Local seismic response and NaTech events: new perspectives of analysis

The interaction between natural hazards and industrial accidents can lead to major fires, explosions or toxic releases, identifying a NaTech (Natural-Technological) event. Among natural hazards, an earthquake simultaneously impacts the entire industrial plant, and can cause the collapse of structures as a direct consequence of the ground shaking severity. This study aims to introduce a novel approach for assessing the local seismic response in a Major Hazard Industrial Plant (MHIP) that could lead to a NaTech event. MHIPs are subject to the Italian Code 105/2015 (Directive 2012/18/EC - Seveso III) which requires a multidisciplinary approach, given the high complexity of the problem and the numerous types of involved equipment. Our research bridges the gap between seismic risk assessment and industrial safety protocols by systematically assessing the equipment stressed by different seismic conditions. Assessing the potential seismic amplification phenomena in MHIP plants involves reconstructing the geology of the site. For this reason, a 3D geological reference model needs to be implemented, and it has to be supported by a geo-database based on the collected boreholes of the site. Each identified geological unit is associated with geotechnical parameters (Vs, G/G0, D) necessary to simulate seismic hazard scenarios. For some sites, analyzing the data related to seismic hazard disaggregation, it is possible to highlight a dual contribution to the hazard related to different Magnitude - Distance ranges. In these cases, it is necessary to analyze both contributions separately in order to evaluate the different results of local seismic responses obtained using seismic input derived from different Magnitude - Distance ranges. In this study, we simulated two local seismic response scenarios taking into account these clusters Magnitude - Distance associated respectively with severe and moderate earthquakes and assuming the same geological and topographical conditions. It is important to emphasize and divide the analysis for different seismic hazard scenarios regarding the type of plants and all associated risks. These analyses can simulate different degrees of damage to various pieces of equipment, characterized by their natural vibration period. Therefore, for each seismic simulation, we will derive the corresponding NaTech scenario, potentially leading to the loss of containment of hazardous substances. Through comprehensive seismic scenario-based assessments, we attempt to highlight the different effects on the industrial facilities, thereby informing appropriate strategies and risk management protocols. In addition, the impact of the NaTech event on ordinary and special maintenance on the pieces of equipment and components can be assessed using this methodology.