Seismic microzonation

The seismic zone applies to the whole municipality and the PGA is calculated on rock. Above the rock, though, there is almost always something else: sands, clays, made ground, slopes. Seismic microzonation measures exactly this, and it can give different answers for two houses a few hundred metres apart.

Under the national guidelines approved in 2008 by the Civil Protection Department and the Regions, the study divides the territory into three kinds of zone: stable, stable but liable to amplify shaking, and unstable (landslides, ground rupture along a fault, soil liquefaction). It is done in levels. Level 1 gathers existing data and maps zones that are uniform by type of ground, without figures. Levels 2 and 3 add field surveys and give the amplification factor: how many times the ground at that point multiplies the shaking compared with rock. In Rimini, for example, the studies give values between 1.25 and 1.55.

The studies are collected by the Civil Protection Department and published by the CNR (National Research Council) on the WebMS portal. Many municipalities do not have one yet. In the report the block "How the ground under the house shakes" gives the factor, the level of the study and any instability recorded on the plot and nearby; where there is no study it says so, because not knowing does not mean the ground is good. Method in Earthquakes; what to check on the building in Seismic zone of a house.