Subsurface imaging of the Harrat Lunayyir 2007–2009 earthquake swarm zone, western Saudi Arabia, using potential field methods

The Harrat Lunayyir (HL) 2007–2009 earthquake swarm has attracted much scientific interest mainly due to its occurrence within the Large Igneous Province (LIP) of Saudi Arabia. Qualitative interpretation of aeromagnetic anomalies is suggestive of linear dike-like anomalies, striking NW-SE, across the earthquake swarm zone (ESZ). Residual Bouguer anomaly data for HL range between −4.3 and 1.5 mgal and show a 20 km-wide gravity low flanked by lateral gravity highs. The axial-zone of low gravity corresponds to the ESZ cluster. Employing a three-dimensional (3D) inversion of the residual gravity field with a one-dimensional (1D) P-wave velocity model as a proxy for background density variations with depth, we imaged a mid-to-upper crustal low density zone (>10 km depth). We interpret this zone as the source area of ascending magma that intruded into the upper crust underlying the HL during the 2009 seismic crisis. The inversion also highlights lower than average density rocks beneath the western parts of the HL whereas its eastern part is composed of rocks of higher density. The 3D magnetic inversions lead us to infer magmatic intrusion of lower magnetic susceptibility at this locality extending vertically from 1.57 km altitude (above sea level) to its bottom edge at 10.8 km depth (below sea level). Based on these results, the 2007–2009 ESZ is likely associated with rift-related extension associated with the Red Sea spreading, where a trans-crustal magmatic plumbing system extends to depths of about 10 km beneath the centre of HL.