A new perspective of the 2009 Al-Ays earthquake episode at western Arabia

Abstract

The 2009 earthquake swarm that occurred in Al-Ays area, Saudi Arabia was believed to be of magmatic origin and was caused by magma rising underneath the area and installment of a subsurface dyke that reached up to 3.5 km below the ground surface. This study uses the results of an integrated geophysical survey conducted across the major surface rupture, which appeared in the area after the earthquake swarm, along with other evidence, to suggest a different scenario. Contradicting focal mechanisms, temporal and spatial distribution of earthquakes, mixed frequency earthquakes, a previous earthquake swarm, recent similar swarms along the Red Sea coast, and the alignment of earthquakes along a major northwest-southeast trending fault are among the evidence presented to refute the one-decade old subsurface dyke intrusion model. Last but most important, the geophysical data we acquired across the surface rupture in the area show clear evidence of shallow negative flower structure in the ruptured zone, implicating the occurrence of a transtensional tectonic movement, that may have caused the rupture and triggered the associated earthquake swarm. The evidence presented in this article prove that the 2009 earthquakes episode in the Al-Ays area was triggered by regional tectonics readjustment in the area, rather than magma upwelling and subsurface dyke intrusion, that if present, could be a secondary effect of the tectonic movement.