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Earthquake

Probable maximum intensity (MM: modified Mercalli scale) with an exceedance probability of 10% in 50 years (equivalent to a „return period“ of 475 years) for medium subsoil conditions.

The earthquake map is graded according to the intensity that is to be expected once in a period of 475 years.

Intensity integrates a number of parameters such as ground acceleration and earthquake duration. The return period of 475 years corresponds to a 10% exceedance probability in 50 years, which repre- sents the mean service life of modern buildings. The intensity is expressed in terms of the modified Mercalli scale (MM). The earthquake map is based on an assemblage of existing hazard maps of individual countries. The source maps show:

• The minimum intensity or peak acceleration to be expected for an exceedance probability of 10% in 50 years
• The same parameters but for a different reference period
• The maximum intensity observed
• Active or potentially active faults
• Epicentres of earthquakes recorded by instruments and/or historical earthquakes

Merging such heterogeneous sources presents enormous problems, beginning with the process of converting acceleration values into macroseismic intensity, for which various formulas have been proposed (e.g. Trifunac and Brady 1975, Murphy and O’Brien 1977).

Global Earthquake Model (GEM)

The Global Earthquake Model (GEM) Global Seismic Hazard Map (version 2018.1) depicts the geographic distribution of the Peak Ground Acceleration (PGA) with a 10% probability of being exceeded in 50 years, computed for reference rock conditions (shear wave velocity, VS30, of 760–800 m/s).

The map was created by collected maps computed using national and

regional probabilistic seismic hazard models developed by various institu- tions and projects, and by GEM Foundation scientists. The OpenQuake

engine, an open-source seismic hazard and risk calculation software devel- oped principally by the GEM Foundation, was used to calculate the hazard

values. A smoothing methodology was applied to homogenize hazard values along the model borders. The map is based on a database of hazard models described using the

OpenQuake engine data format (NRML); those models originally imple- mented in other software formats were converted into NRML. While trans- lating these models, various checks were performed to test the compatibil- ity between the original results and the new results computed using the

OpenQuake engine. Overall the differences between the original and

translated model results are small, notwithstanding some diversity in mod- elling methodologies implemented in different hazard modelling software.

The hashed areas in the map (e.g. Greenland) are currently not covered by a hazard model. The map and the underlying database of models are a dynamic framework, capable of incorporating newly released open models. Due to possible model limitations, regions portrayed with low hazard may still experience potentially damaging earthquakes.