Morphotectonic and seismological evidence of active tectonics in the Umbria-Marche Apennines, central Italy

Abstract

During the last millennium, several moderate to strong earthquakes occurred in the Umbria-Marche Apennines and Marche foredeep, making this region one of the most seismically active areas of Europe. Seismic activity is mainly clustered in the southern part of the region. Nevertheless, beside the presence of high magnitude historical earthquakes, the lack of significant instrumental seismicity (i.e. ML > 2.5 earthquakes) makes the northern sector of the study area as a prominent seismic gap. Such a northern area is highly challenging for the detection of active, often blind, faults that are essential to a reliable seismic hazard assessment of the region. To tackle this issue, we test an integrated methodological approach based on available geological information together with the analysis of high-resolution seismological data and stress inversion analysis. Furthermore, GIS-based morphotectonic analyses are also used to study the correlation between the identified structures and neotectonic processes. Topography analysis includes the creation of seven swath profiles (20 km in width) and of maximum, minimum, mean elevation and relief maps. River network analysis is carried out using river long profiles and transformed (𝝌) long profiles coupled with slope/area diagrams. The former allowed the location of knickpoints/knickzones, whereas the latter allowed the spatial distribution of the Ksn (normalized steepness) index to be obtained. Combined data from topography and river network mark the different neotectonic behaviour of the northern and southern sectors of the Umbria-Marche Apennines. In fact, the southward increase of (i) mean and minimum elevations, (ii) local relief, (iii) abundance of non-lithology controlled knickpoints, and (iv) Ksn values, suggest the presence of a locus of enhanced vertical motion (i.e., surface uplift) in the south. Here, the largest historical earthquakes and the strongest instrumental seismicity also occur, thus suggesting active tectonics is more intense in this area of stronger uplift.The results of our multidisciplinary analysis are consistent with evidence of along-strike segmentation of the fold and thrust belt where each sector is characterised by different active tectonic behaviour and thickness of Pliocene deposits. Boundaries of sectors roughly coincide with major transversal fault systems, segmenting the outer portion of the fold and thrust belt in the Marche foothills. Such transversal systems are accompanied by diffuse seismicity (< M= 5.5) and probably mark the loci of long-lived, deep-seated fault zones that exert a major control on the active tectonic behaviour of large crustal blocks also compartmentalizing the axial zone of the chain.