A first hazard analysis of the Quaternary Harrat Al-Madinah volcanic field, Saudi Arabia

The northern portion of the 20,000 km2 Harrat Rahat basaltic field in NW Saudi Arabia (Harrat Al-Madinah) has hosted some of the most recent volcanic eruptions in the country. Rapid growth of the city of Al-Madinah has spread out onto the lava flows and scoria cones of the Harrat, increasing exposure to any potential renewed volcanism. We present here a first-order probabilistic hazard analysis related to new vent formation and subsequent lava flow from this volcanic field. The 501 visible eruption vent sites were integrated with aeromagnetic survey data (as representative of potential regions of buried volcanic vents) to develop a probability density function for new eruption sites using Gaussian kernel smoothing. This revealed a NNW striking zone of high spatial hazard terminating < 10 km south of the city. Using the properties of the AD1256 eruption lava flows and the spatial PDF, an analysis of lava hazard was carried out. Assuming a future lava-producing eruption, around 25% of the city of Al-Madinah is exposed to a probability of 0.001 to 0.005 of lava inundation. The temporal eruption recurrence rate is estimated at approximately one vent per 3300 years, but the temporal record of the field is so poorly constrained that the lower and upper bounds for the recurrence interval are 13,300 yrs and 70 yrs, respectively. Applying a Poisson temporal model, this results in a worst-case lava inundation recurrence interval of approximately 14,300 years.