HypoDD relocated seismicity in northern Apennines (Italy) preceding the 2013 seismic unrest: seismotectonics implications for the Lunigiana-Garfagnana area

We present the results of a study aimed at defining the geometry and kinematics of seismogenic volumes and structures of the Lunigiana-Garfagnana region (northwestern Apennines) as depicted by background seismicity recorded before the seismic crisis of 2013. In this analysis we profited from earthquakes located with the high precision algorithm HypoDD and the availability of a large set of focal mechanisms. The obtained data set of well-located hypocentres allowed us to define some previouslyunknown, or only poorly-defined, geometric characteristics. We also confirmed, with a finer detail, some already-known first order features such as the presence of two NW-SE-trending zones of seismicity, west and east of the Apennine water divide, separated by a low seismicity corridor. The main findings of this study are: 1) most of the seismicity of the western zone is located in the Lunigiana graben, north-NW of the Apuane Alps; 2) at depth, the Lunigiana seismicity deepens to the east parallel to the top of the basement, which in turn coincides with an extensional detachment (~30° E-dipping); and 3) the Lunigiana seismicity terminates southwards with a dense cluster of epicentres oriented nearly E-W, parallel to the transfer fault zone that delimits the Apuane Alps to the north; south of this cluster, a strong reduction of seismicity is observed and the locations are shifted to the eastern sector. These findings might help in interpreting the seismotectonics of the 1481, 1837, 1920 and 1995 earthquakes, all located within the E-W-trending cluster at the southern termination of the Lunigiana seismicity

Width of surface rupture zone for thrust earthquakes: implications for earthquake fault zoning

Abstract. The criteria for zoning the surface fault rupture hazard (SFRH) along thrust faults are defined by analysing the characteristics of the areas of coseismic surface faulting in thrust earthquakes. Normal and strike–slip faults have been deeply studied by other authors concerning the SFRH, while thrust faults have not been studied with comparable attention. Surface faulting data were compiled for 11 well-studied historic thrust earthquakes occurred globally (5.4 ≤ M ≤ 7.9). Several different types of coseismic fault scarps characterize the analysed earthquakes, depending on the topography, fault geometry and near-surface materials (simple and hanging wall collapse scarps, pressure ridges, fold scarps and thrust or pressure ridges with bending-moment or flexural-slip fault ruptures due to large-scale folding). For all the earthquakes, the distance of distributed ruptures from the principal fault rupture (r) and the width of the rupture zone (WRZ) were compiled directly from the literature or measured systematically in GIS-georeferenced published maps. Overall, surface ruptures can occur up to large distances from the main fault ( ∼ 2150 m on the footwall and  ∼  3100 m on the hanging wall). Most of the ruptures occur on the hanging wall, preferentially in the vicinity of the principal fault trace ( >   ∼  50 % at distances  <   ∼  250 m). The widest WRZ are recorded where sympathetic slip (Sy) on distant faults occurs, and/or where bending-moment (B-M) or flexural-slip (F-S) fault ruptures, associated with large-scale folds (hundreds of metres to kilometres in wavelength), are present. A positive relation between the earthquake magnitude and the total WRZ is evident, while a clear correlation between the vertical displacement on the principal fault and the total WRZ is not found. The distribution of surface ruptures is fitted with probability density functions, in order to define a criterion to remove outliers (e.g. 90 % probability of the cumulative distribution function) and define the zone where the likelihood of having surface ruptures is the highest. This might help in sizing the zones of SFRH during seismic microzonation (SM) mapping. In order to shape zones of SFRH, a very detailed earthquake geologic study of the fault is necessary (the highest level of SM, i.e. Level 3 SM according to Italian guidelines). In the absence of such a very detailed study (basic SM, i.e. Level 1 SM of Italian guidelines) a width of  ∼  840 m (90 % probability from "simple thrust" database of distributed ruptures, excluding B-M, F-S and Sy fault ruptures) is suggested to be sufficiently precautionary. For more detailed SM, where the fault is carefully mapped, one must consider that the highest SFRH is concentrated in a narrow zone,  ∼ 60 m in width, that should be considered as a fault avoidance zone (more than one-third of the distributed ruptures are expected to occur within this zone). The fault rupture hazard zones should be asymmetric compared to the trace of the principal fault. The average footwall to hanging wall ratio (FW  :  HW) is close to 1  :  2 in all analysed cases. These criteria are applicable to "simple thrust" faults, without considering possible B-M or F-S fault ruptures due to large-scale folding, and without considering sympathetic slip on distant faults. Areas potentially susceptible to B-M or F-S fault ruptures should have their own zones of fault rupture hazard that can be defined by detailed knowledge of the structural setting of the area (shape, wavelength, tightness and lithology of the thrust-related large-scale folds) and by geomorphic evidence of past secondary faulting. Distant active faults, potentially susceptible to sympathetic triggering, should be zoned as separate principal faults. The entire database of distributed ruptures (including B-M, F-S and Sy fault ruptures) can be useful in poorly known areas, in order to assess the extent of the area within which potential sources of fault displacement hazard can be present. The results from this study and the database made available in the Supplement can be used for improving the attenuation relationships for distributed faulting, with possible applications in probabilistic studies of fault displacement hazard

Seismogenesis in Central Apennines, Italy: an integrated analysis of minor earthquake sequences and structural data in the Amatrice-Campotosto area

We present a seismotectonic study of the Amatrice-Campotosto area (Central Italy) based on an integrated analysis of minor earthquake sequences, geological data and crustal rheology. The area has been affected by three small-magnitude seismic sequences: August 1992 (M=3.9), June 1994 (M=3.7) and October 1996 (M=4.0). The hypocentral locations and fault plane solutions of the 1996 sequence are based on original data; the seismological features of the 1992 and 1994 sequences are summarised from literature. The active WSWdipping Mt. Gorzano normal fault is interpreted as the common seismogenic structure for the three analysed sequences. The mean state of stress obtained by inversion of focal mechanisms (WSW-ENE-trending deviatoric tension) is comparable to that responsible for finite Quaternary displacement, showing that the stress field has not changed since the onset of extensional tectonics. Available morphotectonic data integrated with original structural data show that the Mt. Gorzano Fault extends for ~28 km along strike. The along-strike displacement profile is typical of an isolated fault, without significant internal segmentation. The strong evidence of late Quaternary activity in the southern part of the fault (with lower displacement gradient) is explained in this work in terms of displacement profile readjustment within a fault unable to grow further laterally. The depth distribution of seismicity and the crustal rheology yield a thickness of ~15 km for the brittle layer. An area of ~530 km2 is estimated for the entire Mt. Gorzano Fault surface. In historical times, the northern portion of the fault was probably activated during the 1639 Amatrice earthquake (I = X, M~ 6.3), but this is not the largest event we expect on the fault. We propose that a large earthquake might activate the entire 28 km long Mt. Gorzano Fault, with an expected Mmax up to 6.7

Width of surface rupture zone for thrust earthquakes: implications for earthquake fault zoning

The criteria for zoning the surface fault rupture hazard (SFRH) along thrust faults are defined by analysing the characteristics of the areas of coseismic surface faulting in thrust earthquakes. Normal and strike–slip faults have been deeply studied by other authors concerning the SFRH, while thrust faults have not been studied with comparable attention. Surface faulting data were compiled for 11 well-studied historic thrust earthquakes occurred globally (5.4 ≤ M ≤ 7.9). Several different types of coseismic fault scarps characterize the analysed earthquakes, depending on the topography, fault geometry and near-surface materials (simple and hanging wall collapse scarps, pressure ridges, fold scarps and thrust or pressure ridges with bending-moment or flexural-slip fault ruptures due to large-scale folding). For all the earthquakes, the distance of distributed ruptures from the principal fault rupture (r) and the width of the rupture zone (WRZ) were compiled directly from the literature or measured systematically in GIS-georeferenced published maps. Overall, surface ruptures can occur up to large distances from the main fault ( ∼ 2150 m on the footwall and  ∼  3100 m on the hanging wall). Most of the ruptures occur on the hanging wall, preferentially in the vicinity of the principal fault trace ( &gt;   ∼  50 % at distances  &lt;   ∼  250 m). The widest WRZ are recorded where sympathetic slip (Sy) on distant faults occurs, and/or where bending-moment (B-M) or flexural-slip (F-S) fault ruptures, associated with large-scale folds (hundreds of metres to kilometres in wavelength), are present. A positive relation between the earthquake magnitude and the total WRZ is evident, while a clear correlation between the vertical displacement on the principal fault and the total WRZ is not found. The distribution of surface ruptures is fitted with probability density functions, in order to define a criterion to remove outliers (e.g. 90 % probability of the cumulative distribution function) and define the zone where the likelihood of having surface ruptures is the highest. This might help in sizing the zones of SFRH during seismic microzonation (SM) mapping. In order to shape zones of SFRH, a very detailed earthquake geologic study of the fault is necessary (the highest level of SM, i.e. Level 3 SM according to Italian guidelines). In the absence of such a very detailed study (basic SM, i.e. Level 1 SM of Italian guidelines) a width of  ∼  840 m (90 % probability from "simple thrust" database of distributed ruptures, excluding B-M, F-S and Sy fault ruptures) is suggested to be sufficiently precautionary. For more detailed SM, where the fault is carefully mapped, one must consider that the highest SFRH is concentrated in a narrow zone,  ∼ 60 m in width, that should be considered as a fault avoidance zone (more than one-third of the distributed ruptures are expected to occur within this zone). The fault rupture hazard zones should be asymmetric compared to the trace of the principal fault. The average footwall to hanging wall ratio (FW  :  HW) is close to 1  :  2 in all analysed cases. These criteria are applicable to "simple thrust" faults, without considering possible B-M or F-S fault ruptures due to large-scale folding, and without considering sympathetic slip on distant faults. Areas potentially susceptible to B-M or F-S fault ruptures should have their own zones of fault rupture hazard that can be defined by detailed knowledge of the structural setting of the area (shape, wavelength, tightness and lithology of the thrust-related large-scale folds) and by geomorphic evidence of past secondary faulting. Distant active faults, potentially susceptible to sympathetic triggering, should be zoned as separate principal faults. The entire database of distributed ruptures (including B-M, F-S and Sy fault ruptures) can be useful in poorly known areas, in order to assess the extent of the area within which potential sources of fault displacement hazard can be present. The results from this study and the database made available in the Supplement can be used for improving the attenuation relationships for distributed faulting, with possible applications in probabilistic studies of fault displacement hazard

Fault2SHA Central Apennines database and structuring active fault data for seismic hazard assessment

We present a database of field data for active faults in the central Apennines, Italy, including trace, fault and main fault locations with activity and location certainties, and slip-rate, slip-vector and surface geometry data. As advances occur in our capability to create more detailed fault-based hazard models, depending on the availability of primary data and observations, it is desirable that such data can be organized in a way that is easily understood and incorporated into present and future models. The database structure presented herein aims to assist this process. We recommend stating what observations have led to different location and activity certainty and presenting slip-rate data with point location coordinates of where the data were collected with the time periods over which they were calculated. Such data reporting allows more complete uncertainty analyses in hazard and risk modelling. The data and maps are available as kmz, kml, and geopackage files with the data presented in spreadsheet files and the map coordinates as txt files. The files are available at: https://doi.org/10.1594/PANGAEA.922582

A multidisciplinary approach to the seismotectonics of the Lunigiana and Garfagnana extensional basins (Northern Tuscany, Italy)

New geological and morpho-structural surveys are integrated with seismological and seismic reflection data in order to define the active and potentially seismogenic sources of the Lunigiana and Garfagnana extensional basins (Tuscany, Northern Italy). The seismogenic role of the E-W-striking transfer fault between the Lunigiana and Garfagnana basins, located at the northern termination of the Apuane metamorphic core, and its possible association with a number of historical earthquakes are also discussed

Fault2SHA Central Apennines database and structuring active fault data for seismic hazard assessment

We present a database of field data for active faults in the central Apennines, Italy, including trace, fault and main fault locations with activity and location certainties, and slip-rate, slip-vector and surface geometry data. As advances occur in our capability to create more detailed fault-based hazard models, depending on the availability of primary data and observations, it is desirable that such data can be organized in a way that is easily understood and incorporated into present and future models. The database structure presented herein aims to assist this process. We recommend stating what observations have led to different location and activity certainty and presenting slip-rate data with point location coordinates of where the data were collected with the time periods over which they were calculated. Such data reporting allows more complete uncertainty analyses in hazard and risk modelling. The data and maps are available as kmz, kml, and geopackage files with the data presented in spreadsheet files and the map coordinates as txt files. The files are available at: https://doi. org/10.1594/PANGAEA.922582

Which fault threatens me most? Bridging the gap between geologic data-providers and seismic risk practitioners

The aim of the Fault2SHA European Seismological Commission Working Group Central Apennines laboratory is to enhance the use of geological data in fault-based seismic hazard and risk assessment and to promote synergies between data providers (earthquake geologists), end users and decision-makers. Here we use the Fault2SHA Central Apennines Database where geologic data are provided in the form of characterized fault traces, grouped into faults and master faults, with individual slip rate estimates. The proposed methodology first derives slip rate profiles for each master fault. Master faults are then divided into distinct sections of length comparable to the seismogenic depth to allow consideration of variable slip rates along master faults and the exploration of multi-fault ruptures in the computations. The methodology further allows exploration of epistemic uncertainties documented in the database (e.g. master fault definition, slip rates) as well as additional parameters required to characterize the seismogenic potential of fault sources (e.g. 3D fault geometries). To illustrate the power of the methodology, in this paper we consider only one branch of the uncertainties affecting each step of the computation procedure. The resulting hazard and typological risk maps allow both data providers and end-users (1) to visualize the faults that threaten specific localities the most, (2) to appreciate the density of observations used for the computation of slip rate profiles, and (3) interrogate the degree of confidence on the fault parameters documented in the database (activity and location certainty). Finally, closing the loop, the methodology highlights priorities for future geological investigations in terms of where improvements in the density of data within the database would lead to the greatest decreases in epistemic uncertainties in the hazard and risk calculations. Key to this new generation of fault-based seismic hazard and risk methodology are the user-friendly open source codes provided with this publication, documenting, step-by-step, the link between the geological database and the relative contribution of each section to seismic hazard and risk at specific localities

Geodetic model of the 2016 Central Italy earthquake sequence inferred from InSAR and GPS data

We investigate a large geodetic data set of interferometric synthetic aperture radar (InSAR)and GPS measurements to determine the source parameters for the three main shocks of the 2016Central Italy earthquake sequence on 24 August and 26 and 30 October (Mw6.1, 5.9, and 6.5,respectively). Our preferred model is consistent with the activation of four main coseismic asperitiesbelonging to the SW dipping normal fault system associated with the Mount Gorzano-Mount Vettore-Mount Bove alignment. Additional slip, equivalent to aMw~ 6.1–6.2 earthquake, on a secondary (1) NEdipping antithetic fault and/or (2) on a WNW dipping low-angle fault in the hanging wall of the mainsystem is required to better reproduce the complex deformation pattern associated with the greatestseismic event (theMw6.5 earthquake). The recognition of ancillary faults involved in the sequencesuggests a complex interaction in the activated crustal volume between the main normal faults and thesecondary structures and a partitioning of strain releas

A database of the coseismic effects following the 30 October 2016 Norcia earthquake in Central Italy

We provide a database of the coseismic geological surface effects following the Mw 6.5 Norcia earthquake that hit central Italy on 30 October 2016. This was one of the strongest seismic events to occur in Europe in the past thirty years, causing complex surface ruptures over an area of >400 km 2. The database originated from the collaboration of several European teams (Open EMERGEO Working Group; about 130 researchers) coordinated by the Istituto Nazionale di Geofisica e Vulcanologia. The observations were collected by performing detailed field surveys in the epicentral region in order to describe the geometry and kinematics of surface faulting, and subsequently of landslides and other secondary coseismic effects. The resulting database consists of homogeneous georeferenced records identifying 7323 observation points, each of which contains 18 numeric and string fields of relevant information. This database will impact future earthquake studies focused on modelling of the seismic processes in active extensional settings, updating probabilistic estimates of slip distribution, and assessing the hazard of surface faulting