About Seismic Hazard

How Do We Estimate Seismic Hazard?

There are two principal approaches of seismic hazard assessment, which may be referred to as deterministic and probabilistic seismic hazard assessment. Both approaches are based on geological and seismological data, but the definitions of seismic hazard are fundamentally different. We use a probabilistic approach to cover all possible earthquakes and assess the hazard accordingly.

Probabilistic Seismic Hazard Assessment

To assess the seismic hazard with a probabilistic approach, scientists envision when, where and with which magnitude earthquakes will occur and how the excited seismic wave amplitudes that cause the ground shaking can be described. This is done based on the best available knowledge and latest research results. Therefore, these maps need to be reviewed and regularly updated.

As there are various ways to envision future earthquake occurrence and seismic wave propagation, we are using statistical methods to express future ground shaking in terms of probabilities. A probabilistic seismic hazard map therefore often expresses a 10% exceedance probability of a peak ground acceleration (PGA) in 50 years. This means that the PGA calculated and plotted on a map is exceeded with a 10% chance, but it also implies that it may not be exceeded with a 90% chance in 50y.

For comparison with a weather forecast, this would be phrased in the following way: There is a 10% exceedance probability of 10mm/m2 rain in the next 24h. In this case it means that there is a 10% chance that more than 10mm/m2 rain falls, but a 90% chance that less rain will fall in the next 24h.

What is Seismic Hazard? (Generic Overview)

Seismic hazard is the potential threat to human society that is caused due to earthquakes. Earthquakes cannot be prevented, neither is it possible to prevent human society from secondary effects such as tsunamis, landslides, liquefaction, rockfalls, and / or avalanches cause by the shaking of the ground due to an earthquake. Seismologists, geologists, and engineers therefore work together to assess the potential seismic hazard and develop appropriate measures and means, so that e.g. buildings do not collapse during an earthquake. In a very general way, seismic hazard assessment attempts to assess the level of ground shaking that earthquakes might generate for a time period in the future.

The results of seismic hazard assessments are further used by engineers to appropriately design any type of building, thus the results are widely used and an important information.