Seismogenic nodes as a viable alternative to seismogenic zones and observed seismicity for the definition of seismic hazard at regional scale

TL;DR

This paper compares traditional seismogenic zone hazard assessment with a novel seismogenic node approach in Italy, introducing a safety factor to align hazard maps and validate the new method.

Contribution

It proposes using seismogenic nodes with a safety factor as an alternative to seismogenic zones for regional seismic hazard assessment, validated through hazard map comparison in Italy.

Findings

Seismogenic nodes method mostly agrees with traditional hazard maps.

A safety factor of about 2 effectively calibrates the seismogenic node approach.

Discrepancies observed in Central Alps and southeastern Sicily.

Abstract

It is shown that considering a fixed increment of a given magnitude at a fault is equivalent to factoring the mechanical moment at the fault as done in structural engineering with the applied loads, by the most currently used structural engineering standards (e.g. Eurocodes). A special safety factor is introduced and related to the partial factor acting on the mechanical moment representing the fault. A comparison is then made between the hazard maps obtained for Italy with the Neo Deterministic Seismic Hazard Assessment (NDSHA) technique, using two approaches for the definition of the seismic sources considered for the computation of the synthetic seismograms. The first one is based on the magnitude of the events listed in the earthquake catalogue, and located within the active seismogenic zones. This is the standard approach, used in most of the NDSHA computations performed up to now. It is adequate for countries, like Italy, where the catalogue can be reasonably considered complete for events of M=5 and above. When the catalogue completeness is barely adequate, another approach can be adopted for the definition of the earthquake sources. It uses the seismogenic nodes identified by means of pattern recognition techniques applied to morphostructural seismic zonation (MSZ), and increases the reference magnitude by a constant variation tuned thanks to the safety factor. The two approaches have been compared for Italy using the safety factor 2.0: they mostly produce comparable hazard maps. As the two sets are fully independent and the catalogue is very long, this implies a validation of the seismogenic nodes method and a tuning of the safety factor at about 2. Notable exception is seen in the Central Alps, where nodes tend to overestimate the "observed" hazard, and in southeastern Sicily, where the nodes underestimation is only apparent and can be negligible within experimental errors.