region of origin. Waveforms from events in the Gulf of Aqaba, northwest of the network, possess weak Pn, Pg, and Sn but show a prominent Lg phase. In contrast, clear Pn, Sn, and Lg are observed for events located in the Zagros, a region northeast of the network. Events near the Straits of Hormuz also display Pn and Sn but lack a strong high-frequency Lg. Southern Red Sea and African earthquakes have moderate-amplitude body phases with some Lg. For the stacks the data were high-pass filtered at 1 Hz, rectified, binned, and then stacked by time/distance or by time/slowness. The time/distance stacks show clear differences between regions that correspond to the variations observed in individual seismograms. The time/slowness stacks allow comparison of relative phase velocities and amplitudes. Pn velocity under the network was estimated to be 8.0±0.2 km/s, consistent with data from prior refraction profiles. The area of inefficient Pn and Sn propagation coincides with an area of Holocene volcanism and suggests that anomalous upper mantle underlies much of the Arabian Shield.

Source Parameters for Moderate Earthquakes in the Zagros Mountains with Implications for the Depth Extent of Seismicity

Six earthquakes within the Zagros Mountains with magnitudes between 4.9 and 5.7 have been studied to determine their source parameters. These events were selected for study because they were reported in open catalogs to have lower crustal or upper mantle source depths and because they occurred within an area of the Zagros Mountains where crustal velocity structure has been constrained by previous studies. Moment tensor inversion of regional broadband waveforms has been combined with forward modeling of depth phases on short-period teleseismic waveforms to constrain source depths and moment tensors. Our results show that all six events nucleated within the upper crust (<11 km depth) and have thrust mechanisms. This finding supports other studies that call into question the existence of lower crustal or mantle events beneath the Zagros Mountains.

Recent seismic activity in the northern Red Sea

Abstract Recent seismicity and swarm activity in the northern Red Sea have been examined in relation to the tectonics and structures indicated by surface geology and marine magnetic anomalies. Seismicity appears to be low compared to the rest of the Red Sea. Sixty eight earthquakes recorded during the period 1964–1993 had body-wave magnitudes between 3.8 and 6. On 18 February 1992, a swarm of earthquakes began in the northern Red Sea and lasted for about 5 days. More than 180 local earthquakes were detected between 25.79 °–26.89 °N latitudes and 34.74 °–35.57 °E longitudes, 12 of which were discernible enough on seismograms to establish epicenters and magnitudes. The majority of seismic activity of this swarm is clustered in the area located between lat. 26.7 ° and 27.3 °. The most remarkable aspect of this earthquake swarm sequence is the migration of epicenters northward by about 100 km in 5 days with focal depths less than 20 km. This study and historical data confirm that the relatively low level of seismicity should not be used as an argument for minimizing the probability of seismic hazard. This swarm may release energy that can be accumulated to cause larger events in the future. This study, however, does not agree with the previous idea which states that the northern Red Sea is considered to be a seismic gap, where the faults are locked and do not generate earthquakes.