The experience of several failures of equipment in industrial plants due to earthquake has shown that incidental scenarios like explosion, fire or hazardous release can occur. In Italy, industrial plants that operate with hazardous substances are subject to Italian standard D.Lgs. 105/2015, that requires to carry out a risk analysis, also considering the seismic risk. Earthquake is one of the natural events which is part of NaTech events (Natural Hazard Triggering Technological Disasters). The aim of this paper is to discuss a critical aspect about methodology for assessment of the earthquake NaTech risk in major-hazard industrial plants. Particularly, to evaluate the seismic vulnerability of equipment, both data from a structural safety verification and fragility curves can be used. At the end of the seismic vulnerability analysis, it should be identified the possible incidental scenarios involving one or more industrial components and, for each scenario, to evaluate the frequency of occurrence. The attention is focused on a typical industrial equipment, a cylindrical liquid storage tanks with floating roof. Not only failure of structural elements can cause hazardous release, but also the non-structural elements must be considered. Particularly, the presence of floating roof and the seal rim is often neglected in seismic analysis and, consequently, in risk analysis.