Incipient status of dyke intrusion in top crust - evidences from the Al-Ays 2009 earthquake swarm, Harrat Lunayyir, SW Saudi Arabia

Abstract

The 2009 earthquake-swarm in the Al-Ays volcanic zone in Harrat-Lunayyir in NW Saudi-Arabia is unique because of its intense character and focal-depth distribution at two depth bands (5-10 and 15-20 km) in upper crust without volcanic eruption. We investigate an anatomy of the dyke-intrusion model that supports the mechanism for the swarm itself with seismotectonics, pore pressure diffusion process and inference model. Inferred dyke-intrusion initially started at depth had a five-day peak period (15-20 May 2009) since inception of event-recordings, following which the activity diminished. The process of pore pressure perturbation and resultant "r-t plot" with modelled diffusivity (D = 0.01) relates the diffusion of pore pressure to seismic sequence in a fractured poro-elastic fluid-saturated medium. The spatio-temporal b-values show high b-values (&1.3) along the zone of dyke intrusion (length 10 km and height 5 km) at ~20km depth. The main-shock and other prominent earthquakes originated on a moderate b-value zone (~1.0). Temporal b-value analysis indicates an exceptionally low b-value (~0.4) during the main-shock occurrence. The Al-Ays lava-field is inferred to underlie a seismic volume trending NW-SE bounded on both sides by two NW-SE trending fault systems, dipping 40-50° opposite to each other within a proposed nascent rift setting. © 2013 Copyright Taylor and Francis Group, LLC.