Integrating new and traditional approaches for the estimate of slip-rates of active faults: examples from the Mw 6.3, 2009 L’Aquila earthquake area, Central Italy

Abstract

This thesis developed a multidisciplinary and multi-scale investigation strategy based on the integration of traditional and innovative approaches aimed at improving the normal faults seismogenic identification and characterization, focusing mainly on slip-rate estimate as a measure of the fault activity. The L’Aquila Mw 6.3 April 6, 2009 earthquake causative fault was used as a test site for the application, testing, and refinement of traditional and/or innovative approaches, with the aim to 1) evaluate their strength or limitations 2) develop a reference approach useful for extending the investigation to other active faults in the area and 3) translate the results of the methodological approaches into new inputs to local seismic hazard. The April 6, 2009 L’Aquila earthquake occurred on a so far poorly known tectonic structure, considered having a limited seismic potential, the Paganica - San Demetrio fault system (PSDFS), and thus has highlighted the need for a detailed knowledge in terms of location, geometry, and characterization of the active faults that are the potential sources for future earthquakes. To fill the gap of knowledge enhanced by the occurrence of the 2009 L’Aquila earthquake, we developed a multidisciplinary and multiscale‐based strategy consisting of paleoseismological investigations, detailed geomorphological and geological field studies, as well as shallow geophysical imaging and an innovative methodology that uses, as an alternative paleoseismological tool, core sampling and laboratory analyses but also in situ measurements of physical properties. The integration of geomorphology, geology as well as shallow geophysics, was essential to produce a new detailed geomorphological and geological map of the PSDFS and to define its tectonic style, arrangement, kinematics, extent, geometry and internal complexities. Our investigations highlighted that the PSDFS is a 19 km-long tectonic structure characterized by a complex structural setting at the surface and that is arranged in two main sectors: the Paganica sector to the NW and the San Demetrio sector to SE. The Paganica sector is characterized by a narrow deformation zone, with a relatively small (but deep) Quaternary basin affected by few fault splays. The San Demetrio sector is characterized by a strain distribution at the surface that is accommodated by several tectonic structures, with the system opening into a set of parallel, km-spaced fault traces that exhume and dissect the Quaternary basin. The integration of all the fault displacement data and age constraints (radiocarbon dating, optically stimulated luminescence (OSL) and tephrochronology) resulting from paleoseismological, geomorphological, geophysical and geological investigations played a primary role in the estimate of the slip-rate of the PSDFS. Slip-rates were estimated for different time intervals in the Quaternary, from Early Pleistocene (1.8 Ma) to Late Holocene (last 5 ka), yielding values ranging between 0.09 and 0.58 mm/yr and providing an average Quaternary slip-rate representative for the PSDFS of 0.27 - 0.48 mm/yr. We contributed also to the understanding of the PSDFS seismic behavior and of the local seismic hazard by estimating the max expected magnitude for this fault on the basis of its length (ca. 20 km) and slip per event (up to 0.8 m), and identifying the two most active fault splays at present. Our multidisciplinary results converge toward the possibility of the occurrence of past surface faulting earthquakes characterized by a moment magnitude between 6.3 and 6.8, notably larger than the 2009 event, but compatible with the M range observed in historical earthquakes in the area. The slip-rate distribution over time and space and the tectonic style of the PSDFS suggested the occurrence of strain migration through time in the southern sector, from the easternmost basin-bounding fault splay toward the southwestern splays. This topic has a significant implication in terms of surface faulting hazard in the area, because it can contribute defining the fault splays that have a higher potential to slip during future earthquakes along the PSDFS. By a methodological point of view, the multidisciplinary and multiscale‐based investigation strategy emphasizes the advantages of the joint application of different approaches and methodologies for active faults identification and characterization. Our work suggests that each approach alone may provide sufficient information but only the application of a multidisciplinary strategy is effective in providing robust results and in defining a proper framework of active faults.