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Department of Geological Sciences

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Browsing Department of Geological Sciences by Author "Al-Amri, Abdullah M. S."

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    Investigating the P wave velocity structure beneath Harrat Lunayyir, northwestern Saudi Arabia, using double-difference tomography and earthquakes from the 2009 seismic swarm
    (American Geophysical Union, 2013-09-12) Hansen, Samantha E.; DeShon, Heather R.; Moore-Driskell, Melissa M.; Al-Amri, Abdullah M. S.; University of Alabama Tuscaloosa; Southern Methodist University; King Saud University
    In 2009, a swarm of more than 30,000 earthquakes occurred beneath the Harrat Lunayyir lava field in northwest Saudi Arabia. This event was just one of several seismic swarms to occur in this region over the past decade. Surface deformation associated with the seismicity, modeled in previous studies using Interferometric Synthetic Aperture Radar (InSAR) data, is best attributed to the intrusion of a 10kmlong dyke. However, little is known about the velocity structure beneath Harrat Lunayyir, making assessment of future seismic and volcanic hazards difficult. In this study, we use local double-difference tomography to generate a P wave velocity model beneath Harrat Lunayyir and to more precisely locate earthquakes from the 2009 seismic swarm. A pronounced fast velocity anomaly, centered at similar to 15km depth with a shallower extension to the N-NW, is interpreted as an area of repeated magmatic intrusion. The crust surrounding the fast intrusion is slower than that suggested by broader-scale models for the Arabian Shield. The largest magnitude events occurred early in the swarm, concentrated at shallow depths (similar to 2-8km) beneath northern Harrat Lunayyir, and these events are associated with the dyke intrusion. Later, deep earthquakes (similar to 15km) beneath the southern end of the study region as well as a group of intermediate-depth events connecting the shallow and deep regions of seismicity occurred. These later events likely represent responses to the local stress conditions following the intrusion. Our results are unique since harrat magma systems are rarely imaged, and our observations, coupled with the seismic history in this region, suggest that future volcanic intrusions beneath Harrat Lunayyir are likely.
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    Seismic velocity structure and depth-dependence of anisotropy in the Red Sea and Arabian shield from surface wave analysis
    (American Geophysical Union, 2008-10-14) Hansen, Samantha E.; Gaherty, James B.; Schwartz, Susan Y.; Rodgers, Arthur J.; Al-Amri, Abdullah M. S.; King Saud University; Columbia University; United States Department of Energy (DOE); Lawrence Livermore National Laboratory; University of California System; University of California Santa Cruz; University of Alabama Tuscaloosa
    We investigate the lithospheric and upper mantle shear wave velocity structure and the depth-dependence of anisotropy along the Red Sea and beneath the Arabian Peninsula using receiver function constraints and phase velocities of surface waves traversing two transects of stations from the Saudi Arabian National Digital Seismic Network. Frequency-dependent phase delays of fundamental-mode Love and Rayleigh waves, measured using a cross-correlation procedure, require very slow shear velocities and the presence of anisotropy to depths of at least 180 km in the upper mantle. Linearized inversion of these data produce path-averaged 1D radially anisotropic models with similar to 4% anisotropy in the lithosphere and across the lithosphere-asthenosphere boundary (LAB). Models with reasonable crustal velocities in which the mantle lithosphere is isotropic cannot satisfy the data. The lithosphere, which ranges in thickness from about 70 km near the Red Sea coast to about 90 km beneath the Arabian Shield, is underlain by a pronounced low-velocity zone with shear velocities as low as 4.1 km/s. Forward models of azimuthal anisotropy, which are constructed from previously determined shear wave splitting estimates, can reconcile surface and body wave observations of anisotropy. The low shear velocities extend to greater depth than those observed in other continental rift and oceanic ridge environments. The depth extent of these low velocities combined with the sharp velocity contrast across the LAB may indicate the influence of the Afar hot spot and the presence of partial melt beneath Arabia. The anisotropic signature primarily reflects a combination of plate- and density-driven flow associated with rifting processes in the Red Sea.