74. Carafa, M. M. C., G. Valensise, and P. Bird [2017]: Assessing the seismic coupling of shallow continental faults and its impact on seismic hazard estimates: a case-study from Italy, Geophys. J. Int., 209, 32-47, doi: 10.1093/gji/ggx002.

Abstract. We propose an objective and reproducible algorithmic path to forecast seismicity in Italy from long-term deformation models. These models are appropriate for Italy and its neighboring countries and seas thanks to the availability of rich, reliable and regularly updated historical earthquake and seismogenic fault databases, and to the density of permanent GPS stations.  However, so far little has been done to assess the seismic coupling of Italian active faults, that is, to quantify their ability to release earthquakes. This must be determined in order to use geodetic and active faulting observations in alternative seismicity models, to overcome possible limitations of the earthquake record for the assessment of seismic hazard. We use a probabilistic method to assign upper crustal earthquakes from the historical catalogue to their presumed causative faults, then collect all the events into three subcatalogues corresponding to the compressional, extensional and strike-slip faulting classes. We then determine the parameters of their Gutenberg–Richter frequency/magnitude relations using maximum-likelihood methods and integrate these distributions to estimate the long-term seismic moment rate for each class.  Finally, we compare these seismicity rates to the long-term tectonic deformation based on GPS data, thus determining the coupled thickness (and estimating seismic coupling) for each fault class.  We find that in our study region the seismic coupling and the related coupled thickness is on average two times larger for extensional than for compressional faults.  As for the spatial distribution of earthquake rates, a larger number of events is predicted for the extensional settings of the Apennines chain, in agreement with the inferred seismic coupling but also with the long-term strain rates.  We also find that the frequency/magnitude distributions indicate that the largest earthquakes occur in extensional settings, whereas compressional faults are expected to host comparatively smaller events.

Figure 1.  Black circles: CPTI15 catalogue (1880–2013, m_t = 4.8); blue, yellow and green polylines: surface trace of DISS Composite Seismogenic Sources (DISSWorking Group 2015).  Seismicity is forecast for the region enclosed by the black polyline, whereas earthquakes inside the region enclosed by the dashed red polyline are used for determining the magnitude–frequency distribution parameters.