The four components of seismic risk reduction

To protect against the effects associated with earthquakes, we can consider four main components: forecasting, early warning, warning and prevention.

**The forecast

This corresponds to any estimate of an earthquake event, i.e. location, date and magnitude. Earthquake prediction is not possible at this time.

Scientists are convinced that phenomena must occur before an earthquake but that we are not in the current capacity to see and/or understand them. Several attempts have been made in the past but have not yet been confirmed for operational implementation:

• The method V.A.N : According to the names of the three Greek people proposing this detection of electrical signals in the ground before the earthquake. This method was proposed in 1981 following the earthquakes of 24-25 February and 4 March 1980. After thirty years of investigation and verification, this method is not validated.
  However, the study of electrical and magnetic signals before and during an earthquake should be supported because the migration of fluids during the mechanical phenomenon at play in contact with the two opposing plates could generate signals to be observed but we would have to get out of the natural ambient noise on Earth. It is thought that animal reactions to an approaching earthquake could be associated with these electro-magnetic variations, but there is no confirmation to support this hypothesis at this time.

• Geochemical anomalies : Changes in fluid exchange within the crust before and during an earthquake (excess chlorine, etc.) could be detected, but these changes are highly dependent on fluid flows and remain difficult to detect for the moment.

• Other physico-chemical phenomena not yet identified

**Early warning

This is the dissemination of information that an earthquake has occurred but before the impact of the waves where the information is disseminated. This diffusion allows for the safety of goods and people. The reaction time is variable and can range from a few minutes to several hours.

This new approach is associated with our ability to observe and transmit information via radio waves in near-real time. If we can prevent a danger in an area from a source at a certain distance, then we can take steps to secure property and people. Experimental devices have been implemented with response times reduced by a few minutes and some others are in operation, such as early warning in Mexico City or for TGV trains in southern France.

**The alert

This is the dissemination of all relevant information for individual and collective relief efforts after an earthquake and its effects. This dissemination is mainly directed at the emergency response systems but it can also be useful for the security of property and people following on personal initiatives.

Since we have been able to detect, locate and estimate the magnitude of earthquakes through the recording of seismic waves and a reliable clock in the last quarter of the last century, it is possible to provide information to the authorities but also to the population for any action after the disaster. This is why it is important to have a battery-powered radio to listen to the information broadcast. It is also fundamental that the population be educated and trained: this education is even more important because the initial reactions are critical.

The challenges are to respond as quickly as possible with reaction times of less than 15 minutes and with an update of the information over time.

**The prevention

This corresponds to any prior and essentially long-term provision that reduces our vulnerability to earthquakes. This corresponds to land use planning.

It is an essential and permanent action, i.e. any provision, regulatory or otherwise, that reduces physical vulnerability. Thus, buildings must resist vibrations by oscillating to dissipate seismic energy without reaching the failure threshold. Sensitive constructions such as roads, hydroelectric power plants and nuclear power plants must take specific measures to resist the effects of seismic vibrations.

The integration of combined/cumulated risks in an area is essential to ensure effective prevention.