The seismotectonics of the Po Plain (northern Italy): tectonic diversity in a blind faulting domain

We present a systematic and updated overview of a seismotectonic model for the Po Plain (northern Italy). This flat and apparently quiet tectonic domain is in fact rather active as it comprises the shortened foreland and foredeep of both the Southern Alps and the Northern Apennines. Assessing its seismic hazard is crucial due to the concentration of population, industrial activities and critical infrastructures, but it is also complicated because a) the region is geologically very diverse, and b) nearly all potential seismogenic faults are buried beneath a thick blanket of Pliocene-Pleistocene sediments, and hence can be investigated only indirectly. Identifying and parameterizing the potential seismogenic faults of the Po Plain requires proper consideration of their depth, geometry, kinematics, earthquake potential and location with respect to the two confronting orogens. To this end we subdivided them into four main homogeneous groups. Over the past 15 years we developed new strategies for coping with this diversity, resorting to different data and modeling approaches as required by each individual fault group. The most significant faults occur beneath the thrust fronts of the Ferrara-Romagna and Emilia arcs, which correspond to the most advanced and buried portions of the Northern Apennines and were the locus of the destructive May 2012 earthquake sequence. The largest known Po Plain earthquake, however, occurred on an elusive reactivated fault cutting the Alpine foreland south of Verona. Significant earthquakes are expected to be generated also by a set of transverse structures segmenting the thrust system, and by the deeper ramps of the Apennines thrusts. The new dataset is intended to be included in the next version of the Database of Seismogenic Sources (DISS; http://diss.rm.ingv.it/diss/, version 3.2.0, developed and maintained by INGV) to improve completeness of potential sources for seismic hazard assessment

An inventory of river anomalies in the Po Plain, Northern Italy: evidence for active blind thrust faulting

The Po Plain is a low-relief area characterised by active shortening accommodated by blind thrust faulting. In this almost flat region depositional rates are similar to tectonic rates and deformation is seldom expressed by noticeable surface anticlines. We adopted a geomorphological approach based on the detailed analysis of the drainage network to identify the location of active thrust faults. A total of 36 anomalies represented by sudden river diversions and shifts in channel pattern were accurately mapped. After comparison with the location of subsurface buried anticlines and of historical seismicity, these anomalies could be related to a tectonic origin and included in a database. Their distribution highlights the activity of the buried outer thrust fronts of both the Southern Alps and the Northern Apennines. Among all the anomalies, we identified one related to the seismogenic source responsible for the 12 May 1802 earthquake (Me 5.7), which struck the Oglio River Valley near Soncino (Cremona). We propose that this earthquake was generated by an east-west trending, north-dipping, blind thrust fault that roots into the Alpine system. If this inference is correct, other faults along the Southern Alpine margin are potentially seismogenic

Influence of inherited geometry and fault history on the seismogenic activity and potential of strike-slip fault systems in NW Slovenia: the case study of the Ravne Fault

La zona di faglia Ravne è situata in un area di interazione fra due sistemi regionali di faglie con differente cinematica, entrambi collegati alla convergenza fra Adria e Eurasia: le faglie dinariche orientate NW-SE e le faglie del Sud-alpino orientate E-W. L’analisi di dati di geologia strutturale e di due sequenze sismiche recenti che hanno colpito l’area, ci permette di proporre un modello sismotettonico per la faglia di Ravne, che è stata interessata da diverse fasi tettoniche. La geometria originale e la storia evolutiva della zona di faglia svolgono un ruolo cruciale nella distribuzione recente dell’attività sismica e del potenziale sismogenetico dell’intera struttura. Infatti, la configurazione attuale della faglia Ravne, caratterizzata da fagliazione trascorrente su piani ad alto angolo a profondità crostali, è il risultato dell’iniziale geometria di un thrust orientato NW-SE e avente immersione verso NE, e della sua interazione con i piani di thrust diretti essenzialmente E-W. Partendo dai dati raccolti e tenendo in considerazione sia il quadro geodinamico che le relazioni empiriche, proponiamo tre possibili scenari con relativi potenziali sismogenetici per la possibile futura attività della faglia di Ravne

La sorgente del Terremoto del 1908 nel quadro sismotettonico dello Stretto di Messina

La costruzione di un modello della faglia responsabile del terremoto del 1908 ha impegnato diversi ricercatori negli ultimi decenni e si è basata sulle deformazioni del suolo rilevate grazie alla ripetizione di misure di livellazione geodetica sulle due sponde dello Stretto. Il modello preferito è una faglia estensionale lunga circa 40 km, posta al centro dello Stretto e cieca, ovvero non direttamente visibile in superficie. Questo modello si accorda bene con l’evoluzione recente dello Stretto come un elemento fisiografico ben distinto all’interno dell’Arco Calabro. L’accordo tra la sorgente del terremoto e l’evoluzione geologica dello Stretto suggerisce che l’evento del 1908 sia un “terremoto caratteristico” di quest’area, con lunghi tempi di ritorno

A fresh look at the seismotectonics of the Abruzzi (Central Apennines) following the 6 April 2009 L'Aquila earthquake (Mw 6.3)

This work aims at providing an updated and augmented view of present-day tectonics and seismogenic sources of the Abruzzi Apennines, focusing on its extensional domain. This paper was spurred by the 6 April 2009, L’Aquila earthquake (Mw 6.3), an event from which geologists learned important lessons-including rather surprising ones. Although the earthquake was not major compared with other catastrophic events that occurred in Italy and elsewhere, this destructive earthquake led to a thorough review of the geometry – and style, in some instances – that characterises earthquake faulting in this region. The poorly expressed field evidence of the 6 April event, especially in light of the damage it caused in the mesoseismal area, stressed the intrinsic limitation of the earthquake geologists’ toolbox. Abruzzi is the region of a true “seismological paradox”: despite the rather long earthquake history available for the region, the number of potential sources for earthquakes of M ≥ 6.0 proposed in the literature is two to five times larger than the number of events that appear in the full earthquake record. This circumstance is made even more paradoxical by recent palaeoseismological work that proposed recurrence times of only a few centuries for individual seismogenic sources. Do the evident faults mapped by previous workers all correspond to potential seismogenic sources? We aim at addressing this paradox by drawing an updated seismotectonic model of Abruzzi based on the lessons learned following the 2009 earthquake. The model is based on selected geological, geomorphological, seismological, historical and geodetic data and will ultimately feed an updated version of the DISS database (http://diss.rm.ingv.it/diss/)

Slip-rates of blind thrusts in slow deforming areas: examples from the Po Plain (Italy)

We calculate Plio-Pleistocene slip rates on the blind thrusts of the outer Northern Apennines fronts, that are the potential sources of highly damaging earthquakes, as shown by the MW 6.1-6.0, 2012 Emilia-Romagna seismic sequence. Slip rates are a key parameter for understanding the seismogenic potential of active fault systems and assessing the seismic hazard they pose, however, they are difficult to calculate in slow deforming areas like the Po Plain where faulting and folding is mostly blind. To overcome this, we developed a workflow which included the preparation of a homogeneous regional dataset of geological and geophysical subsurface information, rich in Plio- Pleistocene data. We then constructed 3D geological models around selected individual structures to decompact the clastic units and restore the slip on the fault planes. The back-stripping of the differential compaction eliminates unwanted overestimation of the slip rates due to compactioninduced differential subsidence. Finally, to restore the displacement we used different methods according to the deformation style, i.e. Fault Parallel Flow for faulted horizons, trishear and elastic dislocation modeling for fault-propagation folds. The result of our study is the compilation of a slip rate database integrating former published values with 28 new values covering a time interval from the Pliocene to the present. It contains data on 14 individual blind thrusts including the Mirandola thrust, seismogenic source of the 29 May 2012, MW 6.0 earthquake. Our study highlights that the investigated thrusts were active with rates ranging between 0.1-1.0 mm/yr during the last 1.81 Myr. The Mirandola thrust slipped at 0.86±0.38 mm/yr during the last 0.4 Myr. These rates calculated with an homogeneous methodology through the entire Po Plain can be charged entirely to the thrust activity and not to secondary effects like the differential compaction of sediments across the structures

WP8 Modelling of topographic signal: GIS-BASED data base of potential earthquake sources identified in suitable kei-areas

GIS-based database of potential earthquake sources that were identified in key areas, such as the Provence, France, Po Plain, Italy, Outer Jura, Switzerland. The seismogenic source is defined as in Valensise and Pantosti (2001) following the scheme de-veloped in the framework of the EC project FAUST (Valensise et al., 2002)

The seismogenic structure of the 2013-2014 Matese seismic sequence, Southern Italy: implication for the geometry of the Apennines active extensional belt

Seismological, geological and geodetic data have been integrated to characterize the seismogenic structure of the late 2013-early 2014 moderate energy (maximum local magnitude M-Lmax = 4.9) seismic sequence that struck the interior of the Matese Massif, part of the Southern Apennines active extensional belt. The sequence, heralded by a M-L = 2.7 foreshock, was characterized by two main shocks with M-L = 4.9 and M-L = 4.2, respectively, which occurred at a depth of similar to 17-18 km. The sequence was confined in the 10-20 km depth range, significantly deeper than the 1997-1998 sequence which occurred fewkm away on the northeastern side of the massif above similar to 15 km depth. The depth distribution of the 2013-14 sequence is almost continuous, albeit a deeper (16-19 km) and a shallower (11-15 km) group of events can be distinguished, the former including the main shocks and the foreshock. The epicentral distribution formed a similar to 10 km long NNW-SSE trending alignment, which almost parallels the surface trace of late Pliocene-Quaternary southwest-dipping normal faults with a poor evidence of current geological and geodetic deformation. We built an upper crustal model profile for the eastern Matese massif through integration of geological data, oil exploration well logs and seismic tomographic images. Projection of hypocentres on the profile suggests that the seismogenic volume falls mostly within the crystalline crust and subordinately within the Mesozoic sedimentary cover of Apulia, the underthrust foreland of the Southern Apennines fold and thrust belt. Geological data and the regional macroseismic field of the sequence suggest that the southwest-dipping nodal plane of the main shocks represents the rupture surface that we refer to here as the Matese fault. The major lithological discontinuity between crystalline and sedimentary rocks of Apulia likely confined upward the rupture extent of the Matese fault. Repeated coseismic failure represented by the deeper group of events in the sequence, activated in a passive fashion the overlying similar to 11-15 km deep section of the upper crustal normal faults. We consider the southwest-dipping Matese fault representative of a poorly known type of seismogenic structures in the Southern Apennines, where extensional seismogenesis and geodetic strain accumulation occur more frequently on NE-dipping, shallower-rooted faults. This is the case of the Boiano Basin fault located on the northern side of the massif, to which the 1997-1998 sequence is related. The close proximity of the two types of seismogenic faults at the Matese Massif is related to the complex crustal architecture generated by the Pliocene-early Pleistocene contractional and transpressional tectonics

Is blind faulting truly invisible? Tectonic-controlled drainage evolution in the epicentral area of the May 2012, Emilia-Romagna earthquake sequence (northern Italy)

For decades alluvial plains have been the areas of fastest population growth over most of the globe. Modern societies demand growing extensions of flat and easily accessible land to accommodate swelling urban areas, booming industrial districts, large power plants, and multi-runway airports. But how can we tell if one of such flat areas hides large active faults? How can we assign a significant pre-instrumental earthquake to its causative source? In other words, how can modern societies deal with buried, that is to say, invisible faults, and with the elusiveness of the hazard they pose

WP8 Modelling of topographic signal: Detailed characterisation of individual structures

This deliverable will describe in detail results obtained during the project for four key areas of the Po Plain, the target area for the INGV contribution to the project. The four area are: 1) Coastal Marche region (southeastern Po Plain – see Section 1) 2) Mantova/Mincio River area (central Po Plain – see Section 2) 3) Mirandola/Secchia-Panaro Rivers area (southern-central Po Plain – see Section 3) 4) Soncino/Oglio River area (western Po Plain – see Section 4