Structural seismology of the Arabian Plate and volcano seismology near the Red Sea rift margin
Keywords: Geophysics.

Brittle-failure earthquakes require that rocks deform via a brittle mechanism, instead of a viscous deformation mechanism. This requires that the material is cool, or that strain rates are high, to support such deformation. Exploiting the observation of brittle-failure earthquakes in the mantle lithosphere I was able to perform earthquake geothermometry. Combining this with surface measurements of heat flow, and the thickness of the crust and lithosphere, I determined that Harrat Lunayyir, Saudi Arabia, is out of thermal equilibrium due to having experienced a second stage of lithospheric thinning at 15--12 Ma. This second stage thinned the lithosphere to its present day 60-km thickness, following initial Red Sea extension which occurred ~27 Ma. The thickness of the crust beneath the Kingdom of Saudi Arabia was poorly mapped, with few historic data. I applied a conventional H-k stacking algorithm and included careful attention to stacking weights, sedimentary corrections for stations located on the Arabian Platform, and additional processing for noisy stations. Average crustal thickness (that is, depth to the Mohorovičić discontinuity [Moho] below the surface) beneath the Red Sea coastal plain (the rift margin) is 29 km, beneath the volcanic fields (locally known as harrats) is 35 km, beneath the Arabian Shield (excluding harrats) is 37 km, and beneath the Arabian Platform is 38 km. Crustal thinning appears not to extend east of the rift escarpment, suggesting uniform extension during Red Sea rifting that is no broader at depth than at the surface. I found no statistically significant difference between the total crustal thicknesses of the Arabian Shield and Platform. However, the average subsedimentary crustal thickness (that is, the crystalline crust) for stations on the Arabian Platform is 34 km, 3 km thinner than the crust of the Arabian Shield. There are no statistically significant differences between VP/VS ratios of the different geologic regions of Saudi Arabia. Similar VP/VS ratios, coupled with similar crustal thicknesses for harrats and the Arabian Shield, indicate that Cenozoic magmatism has contributed negligibly to crustal growth. Volcanic earthquakes, in particular those due to the migration of magmatic fluids, elicit much curiosity around the world (e.g., Iceland, Hawaii, Mount St. Helens, etc.) in the hopes of illuminating the internal dynamics of the magmatic system and improving eruption monitoring protocols. It is only recently that volcanic earthquakes in the lower-crust and lithospheric-mantle are being recognized. Harrat Lunayyir exhibits discrete episodic pulses of seismicity in the mantle lithosphere, over a timespan of > 20 years. The seismicity there includes a few earthquakes within the nominally aseismic lower-crust, and ample seismicity during the decade post-dike intrusion. It also exhibited numerous discrete pulses of seismicity that migrated from near the base of the crust to near the surface, for about a decade before the 2009 dike intrusion. This seismicity is driven by asthenosphere derived magmatic fluids (melt, or possibly) and the continued seismicity suggests there has been no appreciable depletion of the asthenospheric magma source