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

First appraisal to define prospective seismogenic sources from historical earthquake damages in southern Upper Rhine Graben

The southern portion ofthe Upper Rhine Graben, a major oblique rift among France, Germany and Switzerland, shows a weak instrumental seismic record despite its remarkable physiographic imprint within the Northern Alpine foreland. Since traces of active deformation can be found in this region and based on experience in other European areas with high seismic hazard and dense population, we searched for past earthquakes recorded in historical catalogues. Based on the fact that tectonic deformation cumulates through geological time and considering that long-term effects tend to leave characteristic signatures on present-day landscape arrangement, our goal was to identify faults that could have caused the damage of recorded historical events. We isolated five main earthquakes, ofmoderate Richter magnitude, essentially located on the E flank of the graben (as is the case with recent seismic activity). To such events, we were able to associate a specific prospective structure through the use ofa procedure thus far successfully employed in Southern European contexts. We concentrated on three events which showed (a) notable sensitivity to the density of the historical felt reports and (b) accordance with on-going subtle deformation pattern. Another, most relevant earthquake (M 5.5) yielded a promising match with the known deformation network in the region. As a template to better constrain earthquake cycle and damage potential, historical seismicity offers an invaluable tool, since it contains a specific record, although not always unambiguous. Cross-checking such data with pertinent geological information allows to devise a realistic fault geometry capable of being responsible for a specific seismic event

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

Shaking Scenarios from Multiple Source Models Shed Light on the 8 September 1905 Mw 7 Calabria Earthquake (Southern Italy)

4noThe earthquake (Mw 7) that struck western Calabria (southern Italy) on 8 September 1905 profoundly struck a broad region, causing 557 deaths, injuring more than 2000 people, and leaving about 300,000 people homeless. Historical documents also reported a tsunami, although not devastating, for which effects were observed both along the coast and offshore. For all the damage it caused, this event was much studied but not fully explained. Literature source models for the 1905 earthquake are numerous and diverse, in fault geometry, location, and even associated magnitude. They also differ in nature, because these solutions are either field-based or derived from tsunami modeling and macroseismic data inversion. Above all, few or none of the previously published source models appear to be fully compatible with the damage pattern caused by this earthquake. To contribute to the identification of the seismogenic source of this destructive event, we computed a series of ground-shaking scenarios based on the different fault-source models that various authors associated with this event. The only documented data available that are suitable for our comparative purposes are the macroseismic intensities associated with localities affected by the event. Our results show that shaking scenarios for two out of seven literature source models are compatible with the damage distribution caused by the 1905 earthquake. The different parameters and boundary conditions constraining these two solutions suggest that either seismogenic source should include further complexities. Alternatively, because these two sources are antithetic and partially form a graben, they might have kinematically interacted, if passively, on 8 September 1905. Also, our critical analysis attempts to take site effects into account, at least qualitatively, allowing a more robust evaluation of damage distribution against numerical models.openembargoed_20160430Sandron, Denis; Loreto, Maria Filomena; Fracassi, Umberto; Tiberi, LaraSandron, Denis; Loreto, Maria Filomena; Fracassi, Umberto; Tiberi, Lar

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/)

Middle Pleistocene to Holocene activity of the Gondola Fault Zone (Southern Adriatic Foreland): deformation of a regional shear zone and seismotectonic implications

Recent seismicity in and around the Gargano Promontory, an uplifted portion of the southern Adriatic Foreland domain, indicates active E-W strike-slip faulting in a region that has also been struck by large historical earthquakes, particularly along the Mattinata Fault. Seismic profiles published in the past two decades show that the pattern of tectonic deformation along the E-W–trending segment of the Gondola Fault Zone, the offshore counterpart of the Mattinata Fault, is strikingly similar to that observed onshore during the Eocene-Pliocene interval. Based on the lack of instrumental seismicity in the south Adriatic offshore, however, and on standard seismic reflection data showing an undisturbed Quaternary succession above the Gondola Fault Zone, this fault zone has been interpreted as essentially inactive since the Pliocene. Nevertheless, many investigators emphasised the genetic relationships and physical continuity between the Mattinata Fault, a positively active tectonic feature, and the Gondola Fault Zone. The seismotectonic potential of the system formed by these two faults has never been investigated in detail. Recent investigations of Quaternary sedimentary successions on the Adriatic shelf, by means of very high-resolution seismic-stratigraphic data, have led to the identification of fold growth and fault propagation in Middle-Upper Pleistocene and Holocene units. The inferred pattern of gentle folding and shallow faulting indicates that sediments deposited during the past ca. 450 ka were recurrently deformed along the E-W branch of the Gondola Fault Zone. We performed a detailed reconstruction and kinematic interpretation of the most recent deformation observed along the Gondola Fault Zone and interpret it in the broader context of the seismotectonic setting of the southern Apennines-foreland region. We hypothesise that the entire 180 km-long Molise-Gondola Shear Zone is presently active and speculate that also its offshore portion, the Gondola Fault Zone, has a seismogenic behaviour

Middle Pleistocene to Holocene activity of the Gondola Fault Zone (Southern Adriatic Foreland): deformation of a regional shear zone and seismotectonic implications

Recent seismicity in and around the Gargano Promontory, an uplifted portion of the Southern Adriatic Foreland domain, indicates active E–W strike-slip faulting in a region that has also been struck by large historical earthquakes, particularly along the Mattinata Fault. Seismic profiles published in the past two decades show that the pattern of tectonic deformation along the E–W-trending segment of the Gondola Fault Zone, the offshore counterpart of the Mattinata Fault, is strikingly similar to that observed onshore during the Eocene–Pliocene interval. Based on the lack of instrumental seismicity in the south Adriatic offshore, however, and on standard seismic reflection data showing an undisturbed Quaternary succession above the Gondola Fault Zone, this fault zone has been interpreted as essentially inactive since the Pliocene. Nevertheless, many investigators emphasised the genetic relationships and physical continuity between the Mattinata Fault, a positively active tectonic feature, and the Gondola Fault Zone. The seismotectonic potential of the system formed by these two faults has never been investigated in detail. Recent investigations of Quaternary sedimentary successions on the Adriatic shelf, by means of very high-resolution seismic–stratigraphic data, have led to the identification of fold growth and fault propagation in Middle–Upper Pleistocene and Holocene units. The inferred pattern of gentle folding and shallow faulting indicates that sediments deposited during the past ca. 450 ka were recurrently deformed along the E–W branch of the Gondola Fault Zone. We performed a detailed reconstruction and kinematic interpretation of the most recent deformation observed along the Gondola Fault Zone and interpret it in the broader context of the seismotectonic setting of the Southern Apennines-foreland region. We hypothesise that the entire 180 km-long Molise–Gondola Shear Zone is presently active and speculate that also its offshore portion, the Gondola Fault Zone, has a seismogenic behaviour

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

Seismotectonics of the Southern Apennines and Adriatic foreland: insights on active regional E-W shear zones from analogue modeling

The active tectonics at the front of the Southern Apennines and in the Adriatic foreland is characterized by E-W striking, right-lateral seismogenic faults, interpreted as reactivated inherited discontinuities. The best studied among these is the Molise-Gondola shear zone (MGsz). The interaction of these shear zones with the Apennines chain is not yet clear. To address this open question we developed a set of scaled analogue experiments, aimed at analyzing: 1) how dextral strike-slip motion along a pre-existing zone of weakness within the foreland propagates toward the surface and affects the orogenic wedge; 2) the propagation of deformation as a function of displacement; 3) any insights on the active tectonics of Southern Italy. Our results stress the primary role played by these inherited structures when reactivated, and confirm that regional E-W dextral shear zones are a plausible way of explaining the seismotectonic setting of the external areas of the Southern Apennines