PGA
Peak Ground Acceleration
The maximum ground acceleration expected during an earthquake, expressed as a fraction of the acceleration due to gravity (g).
PGA measures how strongly the ground can shake. It is expressed in g, the acceleration due to gravity (about 9.81 metres per second squared): a value of 0.15 g is about 1.5 metres per second squared.
INGV's Italian seismic hazard map MPS04 gives, for a grid of points, the PGA that has a 10% probability of being exceeded in 50 years, which means an event with an average return period of about 475 years, on rock or firm ground.
On soft ground the shaking can be amplified. PGA thresholds separate the seismic zones: above 0.25 g is zone 1, between 0.15 and 0.25 g zone 2, between 0.05 and 0.15 g zone 3. In the report the PGA is calculated at the address point, with the model's uncertainty range.
Official source: doi.org
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