Seismic hazard assessment for Harrat Lunayyir - A lava field in western Saudi Arabia

Distribution of earthquakes along the western part of the Arabian Peninsula is related to the Red Sea floor spreading and volcanism. The Arabian Peninsula contains Cenozoic lava fields (harrats) extending from Yemen in the south up to Syria in the north. Harrat Lunayyir that is a very young volcanic area has suffered from two earthquake swarms with maximum magnitude MW 5.7 caused by the dike intrusion. The area was not considered as a seismic source zone when constructing seismic hazard maps used in the current issue of Saudi Building Code (SBC-2007). In this work, probabilistic seismic hazard is estimated for the Harrat Lunayyir area and the town of Alays located a few kilometers to south-east of the Harrat. Two models of seismic source zones describing seismic process in Harrat Lunayyir are considered as alternatives in the logic tree scheme, namely: the areal source model and the fault (characteristic earthquake) source model. The results of probabilistic estimations of seismic hazard show that the area is characterized by significant level of hazard. The expected peak ground amplitudes at rock site and for return period 2475 yrs are larger than 200 cm/s2 and 140 cm/s2 for the central part of the area and the town of Alays respectively. The level of uncertainty quantified in terms of “relative uncertainty” is the largest in the central part of the area.