New Measurements of Shear-wave Splitting at Volcanic Fields, Western Saudi Arabia

Abstract:

The Saudi Geological Survey (SGS) operates a nationwide digital seismic network with more than 160 broadband seismometers that transmit to a central location at the SGS. These seismic data have been used to measure shear-wave splitting in infer anisotropy within and beneath the Arabian plate. We selected for analysis more than 300 teleseismic recordings between January, 2008 and February, 2015. Individual seismometers located on the crystalline rock of the Arabian shield provide 20 to 30 shear-wave splitting results, whereas seismometers located on volcanic rocks provide 2 to 14 reliable measurements. Here we summarize results obtained from the Tertiary volcanic fields (“harrats”) of western Saudi Arabia, in particular Harrat Lunayyir and Harrat Rahat. Both of these volcanic fields have been active in historic times. Eighteen seismic stations with an average inter-station spacing of 10 km are located within Harrat Lunayyir. Seismic stations there have consistent shear-wave splitting directions ranging from N2°E to N20°W and delay times from 0.7 s to 1.6 s. This volcanic field is of particular interest because in 2009 it experienced abundant seismic activity and measureable crustal deformation that was associated with a dike intrusion into the upper crust (Pallister et al., 2010, Nature Geoscience). However, our analysis does not reveal any anomalous splitting results beneath this harrat. Fifteen seismic stations with an average inter-station spacing of 30 km are located in or adjacent to Harrat Rahat. These show very similar splitting directions to Harrat Lunayyir, ranging from N1°W to N16°W, with delay times of 1.0 s to 1.4 s. Following previous studies, we assume that these delay times are dominantly due to mantle anisotropy, with crustal anisotropy being secondary. Our results indicate a highly uniform fast-direction of anisotropy oriented approximately N10°W beneath these two volcanic fields. The measured orientation is inconsistent with the N40°E absolute plate motion of Arabia, but is consistent with the northward asthenospheric flow originating at the Afar hot spot (Camp and Roobol, 1991). We conclude that Harrat Lunayyir and Harrat Rahat are underlain by asthenosphere with a uniform N10°W flow direction.