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Five years of continuously recording GPS observations in the Kingdom of Saudi Arabia together with new continuous and survey-mode GPS observations broadly distributed across the Arabian Peninsula provide the basis for substantially improved estimates of present-day motion and internal deformation of the Arabian plate. We derive the following relative, geodetic Euler vectors (latitude ([degrees]N), longitude ([degrees]E), rate ([degrees]/Myr, counterclockwise)) for Arabia-Nubia (31.7 [+ or -] 0.2, 24.6 [+ or -] 0.3, 0.37 [+ or -] 0.01), Arabia-Somalia (22.0 [+ or -] 0.5, 26.2 [+ or -] 0.5, 0.40 [+ or -] 0.01), Arabia-India (18.0 [+ or -] 3.8, 87.6 [+ or -] 3.3, 0.07 [+ or -] 0.01), Arabia-Sinai (35.7 [+ or -] 0.8, 17.1 [+ or -] 5.0, 0.15 [+ or -] 0.04), and Arabia-Eurasia (27.5 [+ or -] 0.1, 17.6 [+ or -] 0.3, 0.404 [+ or -] 0.004). We use these Euler vectors to estimate present-day stability of the Arabian plate, the rate and direction of extension across the Red Sea and Gulf of Aden, and slip rates along the southern Dead Sea fault south of the Lebanon restraining bend (4.5-4.7 [+ or -] 0.2 mm/yr, left lateral; 0.8-1.1 [+ or -] 0.3 mm/yr extension) and the Owens fracture zone (3.2-2.5 [+ or -] 0.5 mm/yr, fight lateral, increasing from north to south; 1-2 mm/yr extension). On a broad scale, the Arabian plate has no resolvable internal deformation (weighted root mean square of residual motions for Arabia equals 0.6 mm/yr), although there is marginally significant evidence for N-S shortening in the Palmyride Mountains, Syria at [less than or equal to] 1.5 ram/yr. We show that present-day Arabia plate motion with respect to Eurasia is consistent within uncertainties (i.e., [+ or -] 10%) with plate tectonic estimates since the early Miocene when Arabia separated from Nubia. We estimate the time of Red Sea and Gulf of Aden rifting from present-day Arabia motion, plate tectonic evidence for a 70% increase in Arabia-Nubia relative motion at 13 Ma, and the width of the Red Sea and Gulf of Aden and find that rifting initiated roughly simultaneously ([+ or -] 2.2 Myr) along the strike of the Red Sea from the Gulf of Suez to the Afar Triple Junction, as well as along the West Gulf of Aden at 24 [+ or -] 2.2 Ma. Based on the present kinematics, we hypothesize that the negative buoyancy of the subducted ocean lithosphere beneath the Makran and the Zagros fold-thrust belt is the principle driver of Arabia-Eurasia convergence and that resisting forces associated with Arabia-Eurasia continental collision have had little impact on plate motion. Citation: ArRajehi, A., et al. (2010), Geodetic constraints on present-day motion of the Arabian Plate: Implications for Red Sea and Gulf of Aden riffing, Tectonics, 29, TC3011, doi: 10.1029/2009TC002482.

Calculation of earthquake scarp ages from scarp morphology usually assumes that scarp materials reach their angle of repose immediately after a rupture. However, observations of the 1959 Hebgen Lake, Montana, earthquake scarp and similar features worldwide confirm that scarps require a finite period of mass failure to reach the initial conditions for hillslope diffusion, so the age of features less than 1000 yr old cannot be accurately estimated with methods based only on the linear diffusion equation. We apply a numerical model of this interval of mass failure degradation to vertical initial-angle scarps from the 1959 rupture at Hebgen Lake, Montana. The mass failure rate coefficient, RM, ranges from 1.0 x 10-2 to 1.2 x 10-1 m · yr-1 for young scarps at Hebgen Lake and nine other locations, and has little or no dependence on climate conditions such as annual temperature range or average rainfall. Including an interval of mass failure gives more accurate age estimates where a scarp age is of the same order as the characteristic mass failure relaxation time of 10-1000 yr. [ABSTRACT FROM AUTHOR]