Abstract
Distributed "monogenetic" volcanic eruptions commonly occur in continental settings without obvious structural alignments or rifting/extensional structures. Nevertheless, these may develop as fissures, representing the surface expression of dykes with a range of orientations, especially when stress regimes vary over time and/or older crustal features and faults are exploited by rising magmas. Dykes reaching the surface as fissures can last hours to months and produce groups of closely aligned vents, hiding the true extent of the source fissure. Grouped or aligned vents in a distributed volcanic environment add complexity to hazard modelling where the majority of eruptions are single-vent, point-source features, represented by cones, craters or domes; i.e. vent groups may represent fissure events, or single eruptions coincidently located but erupted hundreds to tens of thousands of years apart. It is common practice in hazard estimation for intraplate monogenetic volcanism to assume that a single eruption cone or crater represents an individual eruptive event, but this could lead to a significant overestimate of temporal recurrence rates if multiple-site and fissure eruptions were common. For accurate recurrence rate estimates and hazard-event scenarios, a fissure eruption, with its multiple cones, must be considered as a single multi-dimensional eruptive event alongside the single-vent eruptions. We present a statistical method to objectively determine eruptive events from visible vents, and illustrate this using the 968 vents of the 10 Ma to 0.6 ka volcanic field of Harrat Rahat, Saudi Arabia. A further method is presented to estimate the number of hidden vents in a thick volcanic pile. By combining these two methods for Harrat Rahat, we determined an updated spatial recurrence rate estimate, and an average temporal recurrence rate of 7.5x10(-5) events/year. This new analysis highlights more concentrated regions of higher temporal hazard in parts of Harrat Rahat, which has significant implications for the city of Al-Madinah and surroundings.