Introduction: Risk, hazard and mitigation
Definitions:
Hazard refers to determining the frequency and intensity of a process able to cause damages and/or fatalities. It also refers also to processes, natural or man made, causing the damage. Natural hazards include volcanoes, earthquakes, lanslides, avalanches, tsunamis, freak waves, cyclones, tornadoes, hailstorms, storms, floodings, bushfires, droughts, lightings, etc. Our appreciation of the hazard associated with a particular process depends on its natural frequency. For instance, the hazard of processes that occur once every 30,000 years (explosion of mega volcanoes) is less robust than that associated with processes which occurs every year (lithgting). Interestingly, the hazard associated to many natural processes is a function of the intensity. The lower the frequency the stronger the intensity and therefore the potential damage. For instance large earthquake are less frequent than small ones.
Exposure refers to what is in jeopardy: population (how many life can be lost?), infrastructure (transport network, communication network, powerplants, nuclear reactors, dam), or sites of particular significance (national parks, historic sites, oil fields etc). Often exposure is greater in densely populated regions. It happens that densely populated regions are often regions of high risks like coastal regions (cyclones, flooding, earthquakes, volcanoes), region around volcanoes (rich soils), around lakes often related to an active fault.
Risk = Hazard x Exposure x Vulnerability
Vulnerability refers to the susceptibility to the hazard. Vulnerability can be lowered by better construction design, population and key infrastructure re-location. Vulnerability usualy decreases with experience... but not always. In this context, hazard mitigation refers to all the policies and actions put in place to reduce the vulnerability to a particular geohazard.