Starting at the top event or system failure (the Hurricane Katrina disaster), the fault tree was built downwards from the
undesired event to intermediate events (rectangular boxes) and then to basic events using logical AND/OR gates and deductivelogic, i.e. repeatedly asking ‘what are the reasons for this event?’ The basic events of the fault tree in this case refer to the component failures and human errors in the circular/elliptical shapes at the bottom of the diagram.