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.