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The Italian historical earthquake record is among the richest worldwide; as such it allows for the development of advanced techniques for retrieving quantitative information by calibration with recent earthquakes. Building on a pilot elaboration of northern Italian earthquakes, we developed a procedure for determining the hypocentral depth of all Italian earthquakes from macroseismic intensity data alone. In a second step the procedure calculates their magnitude, taking into account the inferred depth. Hypocentral depth exhibits substantial variability countrywide but has so far received little attention: pre-instrumental earthquakes were routinely “flattened” at the upper-crustal level (∼10 km), on the grounds that the calculation of hypocentral depth is heavily dependent on the largely unknown local propagation properties. We gathered a learning set of 42 earthquakes documented by reliable instrumental data and by numerous macroseismic intensity observations. We observe (1) that within 50 km from the epicenter the ground motion attenuation rate is primarily controlled by hypocentral depth and largely independent of magnitude, (2) that within this distance the fluctuations in crustal attenuation properties are negligible countrywide, and (3) that knowing both the depth and the expected epicentral intensity makes it possible to estimate a reliable magnitude.

The undersea portion of the Northern Apennines is characterized by blind thrust faults running parallel to the Adriatic Sea coastline in northeastern peninsular Italy. These thrusts are buried below a thick cover of syntectonic Quaternary deposits. Their elusive geological signature at shallow depths and the low seismicity associated with them gave rise to diverging interpretations and views concerning the current activity of these thrusts and their earthquake potential.On 9 November 2022, a seismic sequence started with an Mw 5.5 earthquake in the Pesaro Offshore. Hypocentral depth, focal mechanism, and aftershocks location all suggest that the earthquake was generated by one of the outermost thrusts of the Northern Apennines front that was already mapped as a potential seismogenic source in the DISS database (https://diss.ingv.it/diss330/sources.php?ITCS106).We present a 3D reconstruction of the thrust system that caused the Pesaro Offshore seismic sequence obtained through the reinterpretation of publicly available seismic reflection profiles and well logs. The 3D geometry and size of the thrust activated during the seismic sequence suggest that it can also host larger earthquakes. We also present the application of a well-established workflow for calculating the slip rates of this buried thrust already tested in nearby structures. The outcomes of this study represent a step forward for earthquake and tsunami hazard models, the study of the seismic source, the enhancement of earthquake location by mix and match of seismological and geological independent data, and the expected kinematics of future potential earthquake ruptures.These results are particularly relevant in offshore areas, where neither surface co-seismic ruptures nor GPS/InSAR deformation data are available in the aftermath of a significant earthquake. In these cases, multichannel seismic reflection profiles represent the only tool to appraise the subsurface structural setting.

The development of structurally controlled basins is frequently dominated by inherited geological and tectonic structures, especially when the affected region has undergone multiple tectonic phases. In this study we use physically scaled analog models to analyze the impact of inherited faults on the evolution of a new extensional fault system and its associated basin. In our experiments, we introduced inherited faults – bearing diverse geometries and orientations – cut through a homogeneous analog material (wet clay). After each experiment, we compare (a) how the inherited faults affected the inception and development of new faults and (b) the shape of the resulting basins, using a ‘reference model’ run without pre-existing faults. The results show that the orientation of pre-existing faults with respect to the extensional axis does affect the development of the new extensional structures. The main effects show up when the orientation of the pre-existing faults is closer to that expected for a fault that is optimally oriented (perpendicular) with respect to the direction of extension and has a dip close to an Andersonian extensional fault. Conversely, the impact on the resulting basin shape is more spatially complex, especially in the case of misoriented pre-existing faults. We also compare our experimental results with an analytical method based on the slip tendency theory. The application of our findings to selected natural cases demonstrates how one may interpret the occurrence, orientation, and activity of inherited faults by looking at the present-day geometry and wavelength of an extensional basin, particularly when newly formed extensional faults exhibit structurally unexpected trajectories.

We present a review of the assessment of earthquake hazard in Italy, with special reference to the relationships between hazard models and building codes. After early attempts at hazard assessment in the 19th century, the 28 December 1908, Messina Straits earthquake prompted the inception of the first national seismic legislation, passed in early 1909. Nevertheless, the official building code started to be based on a truly scientific background only after 1980, when the catastrophic Irpinia (southern Italy) earthquake forced the qualified authority to accept a science-based assessment (statistics on the earthquake catalogue data) to support the implementation of the new national seismic zonation. Later on, between 1985 and 2000, the two basic components of seismic hazard assessment, namely the earthquake record and the distribution of earthquake sources, were greatly developed through investigations carried out by the Gruppo Nazionale Difesa dai Terremoti and by the Istituto Nazionale di Geofisica. Along with the improvement of basic data, the Italian seismological community started developing a new hazard model (PS4), based on the concept of seismotectonic probabilism, aimed at supplying the Italian Government with a solid reference frame for updating the seismic zonation and building code. Nevertheless, this goal was achieved only two decades later: on 31 October 2002 a moderate-size earthquake caused the death of 27 children and a teacher in a collapsed school of southern Italy, forcing the qualified authority to take a major step of modernization for the second time in 22 years. The entire Italian territory, including areas of rare and sparse seismicity, was subdivided into four seismic zones, mainly on the basis of PS4 results. In 2004, the Italian seismological community developed MPS04, a fully updated hazard model that was initially conceived only in view of updating the seismic zonation. In 2007, MPS04 was extended to provide design spectra for a new building code, which was finally adopted in 2009, following the disastrous L’Aquila (central Italy) earthquake. The experience of the European project for seismic hazard assessment named SHARE, completed in 2013, represented a step forward and put the basis for a new project, termed MPS19, designed specifically to provide a sound basis for updating the Italian building code.

The Italian historical earthquake record is among the richest worldwide; as such it allows the development of advanced techniques for retrieving quantitative information by calibration with recent earthquakes. Building on a pilot elaboration of northern Italy earthquakes, we developed a procedure for determining the focal depth of all Italian earthquakes from intensity data alone. In a second step the procedure calculates their magnitude, taking into account the inferred depth. Focal depth exhibits a substantial variability countrywide, but so far received little attention: pre-instrumental earthquakes were routinely "flattened" at upper crustal level (~10 km), on the grounds that the calculation of focal depth is heavily dependent on the largely unknown local propagation properties. We gathered a learning set of 42 earthquakes documented by reliable instrumental data and by numerous intensity observations. Rather unexpectedly we observe (1) that within 50 km from the epicenter the ground motion attenuation rate is primarily controlled by focal depth and largely independent of magnitude; (2) that within this distance the fluctuations of crustal attenuation properties are negligible countrywide; and (3) that knowing both the depth and the expected epicentral intensity makes it possible to estimate a reliable magnitude.

We investigated the spatial relationships among 18 known seismogenic faults and 1651 wells drilled for gas exploitation in the main hydrocarbon province of northern-central Italy, a unique dataset worldwide. We adopted a GIS approach and a robust statistical technique, and found a significant anticorrelation between the location of productive wells and of the considered seismogenic faults, which are often overlain or encircled by unproductive wells. Our observations suggest that (a) earthquake ruptures encompassing much of the upper crust may cause gas to be lost to the atmosphere over geological time, and that (b) reservoirs underlain by smaller or aseismic faults are more likely to be intact. These findings, which are of inherently global relevance, have crucial implications for future hydrocarbon exploitation, for assessing the seismic–aseismic behaviour of large reverse faults, and for the public acceptance of underground energy and CO2 storage facilities—a pillar of future low carbon energy systems—in tectonically active areas.

The 2013 European Seismic Hazard Model (ESHM13) results from a community-based probabilistic seismic hazard assessment supported by the EU-FP7 project “Seismic Hazard Harmonization in Europe” (SHARE, 2009–2013). The ESHM13 is a consistent seismic hazard model for Europe and Turkey which overcomes the limitation of national borders and includes a through quantification of the uncertainties. It is the first completed regional effort contributing to the “Global Earthquake Model” initiative. It might serve as a reference model for various applications, from earthquake preparedness to earthquake risk mitigation strategies, including the update of the European seismic regulations for building design (Eurocode 8), and thus it is useful for future safety assessment and improvement of private and public buildings. Although its results constitute a reference for Europe, they do not replace the existing national design regulations that are in place for seismic design and construction of buildings. The ESHM13 represents a significant improvement compared to previous efforts as it is based on (1) the compilation of updated and harmonised versions of the databases required for probabilistic seismic hazard assessment, (2) the adoption of standard procedures and robust methods, especially for expert elicitation and consensus building among hundreds of European experts, (3) the multi-disciplinary input from all branches of earthquake science and engineering, (4) the direct involvement of the CEN/TC250/SC8 committee in defining output specifications relevant for Eurocode 8 and (5) the accounting for epistemic uncertainties of model components and hazard results. Furthermore, enormous effort was devoted to transparently document and ensure open availability of all data, results and methods through the European Facility for Earthquake Hazard and Risk (www.efehr.org)., Bulletin of Earthquake Engineering, 13 (12), ISSN:1570-761X, ISSN:1573-1456

We adopted a multidisciplinary approach to investigate the seismotectonic scenario of the 30 October 2016, Mw 6.5, Norcia earthquake, the largest shock of the 2016–2017 central Italy earthquake sequence. First, we used seismological and geodetic data to infer the dip of the main slip patch of the seismogenic fault that turned out to be rather low‐angle (~37°). To evaluate whether this is an acceptable dip for the main seismogenic source, we modeled earthquake deformation using single‐ and multiple‐fault models deduced from aftershock pattern analyses. These models show that the coseismic deformation generated by the Norcia earthquake is coherent with slip along a rather shallow‐dipping plane. To understand the geological significance of this solution, we reconstructed the subsurface architecture of the epicentral area. As the available data are not robust enough to converge on a single fault model, we built three different models encompassing all major geological evidence and the associated uncertainties, including the tectonic style and the location of major décollement levels. In all models the structures derived from the contractional phase play a significant role: from controlling segmentation to partially reusing inherited faults, to fully reactivating in extension a regional thrust, geometrically compatible with the source of the Norcia earthquake. Based on our conclusions, some additional seismogenic sources falling in the eastern, external portions of the Apennines may coincide with inherited structures. This may be a common occurrence in this region of the chain, where the inception of extension is as recent as Middle‐Upper Pleistocene.

In recent years, new approaches for developing earthquake rupture forecasts (ERFs) have been proposed to be used as an input for probabilistic seismic hazard assessment (PSHA). Zone- based approaches with seismicity rates derived from earthquake catalogs are commonly used in many countries as the standard for national seismic hazard models. In Italy, a single zone- based ERF is currently the basis for the official seismic hazard model. In this contribution, we present eleven new ERFs, including five zone-based, two smoothed seismicity-based, two fault- based, and two geodetic-based, used for a new PSH model in Italy. The ERFs were tested against observed seismicity and were subject to an elicitation procedure by a panel of PSHA experts to verify the scientific robustness and consistency of the forecasts with respect to the observations. Tests and elicitation were finalized to weight the ERFs. The results show a good response to the new inputs to observed seismicity in the last few centuries. The entire approach was a first attempt to build a community-based set of ERFs for an Italian PSHA model. The project involved a large number of seismic hazard practitioners, with their knowledge and experience, and the development of different models to capture and explore a large range of epistemic uncertainties in building ERFs, and represents an important step forward for the new national seismic hazard model.

We investigated whether the joint inversion of geodetic and stress direction data can constrain long-term fault slip rates in the central Apennines, and ultimately how extension is partitioned among fault slip and bulk lithosphere permanent strain Geodetic velocities are collected in the fault interseismic stage with steady secular deformation;thus, long-term estimates can be derived with a model of elastically unloading seismogenic faults within a viscously deforming lithosphere As the average spacing of permanent Global Navigation Satellite Systems (GNSS) stations is similar to the average length of seismogenic faults (25?35 km), if not larger, we decided to merge permanent and temporary GNSS measurements, resulting in a denser geodetic data set Given that most normal faults in the Apennines have slip rates around or below 1 mm/a, and most campaign GNSS velocities carry similar uncertainties, simple local back slip models cannot be applied More sophisticated modeling is required to extract reasonable bulk deformation rates and long-term fault slip rates at signal-to-noise ratio of order unity Given the spatial distribution of the GNSS network, we estimated the long-term slip rate of seven major fault systems that are in satisfactory agreement with available geological slip rates The resulting spatial distribution of bulk deformation rates locally fits short-term transients;in other cases, they represent the currently unclear signature of tectonic processes like upper-crustal viscoplastic deformation and aseismic slip, or indicate missing faults in the adopted database We conclude that the time is ripe for determining fault slip rates using geodetic and stress direction data, particularly where fault activity rates are hard to determine geologically

The prototype version of the DISS was launched and published in July 2000. Twenty years later we present an appraisal of how the database started off, how it evolved, and how it served the seismological and engineering communities.During the early years of its development we learned that the three fundamental requirements of any SHA-oriented fault database are:1) the capacity to represent seismogenic sources in 3D, thus providing a standardized quantitative basis for subsequent SHA calculations and stressing the hierarchy relationships among all existing active faults;2) the completeness, i.e. the ability to portray the vast majority of seismogenic sources existing in the region of relevance and to progressively address the emerging lack of knowledge;3) the reliability of the geometrical parameters of each seismogenic source and of the relevant slip and strain rates, and the ability to assess the associated uncertainties.Given these requirements, we found it hard to build a database around existing studies of individual large faults, which are often carried out for non-SHA purposes; as such they do not necessarily involve a 3D delineation and a hierarchization of the master fault. Furthermore, most published studies concern surface-breaking faults occurring onshore; they are most relevant to surface faulting hazard, but in shaking-oriented SHA they are less crucial than deeper, hidden faults.We initially developed the concept of “Individual Seismogenic Source” (ISS), a simplified but geometrically coherent representation of the presumed causative fault of the largest earthquakes of the investigated region. An ISS is based on original observations, seismological/geophysical evidence, and literature data. Since large portions of the Italian territory are characterized by blind or hidden faulting, we developed strategies based on the analysis of geomorphic evidence for cumulative tectonic strain, on the reappraisal of commercial seismic lines and subsurface data, and on geological and geodetic evidence.In 2005 we introduced the “Composite Seismogenic Sources” (CSSs): generalized, unsegmented sources designed to increase the database geographic coverage and completeness, based on the same type of information used for the ISSs and on regional-scale synopses of ongoing tectonic strain. Their identification was progressively extended to offshore areas, often scarcely considered in traditional fault mapping. In 2015 we also introduced the 3D definition of the subduction slabs and associated interfaces for the whole Mediterranean region.The ISSs are routinely used in engineering applications aimed at investigating the shaking scenario associated with known earthquakes or well-identified quiescent fault segments. In contrast, the CSSs are not assumed to be capable of a specific-size earthquake; as such, they can be used in any standard PSHA procedure after estimating their activity rate and frequency magnitude distribution, based on tectonic slip rates integrated with the record of past earthquakes and GPS-determined strains, or derived from regional-scale geodynamic models.DISS also served as a template for developing EDSF, the European Database of Seismogenic Faults. Over the years, DISS and EDSF have become the basic geological input for PSHA and PTHA, both at Italian scale (MPS04, MPS19, MPTS19) and European scale (ESHM13, ESHM20, NEAMTHM18).

Most of the present-day extensional systems formed in areas that already experienced an older phase of tectonic activity. Therefore, understanding how a pre-existing structural setting may affect the development of an extensional basin is a crucial interplay to decipher. Depending on the kinematics of these phases, the resulting inherited faults can be extensional, contractional, or transcurrent. Consequently, a new extensional basin forms atop or across pre-existing faults that can dip at a low- (e.g., inherited thrust faults) or high-angle (e.g., inherited extensional faults). Furthermore, the inherited structures can have a non-optimal attitude with respect to the new extensional stress field, thereby determining different instances for reactivation. In this study, we analyzed the impact of dip and strike of inherited faults on the development of an extensional basin using wet clay (kaolin) analogue modeling. We reproduced sixteen different setups by varying the dip (30°, 45°, 60°) and the strike (15°, 30°, 45°, 60°, 75°) of the pre-existing faults that we introduced in the experiments before applying extension. The results show that the orientation of pre-existing faults has a direct effect onto the shape of the new extensional basins. When the pre-existing faults are reused to accommodate the new extensional phase, the formed basins are asymmetric and the rate of growth of the new faults is lower.

The 2016–2017 Central Italy earthquakes have shown that the local seismic risk is dominated by the extreme vulnerability of the building stock. We attempt to rank the vulnerability of Apennines' settlements based on a combined geological-historical approach. We first discuss the reasons of the apparent paradox caused by the very different seismic response of Amatrice and Norcia, both strongly hit by the 24 August 2016 earthquake (Mw 6.0). Based on the awareness that strong earthquakes force building reconstructions and changes in the individual and societal perception of seismic risk, we assume that the global vulnerability of Italian settlements increases with time since the last significant earthquake. We focus on the very active seismogenic areas straddling Italy's Apennines. We then use data on the local seismogenic sources and earthquake history to (1) select the municipalities that are more likely to suffer from destructive ground shaking, and (2) rank them as a function of the time elapsed since the latest earthquake, i.e. in terms of increasing vulnerability. We hence identified 716 municipalities, totaling about 5% of the Italian population, over 50% of which have not experienced destructive shaking since 1861, when the Kingdom of Italy reunited a number of smaller states. As such they are primary candidates to a poor performance in future significant earthquakes (Mw > 5.5) and should be given priority in any statewide vulnerability reduction plan. All results and elaborations, including the seismic histories of each of the selected localities, are also supplied in a specifically designed web-GIS.

For any scientist working in seismotectonics, the Calabrian Arc represents the most challenging area of Italy. Lying on top of a subduction zone, it is characterised by a complex geological structure largely inherited from the early stages of the collision between the Africa and Eurasia plates. The current and extremely vigorous seismogenic processes, although generated by a mechanism driven by the subduction, are no longer a direct consequence of plate convergence. About one fourth of the largest Italian earthquakes concentrates in a narrow strip of land (roughly 200×70 km) corresponding to the administrative region of Calabria. The present-day seismicity, both shallow and deep, provides little help in detecting the most insidious seismogenic structures, nor does the available record of GPS-detected strains. In addition to its fierce seismicity, the Calabrian Arc also experiences uplift at rates that are the largest in Italy, thus suggesting that active tectonic processes are faster here than elsewhere in the country. Calabrian earthquakes are strong yet inherently elusive, and even the largest of those that have occurred over the past two centuries do not appear to have caused unambiguous surface faulting. The identified active structures are not sufficient to explain in full the historical seismicity record, suggesting that some of the main seismogenic sources still lie unidentified, for instance in the offshore. As a result, the seismogenic processes of Calabria have been the object of a lively debate at least over the past three decades. In this work we propose to use the current geodynamic framework of the Calabrian Arc as a guidance to resolve the ambiguities that concern the identification of the presumed known seismogenic sources, and to identify those as yet totally unknown. Our proposed scheme is consistent with the location of the largest earthquakes, the recent evolution of the regions affected by seismogenic faulting, and the predictions of current evolutionary models of the crust overlying a W-dipping subduction zone.

Determining the hypocentral depth of pre-instrumental earthquakes is a long-standing geophysical issue that still awaits to be elucidated. Using very well documented recent earthquakes we found that the depth of crustal and upper-mantle events correlates well with the slope of the first 50 km of their intensity attenuation curve, regardless of their magnitude. We used this observation to build a magnitude-independent method for calculating the depth of selected historical and early-instrumental earthquakes of northern Italy based on their macroseismic intensity field. Our method relies on both standard intensity data and questionnaire-based data for 20 earthquakes, encompassing a relatively large range of magnitude (Mw 4.0–5.8) and depth (3.0–72.4 km), that occurred in Northern Italy between 1983 and 2019. We then used the method to estimate the depth of 20 older earthquakes that occurred in the same region between 1570 and 1972. Knowing the approximate depth of historical earthquakes is crucial for assigning them to the relevant seismogenic source, especially where seismogenic faults occur at different depths, allowing for a better characterisation of the region’s seismotectonic setting. Knowing the focal depth also allows recalculating the equivalent magnitude, which turns out to be consistently larger for deeper events, suggesting a reassessment of the local seismic hazard.

A key element for assessing seismic hazard and risk is the availability of a comprehensive dataset on past earthquakes. Here we present the rationale, structure and contents of CFTI5Med (10.6092/ingv.it-cfti5), the 2018 version of the Catalogue of Strong Earthquakes in Italy: a large multidisciplinary effort including historians, seismologists and geologists. It was conceived in 1989, following the inception of GIS technology, and first published in 1995 to offer a full account of Italy’s strongest earthquakes, of their territorial impact and associated social and economic upheaval. Subsequent versions (1997, 2000, 2007) entailed a fine tuning of research methodologies, included additional research on Italian earthquakes, and were extended to large earthquakes of the Mediterranean area. CFTI5Med comprised an opportunity to streamline the structure of the Catalogue database and propose a renovated user interface. The new front-end (1) grants an easier, intuitive access to the data, including earthquake effects on the environment, and (2) allows all data to be displayed jointly with relevant topographic, geological and seismological overlays published as web services., Design Type(s)data integration objective • source-based data analysis objectiveMeasurement Type(s)earthquakeTechnology Type(s)digital curationFactor Type(s)geographic location • temporal_intervalSample Characteristic(s)Mediterranean Region Machine-accessible metadata file describing the reported data (ISA-Tab format)

The article by Camassi and Castelli (2013) (hereinafter CC13) deals with the 1346, northern Italy earthquake, one of hundreds of medieval earthquakes that were investigated in Italy from 1983 to 2007. Regrettably, the article does not add any new data but only proposes a revision and a reinterpretation of published materials. CC13 first criticized the variability of magnitude estimates assigned to this earthquake in catalogs published in Italy over the past 20 years, then went so far as to question whether the 1346 earthquake actually occurred. Their analysis, however, is fraught with demonstrable mistakes in the analysis of medieval texts, such that their conclusions are objectionable both from the point of view of historical criticism and from that of historical seismology. Such conclusions may critically affect the assessment of seismic hazard in a heavily populated and industrialized portion of the Po Plain (northern Italy), right at a time when the threat posed by strong earthquakes in this region is being rediscovered by the citizens and by their administrators following the 20 and 29 May 2012, Emilia events ( M w 6.0 and 5.9). As the coauthors of the catalogs being questioned, we feel an obligation to re‐establish what is the evidence supporting 1346 being a real major earthquake and why its magnitude is still uncertain. CC13 stated that three successive versions (Boschi et al. , 1997, 2000; Guidoboni et al. , 2007) of the Catalogo dei Forti Terremoti in Italia (CFTI), referred to as an authoritative study, “…did noticeably alter the epicentral location and estimated energy of the 1346 earthquake, but without explaining why these alterations had become necessary…”. First, we must recall that all of these studies, as well as the first version of CFTI (Boschi et al. , 1995), are not specific studies of the 1346 earthquake, as it might …

To investigate the transfer of elastic energy between a regional stress field and a set of localized faults we project the stress rate tensor inferred from the Italian GNSS velocity field onto faults selected from the Database of Individual Seismogenic Sources (DISS 3.2.0). For given Lamé constants and friction coefficient we compute the loading rate on each fault in terms of the Coulomb Failure Function (CFF) rate. By varying the strike, dip and rake angles around the nominal DISS values, we also estimate the geometry of planes that are optimally oriented for maximal CFF rate. Out of 86 Individual Seismogenic Sources (ISSs), all well covered by GNSS data, 78 to 81 (depending on the assumed friction coefficient) load energy at a rate of 0-4 kPa yr-1. The faults displaying larger CFF rates (4 to 6 ± 1 kPa yr-1) are located in the central Apennines and are all characterized by a significant strike-slip component. We also find that the loading rate of 75% of the examined sources is less than 1 kPa yr-1 lower than that of optimally oriented faults. We also analyzed the 24 August and 30 October 2016, central Apennines earthquakes (Mw 6.0-6.5 respectively). The strike of their causative faults based on seismological and tectonic data and the geodetically inferred strike differ by < 30°. Some sources exhibit a strike oblique to the direction of maximum strain rate, suggesting that in some instances the present-day stress acts on inherited faults. The choice of the friction coefficient only marginally affects this result.

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 preinstrumental times in full, including fore-, mainand 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 forced historical seismologists to focus only on mainshocks rather than also on the foreand aftershocks. To shed light onto major earthquake sequences of the past, rather than onto individual mainshocks, we investigated 10 damaging earthquake sequences (Mw 4.7-7.0) that hit the L’Aquila area and central Abruzzo from the XIV to the XX century. We find that most of the results of historical research are crucial for modern seismology, yet their rendering by the current parametric catalogues causes most information to be lost, or not easily trasnferred 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.

Large earthquakes occur rather orderly in space and time; hence they can be somehow anticipated, and their effects can be projected into the future. The modern practice of seismic hazard assessment rests on these principles and may rely on them, but also requires a detailed knowledge of the location and characteristics of individual earthquake sources. We discuss how this knowledge base can be constructed, with an eye on the geological history, which provides a record of the faults capable of generating large earthquakes, and one on the human history, which supplies evidence for whether, where and how these faults have actually caused damaging earthquakes in the past. How do these two records interact with each other? It is now accepted that identifying active and potentially seismogenic faults in Italy is especially hard. The geological record may be clear and honest when dealing with processes at the scale of several million years, but can be very difficult to decipher if we are concerned with contemporary geological processes, such as earthquakes. Shortening the tim E-W indow of observation of earthquake activity is why Historical Seismology is so crucial for constructing a seismogenic source model. To this end we exploited a number of key Italian destructive earthquakes, each of which illuminates a recent geological process that may not offer a discernible surface signature. Our findings led us to reconsider the tectonic style of large areas, changed our perception of their earthquake potential, hinted at the existence of unknown seismogenic zones, and even led to downsizing the magnitude of the largest Italian historical earthquakes. We maintain that the complexity of the geological setting may be counterbalanced by the richness of the historical earthquake record. We also believe that our experience in the combined investigation of Italy’s historical earthquakes and seismogenic sources may be replicated in all earthquake-prone countries that exhibit a history of human settlement throughout the millennia.

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 thus 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 Individual 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.

Over the past few years the assessment of the earthquake potential of large continental faults has increasingly relied on field investigations. State-of-the-art seismic hazard models are progressively complementing the information derived from earthquake catalogs with geological observations of active faulting. Using these observations, however, requires full understanding of the relationships between seismogenic slip at depth and surface deformation, such that the evidence indicating the presence of a large, potentially seismogenic fault can be singled out effectively and unambiguously. We used observations and models of the 6 April 2009, Mw 6.3, L'Aquila, normal faulting earthquake to explore the relationships between the activity of a large fault at seismogenic depth and its surface evidence. This very well-documented earthquake is representative of mid-size yet damaging earthquakes that are frequent around the Mediterranean basin, and was chosen as a paradigm of the nature of the associated geological evidence, along with observational difficulties and ambiguities. Thanks to the available high-resolution geologic, geodetic and seismological data aided by analog modeling, we reconstructed the full geometry of the seismogenic source in relation to surface and sub-surface faults. We maintain that the earthquake was caused by seismogenic slip in the range 3–10 km depth, and that the slip distribution was strongly controlled by inherited discontinuities. We also contend that faulting was expressed at the surface by pseudo-primary breaks resulting from coseismic crustal bending and by sympathetic slip on secondary faults. Based on our results we propose a scheme of normal fault hierarchization through which all surface occurrences related to faulting at various depths can be interpreted in the framework of a single, mechanically coherent model. We stress that appreciating such complexity is crucial to avoiding severe over- or under-estimation of the local seismogenic potential.

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 (M w 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/).

The 31 October and 1 November 2002 Molise earthquakes (both M w 5.7) were caused by right-lateral slip between 12 and 20 km depth. These earthquakes are the result of large-scale reactivation of preexisting, left-lateral, regionally extended east–west structures of Mesozoic age. Although recorded ground motions were generally smaller than expected for typical Italian earthquakes, a recent paper attributes a stress drop as high as 180 bars to the Molise earthquakes. We remark that a high stress drop is in contrast both with the relatively long source duration inferred in previous investigations and with geodetic evidence for a significantly smaller fault slip compared with other Apennine earthquakes having similarly large rupture area (e.g., 1997 Umbria–Marche earthquakes). We analyzed both ground acceleration spectra of the mainshocks and single-station spectral ratios of broadband seismograms in an extended magnitude range (2.7≤ M w≤5.7). Our results show that neither the spectral amplitudes of recorded ground motions nor the spectral ratios can be fit by a high stress-drop source. Instead we find that the observations are consistent with a low stress drop, our best estimates ranging between 6 and 25 bars, in agreement with the relatively long source duration and small coseismic slip. We interpret the low stress of the 2002 Molise earthquakes in terms of lower energy release mechanisms due to the reutilization of faults reactivated opposite to their original sense of slip.

The outermost, NE-verging fronts of the Northern Apennines (Italy) are overlain by a thick syntectonic sedimentary wedge filling up the basin beneath the Po Plain. Due to fast sedimentation rates and comparatively low tectonic rates, the fronts are generally buried. Evidence for their activity includes scattered historical and instrumental earthquakes and drainage anomalies controlled by growing buried anticlines. The largest earthquakes, up to M w 5.8, are associated with active compression, with a GPS-documented shortening rate

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 (Figure 1). 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. Of the population at the time, estimated to number 140,000 at Messina and 45,000 at Reggio Calabria, a significant fraction was reported dead. Assessing the total number of victims has been problematic, as fatality estimates range from 60,000 to more than 100,000, yet 1908 was undeniably the deadliest European earthquake ever and one of the deadliest worldwide. Guidoboni et al. (2007) contend that 80,000 people were killed by the earthquake, including as many as 2,000 who died as a result of the tsunami. Waves up to 12 m struck the shorelines south of Messina and south of Reggio Calabria, displacing the rubble from collapsed buildings and completing the destruction. All communications in the affected area were disrupted, and rescue operations had to rely on access from the sea. Units of the Russian and English navies, already in the area, were the first to offer immediate relief. In particular, medical officers of the Baltic Guard-Marine brought the first medical aid to the …

We calculated the expected impact on the Italian coast of the Adriatic Sea of a large set of tsunamis resulting from potential earthquakes generated by major fault zones. Our approach merges updated knowledge on the regional tectonics and scenario-like calculations of expected tsunami impact. We selected six elongated potential source zones. For each of them we determined a Maximum Credible Earthquake and the associated Typical Fault, described by its size, geometry and kinematics. We then let the Typical Fault float along strike of its parent source zone and simulated all tsunamis it could generate. Simulations are based on the solution of the nonlinear shallow water equations through a finite-difference technique. For each run we calculated the wavefields at specified simulation times and the maximum water height field (above mean sea level), then generated travel-time maps and maximum water height profiles along the target coastline. Maxima were also classified in a three-level code of expected tsunami threat. We found that the southern portion of Apulia facing Albania and the Gargano promontory are especially prone to the tsunami threat. We also found that some bathymetric features are crucial in determining the focalization-defocalization of tsunami energy. We suggest that our results be taken into account in the design of early-warning strategies.

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.

On 16 December 1857, a strong earthquake ( M ∼7) struck a large portion of the southern Apennines about 150 km southeast of Naples. The earthquake was thoroughly investigated by Irish engineer Robert Mallet, who wrote an extensive report that is still regarded as a landmark in observational seismology. Because of the concentration of damage in the High Agri valley, and contrary to Mallet’s own findings, for many years the earthquake was referred to as the “Val d’Agri earthquake” and was believed by most investigators to have ruptured the 20–25-km normal fault lying beneath this intermontane basin. The magnitude of the earthquake, however, and evidence for earthquake complexity suggest that the true rupture length has been so far underestimated. We contend that the 1857 earthquake ruptured in a cascade fashion two adjacent and relatively well-known faults: first the smaller Melandro–Pergola fault, commonly believed to represent a seismic gap between the causative faults of the 1857 and of the 1980 Irpinia ( M w 6.9) earthquakes; and then the larger Agri valley fault proper. Contemporary chronicles reported a time lapse of 2–3 min between the two ruptures, thus effectively making them two independent shocks. The rupture must have proceeded unilaterally from the northwestern edge of the Melandro–Pergola fault, where Mallet placed the earthquake epicenter, thus explaining the concentration of damage—and attention by rescue crews and subsequent investigators—in the southeastern portion of the High Agri valley.

We reconstruct the tectonic framework of the 24 August 2016, Amatrice earthquake. At least three main faults, including an older thrust fault (Sibillini Thrust), played an active role in the sequence. The mainshock nucleated and propagated along an extensional fault located in the footwall of the Sibillini Thrust, but due to the preliminary nature of the data the role of this thrust is still unclear. We illustrate two competing solutions: 1) the coseismic rupture started along an extensional fault and then partially used the thrust plane in extensional motion; 2) the thrust fault acted as an upper barrier to the propagation of the mainshock rupture, but was partially reactivated during the aftershock sequence. In both cases our tectonic reconstruction suggests an active role of the thrust fault, providing yet another example of how structures inherited from older tectonic phases may control the mainshock ruptures and the long-term evolution of younger seismogenic faults.

We use wet-clay analog models to investigate how pre-existing discontinuities (i.e. structures inherited from previous tectonic phases) affect the evolution of a normal fault at the Earth's surface. To this end we first perform a series of three reference experiments driven by a 45° dipping master fault unaffected by pre-existing discontinuities to generate a mechanically isotropic learning set of models. We then replicate the experiment six times introducing a 60°-dipping precut in the clay cake, each time with a different attitude and orientation with respect to an initially-blind, 45°-dipping, master normal fault. In all experiments the precut intersects the vertical projection of the master fault halfway between the center and the right-hand lateral tip. All other conditions are identical for all seven models. By comparing the results obtained from the mechanically isotropic experiments with results from experiments with precuts we find that the surface evolution of the normal fault varies depending on the precut orientation. In most cases the parameters of newly-forming faults are strongly influenced. The largest influence is exerted by synthetic and antithetic discontinuities trending respectively at 30° and 45° from the strike of the master fault, whereas a synthetic discontinuity at 60° and an antithetic discontinuity at 30° show moderate influence. Little influence is exerted by a synthetic discontinuity at 45° and an antithetic discontinuity at 60° from the strike of the master fault. We provide a ranking chart to assess fault-to-discontinuity interactions with respect to essential surface fault descriptors, such as segmentation, vertical-displacement profile, maximum displacement, and length, often used as proxies to infer fault properties at depth. Considering a single descriptor, the amount of deviation induced by different precuts varies from case to case in a rather unpredictable fashion. Multiple observables should be taken into consideration when analyzing normal faults evolving next to pre-existing discontinuities.

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 increasing 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.

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.

Five large earthquakes shook southern Calabria in February–March 1783. We focused on the first shock (Me 6.9), which occurred on 5 February in the Gioia Tauro Plain. Most investigators attribute the event to a W-dipping, high-angle fault running at the base of the Aspromonte crystalline bedrock on the ESE side of the Plain (Aspromonte Fault). Other workers contend that the earthquake was generated by an E-dipping, low-angle blind fault (Gioia Tauro Fault) similar to the adjacent Messina Straits Fault. In 1999–2000 we carried out four geochemical surveys in the Gioia Tauro Plain with the aim of contributing to this debate with an independent line of evidence. We sampled 240 groundwater sites and measured a suite of in-situ physical and chemical parameters. Our goal was to gain new insight into the seismogenic source by identifying geochemical anomalies associated with the deepening of the hydrological circuits due to the presence of enhanced faulting/fracturing. The deep-fluid signatures are mainly represented by temperature, salinity, total carbon and radon anomalies. We identified three zones of dominant deep fluid discharge: the Nicotera-Galatro area (along the Nicotera-Galatro portion of the NW-trending Nicotera-Gioiosa Jonica lineament), a small NW-SE trending area between Gioia Tauro and Seminara, and the coastline between Rosarno and Palmi. This latter sector locates just above the upper edge of the hypothesised Gioia Tauro Fault. Most of the geochemical anomalies are recorded around Rosarno, at the intersection between the Gioia Tauro Fault and the Nicotera-Gioiosa Jonica lineament. In contrast, no evidence of groundwater deepening and active fracturing was found along the Aspromonte Fault. Based on our new findings we updated the concepts of Geochemically Active Fault Zone and Geochemical Interaction Fault Zone in view of the modern understanding of the hydro-mechanical properties of fault zones and the faulting mechanisms promoting fracture permeability in the crust.

The Northern Marche coastal belt is characterised by a series of NW-SE trending, NE verging folds forming the easternmost edge of the Apennines thrust front. Several geomorphic features suggest that the folds are still growing and hence that the thrust front is active. The occurrence of several historical and instrumental earthquakes (e.g. 1672, 1690, 1786, 1875, 1916, 1930, 1972, all having Me≥ 5.2) suggests that the thrust faults are also seismogenic.

Two Mw 5.7 earthquakes struck a sparsely populated region of southern Italy, on October 31 and November 1, triggering a swarm-like sequence that lasted for several days. The earthquakes were caused by pure right-lateral slip between 10 and 24 km depth over a nearly vertical, previously undetected east-west fault. This mechanism is not typical for southern Italy, where normal faulting in the uppermost 12 km of the crust seems to dominate. However, east-west strike-slip faulting is kinematically consistent with the widely documented Apennines extension. The earthquake-causative fault appears to connect the Mattinata fault, a major active strike-slip feature cutting across the Gargano promontory, with east-west structures known beneath the axial part of the Apennines. The 2002 earthquakes thus highlighted a mode of earthquake release that may explain several large yet poorly understood historical earthquakes (e.g., 1361, 1456, 1731, 1930) located between the crest of the Apennines and the Adriatic coastline.

SUMMARY We analysed elevation changes induced by the 1997‐1998 Umbria-Marche, central Apennines (Italy) earthquakes. We employed data from a first-order geodetic levelling line measured in 1951, 1992 and 1998. The line bears a record of pre-seismic and coseismic strains associated with the causative fault of the 1997 September 26, 09:40 mainshock (Mw = 6.0). A first level analysis performed under the assumption of slip homogeneity of coseismic slip shows misfits that cannot be reduced simply by altering the fault size and geometry. A more detailed analysis based on a distribution of coseismic slip obtained from broad-band seismograms provides a better fit and is in agreement with 1951‐1992 elevation changes interpreted as precursory slip by previous investigators. The levelling data sets new constraints on the location, extent, dip and depth of the fault, in full agreement with seismological evidence and images from SAR interferometry. The data show no evidence for slip in the uppermost 3 km of the crust, suggesting that a major and widely recognized normal fault that exists in the area is no longer active and showing a tendency of present tectonic strains to revert the current topographic setting.

Several mountainous regions are currently affected by syn- or post-orogenic active extension. We investigate how a newly-formed normal fault interacts with structures inherited from a previous contractional phase. To this end, we use analog models that adopt an innovative technique for performing a precut that mimics such inherited structures into a clay layer; this clay layer is laid on top of a master fault simulated by two rigid blocks sliding along an inclined plane. We carry out six experiments with variously oriented precuts and compare the results with those obtained in a reference isotropic experiment. All other conditions are identical for all seven realizations. Fault evolution is monitored by taking closely-spaced snapshots analyzed through the Digital Image Correlation method. We find that the upward propagation of the normal fault can be either accelerated or decelerated depending on the presence of a precut and its orientation. Such precuts can also promote or inhibit the formation of bending-moment faults. These interactions between master fault and precut also affect the shape of the fault-related syncline–anticline pair.