Geodetic Strain Rates and Seismicity Rates Along the Apennines (Italy)

Abstract

The Apennines are a tectonically active belt that has experienced significant earthquakes (Mw$>$6). The largest events primarily occurred along the chain axis, where a complex system of normal faults accommodates 2–3 mm/yr of SW-NE oriented extension, as precisely measured by a dense Global Navigation Satellite System network. Geodetic strain rates are now frequently used in earthquake hazard models; however, the impact of using such estimates, computed through different methods, for seismic hazard assessments may be difficult to evaluate. This study explores the relationship between geodetic strain rates and seismicity rates in the Apennines using three distinct horizontal strain rate maps and an instrumental seismicity catalog. We find that the principal directions of geodetic strain rate are kinematically consistent with those of strain release. We estimate a spatially heterogeneous seismogenic thickness using the distribution of earthquake depths, and we isolate likely independent seismicity using three different declustering methods. We observe a correlation between independent seismicity rates and the magnitude of strain rate, which can be represented by either a linear or, more accurately, by a power-law relationship. The variability in the strain-seismicity relationship depends on the combination of independent seismic catalogs and strain rate maps. This relationship is primarily influenced by the declustering technique more than the choice of the strain rate map and, in particular, by the number of aftershocks excluded during declustering. Seismicity models derived from these combinations were used to estimate and compare the seismic moment release rate with the tectonic moment rate estimated from strain rate maps and seismogenic thickness. Findings indicate that the tectonic moment rate exceeds the seismic moment release rate. We highlight uncertainties and potential causes, one of which could be a possible aseismic release of part of the moment rate.