Imaging of magma intrusions beneath Harrat Al-Madinah in Saudi Arabia Mohamed F. Abdelwahed a,e,⇑ , Nabil El-Masry a,f , Mohamed Rashad Moufti a , Catherine Lewis Kenedi b , Dapeng Zhao c , Hani Zahran d , Jamal Shawali d a Geohazards Research Center, King Abdulaziz University, Saudi Arabia b School of Environment, University of Auckland, Auckland, New Zealand c Department of Geophysics, Tohoku University, Sendai, Japan d Saudi Geological Survey (SGS), Saudi Arabia e National Research Institute of Astronomy and Geophysics, NRIAG, Egypt f Geology Department, Faculty of Science, Suez Canal University, Ismailia 41522, Egypt article info Article history: Received 10 December 2015 Received in revised form 24 January 2016 Accepted 26 January 2016 Available online 27 January 2016 Keywords: Seismic tomography Magma intrusions Harrat Rahat Al-Madinah Saudi Arabia abstract High-resolution tomographic images of the crust and upper mantle beneath Harrat Al-Madinah, Saudi Arabia, are obtained by inverting high-quality arrival-time data of local earthquakes and teleseismic events recorded by newly installed borehole seismic stations to investigate the AD 1256 volcanic erup- tion and the 1999 seismic swarm in the study region. Our tomographic images show the existence of strong heterogeneities marked with low-velocity zones extending beneath the AD 1256 volcanic center and the 1999 seismic swarm area. The low-velocity zone coinciding with the hypocenters of the 1999 seismic swarm suggests the presence of a shallow magma reservoir that is apparently originated from a deeper source (60–100 km depths) and is possibly connected with another reservoir located further north underneath the NNW-aligned scoria cones of the AD 1256 eruption. We suggest that the 1999 seis- mic swarm may represent an aborted volcanic eruption and that the magmatism along the western mar- gin of Arabia is largely attributed to the uplifting and thinning of its lithosphere by the Red Sea rifting. Ó 2016 Elsevier Ltd. All rights reserved. 1. Introduction Large areas on the western margin of the Arabian plate are cov- ered with intraplate basaltic volcanic fields that are commonly related to the upwelling of the Afar mantle plume (30 Ma) and the consequent formation of the East African, Gulf of Aden, and Red Sea rift systems (Coleman et al., 1983; Bosworth et al., 2005; Pallister et al., 2010; Chang et al., 2011). Thirteen of these volcanic fields, known locally as the harrat, are located in western Saudi Arabia (Coleman et al., 1983). One of these volcanic fields is Harrat Rahat. Its importance stems from the fact that its northern extrem- ities extend to the south-eastern and eastern suburbs of the holy city of Al-Madinah Al-Munawarah. Harrat Rahat forms a 50–75 km-wide, 300 km-long, N20–25°W- trending plateau (Durozoy, 1970; Berthier et al., 1981; Coleman et al., 1983). It comprises four coalesced smaller volcanic fields attaining an approximate area of 19,830 km 2 and an estimated vol- ume of 1999 km 3 (Camp and Roobol, 1989). In previous studies, the northern part of Harrat Rahat was referred to as Harrat Al-Madinah (Coleman et al., 1983; Moufti, 1985; Moufti et al., 2012). In this study, Harrat Al-Madinah is used to describe the volcanic terrains of Harrat Rahat located north of latitude 24°N. It is mainly com- posed of olivine basalts and hawaiites with minor silicic differenti- ates of mugearites, benmoreites, and trachytes (Moufti et al., 2012). The northern part of Harrat Al-Madinah (NHM), however, is covered mainly with basaltic scoria cones, tephra fall deposits, and lava flows (Camp and Roobol, 1989). On the contrary, the southern part of Harrat Al-Madinah (SHM) is uniquely marked with the development of trachytic lava domes, tuff rings, explosion craters, and pyroclastic deposits (Moufti and Németh, 2013). The most recent volcanic eruption took place in NHM in AD 1256, when an eruption lasted for 52 days, extruded 0.5 km 3 of alkali-olivine basalt from a 2.25 km-long fissure and produced at least 7 scoria cones and a 23 km-long lava flow that came to within 8 km from Al-Madinah (Camp et al., 1987; Camp and Roobol, 1989; Ambraseys et al., 1994; El-Masry et al., 2013; Murcia et al., 2014b). In the SHM, an earthquake swarm of 500 events occurred at the http://dx.doi.org/10.1016/j.jseaes.2016.01.023 1367-9120/Ó 2016 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: Geohazards Research Center, King Abdulaziz University, Saudi Arabia. E-mail addresses: mfibrahim@kau.edu.sa, mfarouk40@yahoo.com (M.F. Abdelwahed). Journal of Asian Earth Sciences 120 (2016) 17–28 Contents lists available at ScienceDirect Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes