Seismic hazard describes what level of ground shaking at the earth’s surface is expected due to potential future earthquake activity. It is based on knowledge of past earthquakes, geology, and tectonics, and takes into account diverse factors that may affect the strength of the shaking at any given location. Seismic hazard assessment as such does not quantify the potential threat to human society due to earthquakes – the hazard ‘numbers’ are not affected by who lives or what is built in a particular location. The inclusion of these ‘values’ that may be affected by earthquake shaking, be it human lives or infrastructures, and their vulnerability, then leads to the estimation of seismic risk.

Earthquakes cannot be prevented, and also secondary effects caused by them, such as tsunamis, landslides or rockfalls, avalanches, or liquefaction of soil cannot be prevented. A proper assessment of seismic hazard is the mandatory foundation that allows seismologists, geologists and engineers to develop meaningful guidelines and models allowing a society to develop appropriate measures and means to reduce the impact of earthquakes. Engineers for example use the seismic hazard assessment to derive codes for earthquake-resistant construction of buildings and other infrastructures.

There are two principal approaches of seismic hazard assessment: *deterministic* and *probabilistic* seismic hazard assessment. Both approaches are based on geological and seismological data, but the definitions of seismic hazard are fundamentally different. In the deterministic approach one or a few ‘controlling earthquakes’ are defined, for which the expected ground motion at a particular location is then determined. EFEHR provides models and results of a probabilistic approach, in which all earthquakes that may affect a given location are considered together with various possible models of ground shaking, in a statistical framework.

To assess seismic hazard with a probabilistic approach, scientists try to envision and model when, where and with what magnitude earthquakes will occur and how the ground shaking caused by these earthquakes can be described. This is done based on available datasets and information, scientific knowledge and research results. Therefore, seismic hazard assessments need to be regularly reviewed and updated.

EFEHR provides access to probabilistic estimates of future ground shaking, which take into account various approaches to model future earthquake occurrence and seismic wave propagation, for different regional hazard models.

A probabilistic seismic hazard map therefore expresses the spatial distribution of peak ground acceleration (PGA) levels for a given exceedance probability (e.g. 10%) in a specific time interval (e.g. of 50 years).

This means that the PGA calculated and displayed on the map is exceeded with a 10% chance, but it also implies that it may not be exceeded with a 90% chance during 50 years (this time interval is often selected because it corresponds to the average expected lifetime of a standard residential building).

For comparison with a weather forecast, this would be phrased in the following way: There is a 10% exceedance probability of 10mm rain per m^{2} in the next 24 hours. In other words, there is a 10% chance that more than 10mm rain will fall per m^{2}, but a 90% chance of getting at most 10mm per m^{2} rain in that time period.