Temps de lecture : 5 minutes

Seismic risk

On a scale of 5 seismicity zones, from very weak to strong, the entire commune of Montpellier is classified in seismicity zone 2 (weak).

What is an earthquake?

An earthquake is caused by a sudden rupture of rock along a fault plane. This rupture generates seismic waves. The passage of the waves through the ground causes vibrations that can be felt at the surface. At the surface, an earthquake can damage or destroy buildings, producing shifts in the ground surface on either side of faults. The duration and frequency of the vibrations transmitted to buildings determine the extent of the damage.

An earthquake is characterized by:

  • Focus: the region of the fault where the rupture occurs and the seismic waves originate. It can be located anywhere from a few kilometers to several tens of kilometers from the place where the earth shook
  • Its epicenter: point located vertically above the focus. The epicenter is not necessarily the site of the greatest damage, as ground movement can vary according to topography and the constitution of the subsoil (site effects)
  • His magnitude: expresses the energy released by the earthquake. It is generally measured by the open Richter scale
  • Its intensity: measures the effects and damage of the earthquake in a given location. It is not a direct measurement, but an assessment of how the earthquake translates on the surface. We usually use the MSK scale, which has twelve degrees
  • The frequency and duration of vibrations: these two parameters play an important role in the effects on the surface
    The fault caused: can propagate on the surface

Consequences: houses, buildings and networks (roads, electricity, water, ...) can be destroyed; many people can be left homeless; landslides can be generated...

Adopt the right reflexes

Before

  • Get informed about the risks, know the instructions and the alert signal
  • Prepare the minimum equipment (battery-operated radio, drinking water supply, personal papers, medicines, ...)
  • Fix heavy appliances and furniture

During

Inside:

  • During the tremor, take shelter under a solid piece of furniture or in the corner of two load-bearing walls
  • Get away from windows
  • Evacuate buildings by the stairs and don't go back in. Do not use the elevator

Outdoors:

  • Get away from buildings, pylons, trees...
  • Beware of falling objects
  • Stop and don't get out of your vehicle until the shaking has stopped
  • Move to a secluded spot or gathering place

After

  • Beware of aftershocks, there may be further tremors
  • Respect the authorities' instructions
  • Listen to France Bleu Hérault radio (100.6)
  • Don't phone, clear lines for rescue services
  • Don't touch downed electrical wires
  • Don't smoke. No flames or sparks
  • Close off gas, electricity and water
  • Assess damage and hazards
  • Make an inventory of your damage and prepare your insurance files

Seismic risk in Montpellier

Decree no. 2010-1255 of October 22, 2010 redefines the seismic zoning of the French territory, taking into account improved seismological knowledge. The zoning is based on a probabilistic, rather than deterministic, assessment of the seismic hazard, in line with the recommendations of European standards Eurocode 8.

On a scale of 5 seismicity zones, from very low to high, the entire commune of Montpellier is classified in seismicity zone 2 (low). This zoning is not just a seismic hazard map, it also meets an objective of earthquake protection within economic limits that are bearable for the community.

Preventive and protective measures

Forecasting

In the absence of short-term forecasting (there is no reliable way of predicting where, when and how powerful an earthquake will be), earthquake forecasting is based on probabilism and statistics. It is based on the study of past events, from which the probability of occurrence of a given phenomenon is calculated (probabilistic method). In other words, the past is the key to the future! In this respect, so-called "probabilistic" hazard maps, based on the return periods of given events, are indicators of the potential occurrence of earthquakes over time.

Monitoring

Real-time seismicity monitoring and warning (locating the focus, determining the magnitude and possibly warning) is provided by the Réseau national de surveillance sismique (RéNaSS).

Prevention

The requirements of this European regulation, Eurocode 8, vary according to seismicity zone and building importance category. The objectives of earthquake-resistant construction are:

  • Safeguarding human life. In the event of a "nominal" tremor, the building may suffer irreparable damage, but must not collapse on its occupants.
  • Limiting damage to infrastructure and therefore economic impact. What's more, the start-up stakes for event management (fire stations, hospitals, etc.) must remain operational.

This is why so-called "normal risk" buildings are classified into four categories of importance according to the stakes to be protected:

  • category 1: includes buildings in which there is no human activity requiring a long-term stay.
  • category 2: individual dwellings...
  • category 3: educational establishments...
  • category 4: strategic structures essential to crisis management (fire stations, hospitals, etc.).

In Montpellier, located at low seismicity risk, construction rules apply only to new or existing buildings in importance categories 3 and 4.