From Historical Seismology to seismogenic source models, 20 years on: Excerpts from the Italian experience

Abstract

Large earthquakes occur rather orderly in space and time; hence they can be somehow anticipated, and their effects can be projected into the future. The modern practice of seismic hazard assessment rests on these principles and may rely on them, but also requires a detailed knowledge of the location and characteristics of individual earthquake sources.

We discuss how this knowledge base can be constructed, with an eye on the geological history, which provides a record of the faults capable of generating large earthquakes, and one on the human history, which supplies evidence for whether and how such damaging earthquakes have occurred in the past.

How do these two records interact with each other? It is now accepted that identifying active and potentially seismogenic faults in Italy is especially hard. The geological record may be clear and honest when dealing with processes at the scale of several million years, but is very difficult to decipher if we are concerned with contemporary geological processes, such as the earthquakes.

Shortening the time-window of observation of earthquake activity is why Historical Seismology is so crucial for constructing a seismogenic source model. To this end we exploited a number of key Italian destructive earthquakes, each of which illuminates a recent geological process that may not offer a discernible surface signature. Our findings led us to reconsider the tectonic style of large areas, changed our perception of their earthquake potential, hinted at the existence of unknown seismogenic zones, and even led to downsizing the magnitude of the largest Italian historical earthquakes.

We maintain that the complexity of the geological setting may be counterbalanced by the potential richness of the historical earthquake record. We also believe that our experience in the combined investigation of Italy's historical earthquakes and seismogenic sources may be replicated in all earthquake-prone countries.