Unveiling the Sources of the Catastrophic 1456 Multiple Earthquake: Hints to an Unexplored Tectonic Mechanism in Southern Italy

We revisited data related to the 1456 seismic crisis, the largest earthquake to have ever occurred in peninsular Italy, in search of its causative source(s). Data about this earthquake consist solely of historical reports and their intensity assessment. Because of the age of this multiple earthquake, the scarcity and sparseness of the data, and the unusually large damage area, no previous studies have attempted to attribute the 1456 events to specific faults. Existing analytical methods to identify a likely source from intensity data also proved inappropriate for such a sparse dataset, since historical evidence suggests that the cumulative damage pattern contains at least three widely separated events. We subdivided the 1456 damage pattern into three independent mesoseismal areas; each of these areas falls onto east–west tectonic trends previously identified and marked by deep (>10 km) right-lateral slip earthquakes. Based on this evidence we propose (1) that the 1456 events were generated by individual segments of regional east–west structures and are evidence of a seismogenic style that involves oblique dextral reactivation of east–west lower crustal faults; (2) that each event may have triggered subsequent but relatively distant events in a cascade fashion, as suggested by historical accounts; hence (3) that the 1456 sequence reveals a fundamental but unexplored mechanism of tectonic deformation and seismic release in southern Italy. This style dominates the region that lies between the northwest–southeast system of large extensional faults straddling the crest of the southern Apennines and the buried outer front of the chain. Although the quality of the available information concerning the 1456 earthquake is naturally limited, we show that the overlap of the damage distribution, the orientation and characteristics of regional tectonic structures, the seismicity patterns, and the focal mechanisms all concur with our interpretations and would be difficult to justify otherwise

On the complexity of earthquake sequences: a historical seismology perspective based on the L'Aquila seismicity (Abruzzo, Central Italy), 1315-1915

Most damaging earthquakes come as complex sequences characterized by strong aftershocks, sometimes by foreshocks and often by multiple mainshocks. Complex earthquake sequences have enormous seismic hazard, engineering and societal implications as their impact on buildings and infrastructures may be much more severe at the end of the sequence than just after the mainshock. In this paper we examine whether historical sources can help characterizing the rare earthquake sequences of pre-instrumental times in full, including fore-, main- and aftershocks. Thanks to the its huge documentary heritage, Italy relies on one of the richest parametric earthquake catalogues worldwide. Unfortunately most current methods for assessing seismic hazard require that earthquake catalogues be declustered by removing all shocks that bear some dependency with those identified as mainshocks. We maintain that this requirement has led most modern historical seismologists to focus mainly on mainshocks rather than also on the fore- and aftershocks. To shed light onto major earthquake sequences of the past, rather than onto individual mainshocks, we investigated 10 damaging earthquake sequences ($M_w$ 수식 이미지 4.7-7.0) that hit the L'Aquila area and central Abruzzo from the 14th to the 20th century. We find that most of the results of historical research are important for modern seismology, yet their rendering by the current parametric catalogues causes most information to be lost or not easily transferred to the potential users. For this reason we advocate a change in current strategies and the creation of a more flexible standard for storing and using all the information made available by historical seismology

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

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

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

Il quadro sismotettonico del grande terremoto del 1905

La storia della Calabria è una storia lunga – molto più lunga di quanto qualunque essere umano possa immaginare, aggiungeremmo noi geologi – di grandi e piccoli terremoti. Catastrofi improvvise e catastrofi parzialmente annunciate, terremoti improvvisi e isolati e sequenze interminabili che sembravano non voler lasciare in piedi nulla di questa regione. Lo spaventoso livello di sismicità della Calabria, di cui qualunque calabrese è testimone almeno indiretto, è oggi quantomeno ben accertato da ricerche sempre più specialistiche e dettagliate. Due degli elementi fondamentali per descrivere la sismicità calabrese consistono nello studio dei terremoti del passato e nello studio della geologia e tettonica di questa regione, riconosciuta da sempre come uno dei luoghi maggiormente attivi di tutto il Mediterraneo. Questi elementi confluiscono in modelli di pericolosità sismica (Gruppo di Lavoro MPS, 2004; fig. 1), che puntualmente fotografano una propensione di questa terra a dare terremoti più forti e più frequenti di quanto non avvenga in qualunque altra zona della penisola. Questa relazione tenta di tratteggiare sinteticamente questa forte propensione alla sismicità, muovendosi tra le caratteristiche geologiche della Calabria e la sua poco invidiabile storia sismica. La relazione si avvale di risultati di ricerche recenti e recentissime, condotte sia presso l’Istituto Nazionale di Geofisica e Vulcanologia (INGV), sia dalla comunità sismologica nazionale che fa riferimento alle università. La relazione privilegia ampiamente il materiale iconografico basato su tali ricerche. Per ulteriori approfondimenti si raccomandano i lettori di consultare il sito Internet dell’INGV (www.ingv.it), che nelle sue pagine interne offre numerosissime informazioni di facile accessibilità e comprensione

The 28 December 1908 Messina Straits Earthquake (Mw 7.1): A Great Earthquake throughout a Century of Seismology

Early in the morning on 28 December 1908, just a few days after Christmas, a severe earthquake struck the Messina Straits, a rather narrow sound that separates Calabria, in southern Italy, from Sicily. The shaking was distinctly felt in Albania, Montenegro, and the Greek Ionian islands, about 400 km to the east and northeast of the Straits; in Malta, about 250 km to the south; and as far as Ustica Island, about 220 km to the west. The earthquake was catastrophic in the epicentral area and was immediately followed by fires and a large tsunami. Messina (Sicily) and Reggio Calabria (Calabria), two significant cities located less than 10 km apart on the two facing shores of the straits, were almost completely destroyed, and buildings were severely damaged over an area in excess of 6,000 km2

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)