Equivalent Hazard Magnitude Scale

Abstract. Hazard magnitude scales are widely adopted to facilitate communication regarding hazard events and the corresponding decision making for emergency management. A hazard magnitude scale measures the strength of a hazard event considering the natural forcing phenomena and the severity of the event with respect to average entities at risk. However, existing hazard magnitude scales cannot be easily adapted for comparative analysis across different hazard types. Here, we propose an equivalent hazard magnitude scale, called the Gardoni Scale after Professor Paolo Gardoni, to measure hazard strength across multiple types of hazards. Using global historical records of hazard magnitude indicators and impacts of events of 12 hazard types from 1900 to 2020, we demonstrate that an equivalent hazard magnitude on the Gardoni Scale can be derived as correlated with the expectation of an impact metric of hazard event. In this study, we model the impact metric as a function of fatalities, total affected population, and total economic damage. Our results show that hazard magnitudes of events can be evaluated and compared across hazard types. For example, we find that tsunami and drought events tend to have large hazard magnitudes, while tornadoes are relatively small in terms of hazard magnitude. In addition, we demonstrate that the scale can be used to evaluate hazard equivalency of historical events. For example, we show that the hazard magnitude of the February 2021 North American cold wave event affecting the southern states of the United States of America was equivalent to the hazard magnitude of Hurricane Harvey in 2017 or a magnitude 7.5 earthquake. Future work will expand the current study in hazard equivalency to modelling of local intensities of hazard events and hazard conditions within a multi-hazard context.