Comparison between single-event effects and cumulative effects for the purpose of seismic hazard assessment. A review from Greece

When compiling a database of active and capable faults, or more in general when collecting data for Seismic Hazard Assessment (SHA) purposes, the exploitation of the numerous and different sources of information represents a crucial issue. Also the understanding of their potential and limitations is essential. For example, using only information deriving from historically and/or instrumentally recorded earthquakes, as it has been commonly applied in the past, it is not sufficient and it could be, sometimes, even misleading in terms of SHA. In the present paper, the importance of using geological information for better defining the principal seismotectonic parameters of a seismogenic source is discussed and emphasized. In order to show this, four case studies of active faults recently reactivated by strong earthquakes have been selected from the Greek Database of Seismogenic Sources (GreDaSS). Each seismogenic source is analyzed twice and separately for the two sources of information: firstly, on the basis of the single-event effects as mainly provided by historically or instrumentally recorded data, and secondly, on the basis of the cumulative effects consisting of any, mainly geological, evidence caused by multiple and repeated fault reactivations of the specific seismogenic source. The quality and accuracy of the produced results from both sources of information are then discussed in order to define the reliability of the outcomes and especially for calibrating the methodological approaches based on geological data, which have an intrinsically different degree of uncertainty and resolution, but also a greater potential exploitability. As a matter of fact, an improved geological, in its broader sense, knowledge will help to fill in the gap of the geodetically and/or seismologically determined tectonic activity of hazardous regions. Moreover, including in a catalogue also the seismogenic sources that are not associated with historical and/or instrumental earthquakes will have a remarkable impact in future SHA analyses either probabilistic and deterministic ones.