Seismogenic Normal Faults Research Articles

Interseismic creep of carbonate-hosted seismogenic normal faults: Insights from central Italy

Understanding fault behavior in carbonates is critical because they represent loci of earthquake nucleation. Models of fault-slip mode generally assume: (1) seismic sliding and aseismic sliding occur in different fault patches, (2) creep is restricted to lithology-controlled weak domains, and (3) rate-weakening patches are interseismically locked. We studied three carbonate-hosted seismogenic normal faults in central Italy by combining (micro)structural and geochemical analyses of fault rocks integrated with new seismic coupling estimates. The (upper bound) seismic coupling was estimated to be ∼0.75, which indicates that at least 25% of the long-term deformation in the study area is released aseismically in the upper crust. Microscopy and electron-backscatter diffraction analyses revealed that whereas the localized principal slip zone records seismic slip (as ultracataclastic material, calcite crystallographic preferred orientation (CPO), and truncated clasts), the bulk fault rock below behaves differently. Cataclasites in massive limestones deform by cataclastic flow, pressure solution, and crystal plasticity, along with CPO development. Foliated tectonites in micritic limestones deform by pressure solution and frictional sliding, with CPO development. We suggest these mechanisms accommodate on-fault interseismic creep. This is consistent with experimental results reporting velocity-strengthening behavior at low slip rates. We present multiscale evidence of coexisting seismic and aseismic slip along the same fault in limestones during the seismic cycle. Our results imply that on-fault aseismic motion must be added to seismic slip to reconcile the long-term deformation rates and that creep is not exclusive to phyllosilicate-bearing units. Our work constitutes a step forward in understanding fault behavior and the seismic cycle in carbonates, and it may profoundly impact future studies on seismogenic potential and earthquake hazard assessment.

The Surface Process and Seismogenic Structure of the 2021 Ms 6.1 Biru, Central Tibet Earthquake

Abstract On 19 March 2021, the Ms 6.1 Biru earthquake occurred in central Tibet. Because of the limited number of stations, researchers have different understandings of the epicenter and the focal mechanism. In addition, when the earthquake occurred in a key location at the intersection of the strike-slip structure and the extensional structure in the Tibetan plateau, no seismogenic fault is associated with this earthquake, according to the known active faults. In this article, the focal mechanism solution, the meizoseismal area, the orientation of the long axis of the isoseismal lines, and the spatial distribution and geometric characteristics of earthquake fissures were obtained based on the Interferometric Synthetic Aperture Radar coseismic deformation field, inversion of the focal mechanism solution, and field investigation. The macroscopic epicenter was located approximately 13.17 km southeast from the instrumental epicenter, and the earthquake occurred in the seismotectonic background of south–north extrusion, east–west extension, and a mixture of strike-slip and normal faulting. The focal mechanism solution is the normal fault rupture of the northeast-trending main nodal plane. The earthquake caused regularly arranged coseismic ground fissures and damaged a myriad of buildings. The maximum intensity was VIII. The long axis of the isoseismal lines was in the northeast–southwest direction, thus showing a distinct hanging-wall effect. The seismogenic structure was a newly discovered northeast-trending normal fault with a partial sinistral component. The newly discovered seismogenic normal fault, together with the normal faults that have been identified in adjacent areas, is vital to the crustal deformation of the central Tibetan plateau.

Assessing active and capable faulting as best practice for post-earthquake reconstruction activities: the Sant’Eutizio Abbey case study, in the epicentral area of the 2016 central Italy seismic sequence

Surface faulting is, together with strong ground shaking, a hazard associated with major earthquake faults. Assessing surface faulting potential of a given active tectonic structure is a fundamental prerequisite to adequately plan the use of territories and to perform new constructions, in order to act practices aimed to mitigate the associated risk. Assessing the surface faulting potential represents also ground for correctly performing re-construction and retrofitting of buildings and infrastructures during post-earthquake activities. We investigated a branch of a major seismogenic normal fault in the central Apennines of Italy, the Campi-Preci fault, along which the monumental Sant’Eutizio Abbey is located. The medieval Abbey is one of the most important cultural/religious edifices of the central Apennines, heavily damaged by the MW 6.5 October 30, 2016, earthquake, focused a few km to the south. Our study, based on field geological, geomorphological and structural survey and trenching investigations revealed that I) the trace of the Campi-Preci active fault branch is not actually located where presently reported in the available literature, II) the supposed morpho-tectonic features (basically, some km-long scarp carved on the Meso-Cenozoic carbonate bedrock), that suggested the presence of the fault segment in the area of the Sant’Eutizio Abbey, are not related to the active fault but are probably associated to a presently inactive reverse fault and III) the Sant’Eutizio Abbey is likely not potentially affected by primary surface faulting. Our work highlights that only a comprehensive multidisciplinary approach allows to correctly assess surface faulting potential in both seismotectonic and engineering perspectives.

Active Faulting and Deep-Seated Gravitational Slope Deformation in Carbonate Rocks (Central Apennines, Italy): A New "Close-Up" View.

Active faulting and deep‐seated gravitational slope deformation (DGSD) are common geological hazards in mountain belts worldwide. In the Italian central Apennines, kilometer‐thick carbonate sedimentary sequences are cut by major active normal faults that shape the landscape, generating intermontane basins. Geomorphological observations suggest that the DGSDs are commonly located in fault footwalls. We selected five mountain slopes affected by DGSD and exposing the footwall of active seismogenic normal faults exhumed from 2 to 0.5 km depth. Field structural analysis of the slopes shows that DGSDs exploit preexisting surfaces formed both at depth and near the ground surface by tectonic faulting and, locally, by gravitational collapse. Furthermore, the exposure of sharp scarps along mountain slopes in the central Apennines can be enhanced either by surface seismic rupturing or gravitational movements (e.g., DGSD) or by a combination of the two. At the microscale, DGSDs accommodate deformation mechanisms similar to those associated with tectonic faulting. The widespread compaction of micro‐grains (e.g., clast indentation), observed in the matrix of both normal faults and DGSD slip zones, is consistent with clast fragmentation, fluid‐infiltration, and congruent pressure‐solution active at low ambient temperatures (<60°C) and lithostatic pressures (<80 MPa). Although clast comminution is more intense in the slip zones of normal faults because of the larger displacement accommodated, we are not able to find microstructural markers that allow us to uniquely distinguish faults from DGSDs.

The Campotosto linkage fault zone between the 2009 and 2016 seismic sequences of central Italy: Implications for seismic hazard analysis

In the last decade central Italy was struck by devastating seismic sequences resulting in hundreds of casualties (i.e., 2009-L′Aquila moment magnitude [Mw] = 6.3, and 2016-Amatrice-Visso-Norcia Mw max = 6.5). These seismic events were caused by two NW-SE−striking, SW-dipping, seismogenic normal faults that were modeled based on the available focal mechanisms and the seismic moment computed during the relative mainshocks. The seismogenic faults responsible for the 2009-L′Aquila Mw = 6.3 (Paganica Fault—PF) and 2016-Amatrice-Visso-Norcia Mw max = 6.5 (Monte Vettore Fault—MVF) are right-stepping with a negative overlap (i.e., underlap) located at the surface in the Campotosto area. This latter was affected by seismic swarms with magnitude ranging from 5.0 to 5.5 during the 2009 seismic sequence and then in 2017 (i.e., a few months later than the mainshocks related with the 2016 seismic sequence). In this paper, the seismogenic faults related to the main seismic events that occurred in the Campotosto Seismic Zone (CSZ) were modeled and interpreted as a linkage fault zone between the PF and MVF interacting seismogenic faults. Based on the underlap dimension, the seismogenic potential of the CSZ is in the order of Mw = 6.0, even in the case that all the faults belonging to the zone were activated simultaneously. This has important implications for seismic hazard assessment in an area dominated by the occurrence of a major NW-SE−striking extensional structure, i.e., the Monte Gorzano Fault (MGF). Mainly due to its geomorphologic expression, this fault has been considered as an active and silent structure (therefore representing a seismic gap) able to generate an earthquake of Mw max = 6.5−7.0. However, the geological evidence provided with this study suggests that the MGF is of early (i.e., pre- to syn-thrusting) origin. Therefore, the evaluation of the seismic hazard in the Campotosto area should not be based on the geometrical characteristics of the outcropping MGF. This also generates substantial issues with earthquake geological studies carried out prior to the recent seismic events in central Italy. More in general, the 4-D high-resolution image of a crustal volume hosting an active linkage zone between two large seismogenic structures provides new insights into the behavior of interacting faults in the incipient stages of connection.

On the rotation and frictional lock-up of normal faults: Explaining the dip distribution of normal fault earthquakes and resolving the low-angle normal fault paradox

Classical rock mechanics predicts that normal faults should form at 60–65°, rotating to 30–40° before locking, so widespread slip at around 20° is considered paradoxical. Furthermore, the dip distribution of normal fault earthquakes has a distinct, unexplained peak at ~45°. For both problems, some combination of low friction, high fluid pressures, stress rotation and efficiency of low-angle slip have been suggested but provide at best a partial solution. A simple quantitative model for normal fault rotation (iterating between slip on faults and distributed strain) predicts that faults spend more time at lower angles. Combining this result with Mohr-Coulomb analysis of the range where seismogenic normal faults should lock up predicts the dip distribution of seismogenic normal faults and matches both the range and 45° peak observed for normal fault earthquakes. As even the lowest dips of normal fault ruptures fall within the limits of fault reactivation, slip on seismogenic low-angle normal faults is not paradoxical. A new Mohr-Griffith solution to the limits of fault reactivation extends the analysis into the top few km, where low-angle slip is best constrained by field observations, and shows that low cohesion normal faults can remain active at <20° without recourse to very low friction, high fluid pressures or stress rotation. This analysis thus resolves the long-standing paradox of slip on low-angle normal faults. Where continued rotation or changes in mechanical properties cause low-angle faults to lock in the shallow subsurface, a new fault propagates upward from the point of lock-up, transferring a hanging wall slice to the footwall, to be rafted up and out as a rider block. By recording the dip/depth of lock-up, riders atop detachments constrain the mechanics of faulting and of lock-up.

3D geological reconstruction of the M. Vettore seismogenic fault system (Central Apennines, Italy): Cross-cutting relationship with the M. Sibillini thrust

The 2016–2017 Amatrice-Norcia seismic sequence was triggered by the reactivation of a complex NNW-SSE trending, WSW-dipping normal fault system cross-cutting the Umbria-Marche fold and thrust belt near M. Vettore. This fault system produced clear and impressive co-seismic ruptures on normal faults in the hangingwall of the M. Sibillini thrust, whereas ruptures in the footwall were observed, but less clear. As a result, a strong controversy exists in the literature about the geometry of the seismogenic faults, their relationships with pre-existing thrusts, and the location of normal-faulting rupture tips. In this work, we present a 3D geological model of the M. Vettore area located between the Castelluccio basin and the outcrop of the M. Sibillini thrust, where the most evident co-seismic ruptures have been observed. The model shows the relationship between the ruptured normal faults and the M. Sibillini thrust, and was constructed using a grid of 14 geological cross-sections parallel and orthogonal to the main structural elements (i.e. normal faults and thrusts) down to a depth of 3 km. The model was built using reference structural surfaces, such as the top of the Early Cretaceous Maiolica Fm., the M. Sibillini thrust and the main seismogenic normal faults belonging to the M. Vettore fault system. The 3D model has allowed us to calculate the vertical cumulative throw distribution for the M. Vettore normal faults. The cumulative geological throw of ca. 1300 m across the normal faults in the proximity of the M. Sibillini thrust indicates that the seismogenic fault system continues into the footwall of the thrust, displacing it in the sub-surface. The results of this study provide important constraints on the cross-cutting relationships between active normal and pre-existing compressional structures in seismically active areas, contributing to a better definition of the faults segmentation, and the related seismic hazard.

Strain variations in a seismogenic normal fault (Baza Sub-basin, Betic Chain): Insights from magnetic fabrics (AMS)

AMS and structural analysis are here applied to study the deformed zone associated with a large-scale, active normal fault in the central Betic Cordillera (Spain), namely the Baza fault system, to determine: i) the kinematics of structures and their relation with fault zone architecture and segmentation degree, ii) the correlation between deformational structures and the different types of magnetic fabrics and iii) the evolution of magnetic fabrics patterns, from sedimentary to shear-related, associated with normal faults. Five outcrops (969 samples) were analysed along the fault trace, which shows different degrees of segmentation along strike and strong localization of deformation along narrow fault zones. A first, main set of magnetic fabric data corroborates the normal kinematics of the Baza fault, showing magnetic lineations parallel to the dip-slip, transport direction. A second, secondary set of magnetic lineations, is parallel to the intersection lineation, and can be related to less intense deformation in the fault rocks. Furthermore, a detailed study (523 samples) of a trench excavated across the fault zone, where two fault splays tend to coalesce in a linkage relay zone indicates that i) lithology and distance to fault planes are two factors that control the development of extension-related magnetic fabrics in weakly deformed sediments, ii) the development of shear-related fabrics in fault zones entails the mechanical rotation of minerals, iii) different orientation of magnetic lineations are related to different intensity of bulk deformation and iv) magnetic lineation is useful to define local deviations of deformation axes produced by changes in the local extension direction (from fault-perpendicular to fault-parallel extension) in the linkage zone between adjacent fault splays.

Seismic Reflection Profiles and Subsurface Geology of the Area Interested by the 2016–2017 Earthquake Sequence (Central Italy)

AbstractStarting from 24 August 2016, a long seismic sequence, including nine Mw &gt; 5.0 earthquakes, struck a wide area of the Central Italy. A large amount of geological, geodetic, and seismological data envisages a complex system of NNW‐SSE trending, seismogenic normal faults. These active tectonic structures are well known at the surface and consistent with previous seismotectonic studies. In order to improve the comprehension of the seismotectonic framework of this seismic sequence, we provide a novel reconstruction of the subsurface geology of the area close to the Norcia Mw 6.5 mainshock (30 October 2016), based on previously unpublished seismic reflection profiles and available geological data. All the data have been synthesized along a 47 km long, WSW‐ENE trending geological cross section, interpreted down to a depth of 12 km. Comparing the subsurface geological model with the available seismological data, we find that the majority of seismicity is confined within the sedimentary sequence and does not penetrate the underlying basement. The basement has been constrained at depths of 8 to 11 km and coincides with the cutoff of the seismicity. We have also traced the trajectories of the seismogenic normal faults activated during this seismic sequence, reconciling the high‐angle (dip&gt;65°) normal faults exposed at the surface, with their angle (dip &lt; 50°) at hypocentral depths. The results of this study may be useful for better understanding the rheological properties of the seismogenic rock volume, as well as the coseismic deformations of the topographic surface observed by geodetic techniques and field mapping.

Active faulting, 3-D geological architecture and Plio-Quaternary structural evolution of extensional basins in the central Apennine chain, Italy

Abstract. The general basin and range Apennine topographic characteristic is generally attributed to the presently active normal fault systems, whose long-term activity (throughout the Quaternary) is supposed to have been responsible for the creation of morphological/structural highs and lows. By coupling field geological survey and geophysical investigations, we reconstructed the 3-D geological model of an inner tectonic basin of the central Apennines, the Subequana Valley, bounded to the northeast by the southern segment of one of the major active and seismogenic normal faults of the Apennines, known as the Middle Aterno Valley–Subequana Valley fault system. Our analyses revealed that, since the late Pliocene, the basin evolved in a double half-graben configuration through a polyphase tectonic development. An early phase, Late Pliocene–Early Pleistocene in age, was controlled by the ENE–WSW-striking and SSE-dipping Avezzano–Bussi fault, that determined the formation of an early depocentre towards the N–NW. Subsequently, the main fault became the NW–SE-striking faults, which drove the formation during the Quaternary of a new fault-related depocentre towards the NE. By considering the available geological information, a similar structural evolution has likely involved three close tectonic basins aligned along the Avezzano–Bussi fault, namely the Fucino Basin, the Subequana Valley, and the Sulmona Basin, and it has been probably experienced by other tectonic basins of the chain. The present work therefore points out the role of pre-existing transverse tectonic structures, inherited by previous tectonic phases, in accommodating the ongoing tectonic deformation and, consequently, in influencing the structural characteristics of the major active normal faults. This has implications in terms of earthquake fault rupture propagation and segmentation. Lastly, the morpho-tectonic setting of the Apennine chain results from the superposition of deformation events whose geological legacy must be considered in a wider evolutionary perspective. Our results testify that a large-scale basin and range geomorphological feature – often adopted for morpho-tectonic and kinematic evaluations in active extensional contexts, as in the Apennines – just led by range-bounding active normal faults may be actually simplistic, as it could not be applied everywhere, owing to peculiar complexities of the local tectonic histories.

Stochastic analysis of earthquake activity in two seismogenic fault systems in Greece.

Possible systematic variations in earthquake occurrence and fluctuations in seismicity behaviourof two seismically active regions in Greece which share common seismotectonic properties, is the aim of this study. Mygdonia graben in northern Greeceis characterized by arather moderate background seismicity, with small earthquakes between 2008-2012 whereas Corinth Gulf in southern Greeceexhibits a constantly high seismicity rate with several seismic activations during the recent instrumental period or before. The statistical approach of seismicity was accomplished, regardingthe magnitude, the inter-event time and distance for recent seismicity as a tool to quantify complex earthquake occurrence ordense spatial andtemporal clustering. For this reason, complete catalogues were compiled for the time period of the study. Probabilistic tests such as the smoothed bootstrap test for modality and bump–hunt were employed in order to unveil the complexity of the probability density function distribution of the above parameters. On the other handspatial earthquake distribution wasalso investigated under the frameof their fractal properties since the fractal coefficientcan largely express the clustering degree of seismicity. The goal of this stochasticanalysisis the quantification of the differentiationin seismicity propertiesinthese two important seismogenic normal fault populations in the back arc Aegean area.

Thermobarometry of metamorphosed pseudotachylyte and associated mylonite: Constraints on dynamic Co-seismic rupture depth attending Caledonian extension, North Norway

The exhumed post-Caledonian Eidsfjord and Fiskfjord extensional shear zones of northern Norway exhibit evidence of coseismic rupture propagating into the ductile crust as evidenced by the presence of mylonitic and metamorphosed pseudotachylyte. Geothermobarometric calculations on garnet-bearing mineral assemblages in mylonitic gneisses associated with mylonitic pseudotachylyte and in metamorphosed pseudotachylyte permit determination of the depth and ambient temperature of seismogenic low-angle ductile extension. Average pressures from Eidsfjord (570±115MPa at ca. 650°C) and Fiskfjord (1120±220MPa at ca. 650°C) correspond to faulting depths of 21±4km and 41±9km, respectively. The Fiskfjord results agree with previous thermobarometric calculations on mylonitic Cpx+Grt-bearing pseudotachylyte at Fiskfjord. The calculated depths are 5–25km below the depth of the standard seismogenic zone. These results demonstrate that low angle normal faults may cut through a large portion of continental crust. This occurrence of mylonitic pseudotachylyte in an extensional crustal setting is most easily explained by dynamic downward rupture into the ductile regime and/or unusually high shear stresses to account for coseismic rupture at such depths, implying a direct connection with a seismogenic normal fault in the upper crust.

An Intense Earthquake Swarm in the Southernmost Apennines: Fault Architecture from High-Resolution Hypocenters and Focal Mechanisms

Abstract Between 2010 and 2013, the Pollino Mountains region (south Italy), already proposed as a seismic gap, was affected by a seismic crisis of more than 5000 small-to-moderate earthquakes (maximum magnitude M L 5.0). Preliminary analyses performed in a previous work highlighted that this activity can be ascribed to normal faulting on north-northwest-trending west-dipping dislocation surfaces consistent with the general seismotectonic frame of the southern Apennines. This work contributes additional data and a more sophisticated analyses that highlight new features of the seismic swarm and support a new interpretation for the study area. We obtained high-precision locations and focal mechanisms using the double-difference method and the cut-and-paste waveform inversion method, respectively. The 3D patterns of hypocenters and focal mechanisms consistently image an ∼10-km-long north-northwest-striking and west-dipping fault zone between 5 and 10 km depth, with predominantly extensional kinematics. The high-resolution data show that this zone broadens from north to south as a result of secondary faulting. The depicted geometry, with preliminary geological observation, leads to the hypothesis of multiple seismogenic normal faults rooted into more regional shallow-dipping detachments inherited from the pre-existing Apennine thrust tectonics. Online Material: Table of estimated focal mechanism parameters.

Influence of calcite decarbonation on the frictional behavior of carbonate-bearing gouge: Implications for the instability of volcanic flanks and fault slip

The impact of heat, readily provided by magma, circulating hot fluids, or rapid fault slip on carbonate substrata is an important factor in determining the flank stability of volcanoes and the mechanical behavior of faults in areas where carbonate lithologies are prevalent. The mineralogy and mechanical properties of carbonate rocks are demonstrably altered by thermally induced decarbonation. While previous studies have considered the role of decarbonation in bulk strength loss at subvolcanic conditions and dynamic weakening during coseismic fault slip, little is known about the effects of decarbonation on the frictional properties of carbonate rocks during both the nucleation and inter-seismic phases. Here, we present results from experiments performed on a portlandite-rich material, a typical hydrous product of the decarbonation reaction. To evaluate frictional strength, stability, and healing at shallow crustal conditions, we sheared gouge layers of this material under saturated conditions at room temperature and at velocities comparable to those involved in earthquake nucleation. Our data indicate that the reaction of calcite to portlandite results in a distinct change in the mechanical behavior of the gouge. Decarbonated shear zones are (1) frictionally weaker at higher normal stresses, (2) more frictionally unstable, and (3) likely to regain their frictional strength more quickly than unaltered calcite-rich zones. The occurrence of portlandite could be key for interpreting the stability of volcanic flanks that root into carbonate substrata and for seismogenic normal faults located within thick sedimentary sequences, and thus provide a better understanding of the hazards they pose.

Graviquakes in Italy

We discuss the mechanics of crustal normal fault-related earthquakes, and show that they represent dissipation of gravitational potential energy (graviquakes) and their magnitude increases with the involved volume (delimited by the seismogenic fault and an antithetic dilated wedge in its hangingwall), and the fault dip. The magnitude increases with the deepening of the brittle–ductile transition (BDT), which in turn enlarges the involved volume. The fault dip seems rather controlled by the static friction of the involved crustal layers. We apply the model to the extensional area of the Italian peninsula, whose geodynamics is controlled by the Alpine and Apennines subduction zones. The latter has a well-developed backarc basin and a large part of the accretionary prism is affected by on-going extensional tectonics, which is responsible for most of peninsular Italy seismicity. Analyzing the seismic record of the Apennines, the length of seismogenic normal faults tends to be at most about 3 times the hypocenter depth. We compile a map of the brittle–ductile transition depth and, assuming a fixed 45° or 60° fault dip and a dilated wedge developed during the interseismic period almost perpendicular to the fault plane, we compute the maximum volume of the hangingwall collapsing at the coseismic stage, and estimate the maximum expected magnitude. Lower magnitude values are obtained in areas with thinner brittle layer and higher heat flow. Moreover, lower magnitude relative to those theoretically expected may occur in areas of higher strain rate where faults may creep faster due to lower frictional values.

A soft linkage between major seismogenic fault systems in the central-southern Apennines (Italy): Evidence from low-magnitude seismicity

We investigate the transfer zone and linkage between divergent extensional seismogenic fault systems on the border amid the central and southern Apennines (central Italy). These regional NW–SE striking sets include large seismogenic sources that caused major historical earthquakes (Mw≤7). The faults in the northern part of the study area dip to the southwest; those in the southern part dip to the northeast. The SW-dipping system (Abruzzi Apennines) terminates with the Aremogna–Cinque Miglia source; the NE-dipping system (southern Apennines) terminates with the Boiano Basin source.To test whether the transfer zone model applies to the central-southern Apennines border, we analyzed and relocated seismicity that occurred from 2007 to 2011 between the Aremogna–Cinque Miglia and Boiano Basin sources, where we expect the transfer zone. Seismicity is made of independent events (MD<3.5) and low-magnitude swarms. West of the Apennines, hypocenters are located within the uppermost 12–13km. Events and swarms that occurred east of the axis affect the 13–25km below. West of the chain, focal mechanisms show T-axes striking ~NNW–SSE. East of the chain, T-axes strike ~NE–SW. This trend is consistent with GPS data. The hypocentral distribution of swarms located between the Aremogna–Cinque Miglia and Boiano Basin sources shows a ~NNE–SSW trend, coincident with part of the Ortona–Roccamonfina Line, a regional transverse lineament.The spatial distribution of seismicity, the geometry and kinematics of active faulting in the region, and the results from previous geophysical studies allow us to contend the existence of a transfer zone between these seismogenic normal fault systems. Our data also allow us to recognize the activity of such transfer along the central part of the Ortona–Roccamonfina Line. We infer that reverse in dip polarity between the two normal fault systems could also result from the passage between the diverse tectonic units composing the border between central and southern Apennines.

Earthquake synchrony and clustering on Fucino faults (Central Italy) as revealed from in situ36Cl exposure dating

AbstractWe recover the Holocene earthquake history of seven seismogenic normal faults in the Fucino system, central Italy. We collected 800 samples from the well‐preserved limestone scarps of the faults and modeled their36Cl concentrations to derive their seismic exhumation history. We found that &gt; 30 large earthquakes broke the faults in synchrony over the last 12 ka. The seven faults released strain at the same periods of time, 12–9 ka, 5–3 ka, and 1.5–1 ka. On all faults, the strain accumulation and release occurred in 3–6 ka supercycles, each included a 3–5 ka phase of slow (≤ 0.5–2 mm/yr) strain accumulation in relative quiescence, followed by a cluster of three to four large earthquakes or earthquake sequences that released most of the strain in &lt; 1–2 ka. The large earthquakes repeated every 0.5 ± 0.3 ka during the paroxysmal phases and every 4.3 ± 0.9 ka between those phases. Earthquakes on the northern faults produced twice larger surface slips (~ 2 m) and had larger magnitudes (Mw 6.2–6.7) than those on the southern faults. On most faults, the relative strain level was found to control the amount of slip and the time of occurrence of the next large earthquake. Faults entered a phase of clustered activity once they had reached a specific relative strain threshold. The Tre Monti fault is identified as the most prone to break over the next century. Our data document earthquake synchrony and clustering at a broader space and time scale than has been reported to date.

Characterization of active fault scarps from LiDAR data: a case study from Central Apennines (Italy)

A high-resolution digital elevation model (DEM, 1 ms spacing) derived from an airborne light detection and ranging (LiDAR) campaign was used in an attempt to characterize the structural and erosive elements of the geometry of the Pettino fault, a seismogenic normal fault in Central Apennines (Italy). Four 90- to 280 m-long fault scarp segments were selected and the surface between the base and the top of the scarps was analyzed through the statistical analysis of the following DEM-derived parameters: altitude, height of the fault scarp, and distance along strike, slope, and aspect. The results identify slopes of up to 40° in faults lower reaches interpreted as fresh faces, 34° up the faces. The Pettino fault maximum long-term slip rate (0.6–1.1 mm/yr) was estimated from the scarp heights, which are up to 12–19 m in the selected four segments, and the age (ca. 18 ka) of the last glacial erosional phase in the area. The combined analysis of the DEM-derived parameters allows us to (a) define aspects of three-dimensional scarp geometry, (b) decipher its geomorphological significance, and (c) estimate the long-term slip rate.

Structural Analysis of Coseismic Normal Fault Zones of the 2011 Mw 6.6 Fukushima Earthquake, Northeast Japan

The 2011 M w 6.6 Fukushima earthquake occurred in Iwaki City, ∼250 km southwest of the epicenter of the 2011 M w 9.0 Tohoku (Japan) earthquake, and produced two subparallel ∼15–23‐km‐long surface rupture zones with distinct displacements along two pre‐existing normal faults: the Shionohira and Yunodake faults, which strike north‐northwest–south‐southeast and northwest–southeast (both dipping southwest), respectively. Field investigations and structural analysis of the coseismic shear zones developed within the Shionohira and Yunodake fault zones, reveal that (1) the coseismic shear zones consist of a fault core that includes a narrow fault gouge zone of <10 cm in width (generally 1–5 cm), a fault breccia zone of <50 cm in width, and a damage zone of 5–50 m in width that is composed of cataclastic rocks, fractures, and subsidiary faults; (2) the foliations characterized by S‐C fabrics developed in the shear zones indicate a dominantly normal fault‐slip sense, consistent with that observed along the coseismic surface ruptures; and (3) veinlet cataclastic rocks composed of unconsolidated fault gouges and fine‐grained materials occur as simple discrete veinlets and multiple, cross‐cutting, interconnected networks of veinlets. These newly documented structural characteristics for two coseismic fault/shear zones and associated cataclastic rocks indicate that the locations of coseismic slip zones associated with the 2011 Fukushima earthquake were controlled by pre‐existing fault/shear zones central to the Shionohira and Yunodake faults, which have repeatedly moved as seismogenic normal faults since the timing of formation of associated cataclastic rocks. Our results demonstrate that the structural analyses of coseismic shear/fault zones provide a powerful tool to study the nature and active faulting history of seismological faults.

Time intervals to assess active and capable faults for engineering practices in Italy

The time span necessary to define a fault as ‘active and capable’ can mainly be derived from the framework of the regulations and the literature produced since the 1970s on risk estimation in engineering planning of strategic buildings. Within this framework, two different lines of thought can be determined, which have mainly developed in the USA. On the one side, there is a tendency to produce ‘narrow’ chronological definitions. This is particularly evident in the regulatory acts for the planning of nuclear reactors. The much more effective second line of thought anchors the chronological definitions of the terms ‘active’ and, therefore ‘capable’, to the concept of ‘seismotectonic domain’. As the domains are different in different regions of the World, the chronological definition cannot be univocal; i.e., different criteria are needed to define fault activity, which will depend on the characteristics of the local tectonic domain and of the related recurrence times of fault activation. Current research on active tectonics indicates that methodological aspects can also condition the chronological choice to define fault activity. Indeed, this practice implies the use of earth science methods, the applications of which can be inherently limited. For example, limits and constraints might be related to the availability of datable sediments and landforms that can be used to define the recent fault kinematic history. For the Italian territory, we consider two main tectonic domains: (a) the compressive domain along the southern margin of the Alpine chain and the northern and northeastern margins of the Apennines, which is characterised by the activity of blind thrusts and reverse faults; and (b) the extensional domain of the Apennines and the Calabria region, which is often manifest through the activity of seismogenic normal and normal-oblique faults. In case (a), the general geomorphic and subsurficial evidence of recent activity suggests that a reverse blind fault or a blind thrust should be considered active and potentially capable if showing evidence of activity during the Quaternary (i.e., over the last 2.6Myr), unless information is available that documents its inactivity since at least the Last Glacial Maximum (LGM) (ca. 20ka). The choice of the LGM period as the minimum age necessary to define fault inactivity is related to practical aspects (the diffusion of the LGM deposits and landforms) and to the evidence that ca. 20kyr to assess fault inactivity precautionarily includes a number of seismic cycles. In the extensional domains of the Apennines and Calabria region, the general geological setting suggests that the present tectonic regime has been active since the beginning of the Middle Pleistocene. Therefore, we propose that a normal fault in the Italian extensional domain should be considered active and capable if it displays evidence of activation in the last 0.8 Myr, unless it is sealed by deposits or landforms not younger than the LGM. The choice of the LGM as the minimum age to ascertain fault inactivity follows the same criteria described for the compressive tectonic domain.