A multidisciplinary workflow to assess seismic hazard by active and capable faults when planning railway lines

The planning phase of a railway line has to carefully consider the potential impact of several geohazards, including the seismic hazard associated with active faults capable to cause significant offset (dm to m) of the ground surface. Italian authorities are investing large resources in the construction of new railway lines in Italy, which is a territory that stands out as high-risk due to the presence of active faults, including those accommodating extension within the Apennines belt. Numerous seismogenic sources have been recognized therein by the geophysical and geological community over the last few years. Their identification and characterization represent the foundation of the seismic hazard map of Italy, which is regularly used to assess the seismic hazard of any given area of the country. Active and Capable Faults (ACFs) may contribute to increasing the seismic hazard of an area, though, and may interfere with railway lines. Following the Italian guidelines for the seismic microzonation procedures, an ACF is capable of producing, within a time interval of concern the society, macro-earthquakes and deformation/displacement at or near the ground surface and should have done so during the last 40 ka (upper Late Pleistocene – Holocene).

We propose a multidisciplinary workflow for improving and standardizing the use of the existing Italian geohazard databases. As the area covered by a railway line may extend for tens or hundreds of kilometers, it is crucial to define systematic criteria that make it possible for the intersected ACFs to be sorted into classes of varying hazard, each requiring different approaches and study levels. The seismicity associated with the ACF’s, the fault geometric and kinematic compatibility with the current regional tectonic setting and stress field, the involvement of < 40 ka old rocks and sediments, the proximity to the ground surface of the historical hypocenters, the geometrical relationships between the ACF’s and the orientation of the railway line are just a few of the aspects to be considered by such an approach.

We aim to define and constrain all the input parameters necessary to perform site-specific fault displacement and seismic hazard analysis, since the currently available Italian seismic hazard map is still too coarse in its resolution (with PGA values every 10 km). Our new approach will allow us to include the detection of near-field effects (e.g., the increasing of the vertical component due to seismic acceleration, forward-directivity phenomena, co-seismic rotation) if the trailway line runs within a 15 km zone from the main fault plane.

Knowing ACFs behavior will make it possible, during the planning, to choose the best railroad options, in order to reduce the vulnerability of the planned infrastructure.