Slip-rate variations over multiple seismic cycles play a fundamental role in controlling the behaviour of active fault systems, as they are linked to spatio-temporal earthquake clustering and can influence the recurrence patterns of adjacent faults. However, processes that produce slip-rate fluctuations are yet to be fully defined. Despite their importance, the physical mechanisms responsible for such slip-rate fluctuations remain only partially understood. In this study, we investigate whether interactions between neighbouring along-strike brittle faults and their underlying viscous shear zones can generate slip-rate variability associated with synchronous earthquake clustering and fault system synchronization. We focus on nine normal faults and related shear zones within the Central Apennines fault system (Italy), arranged in six along-strike fault pairs characterized by different fault spacings and strike geometries. We integrate cosmogenic 36Cl dating of tectonically exhumed fault scarps with numerical modelling of differential stress transfer between interacting fault–shear-zone pairs. The results identify a mechanism capable of producing simultaneous earthquake clusters, driven by the synchronization of high driving stresses within the viscous shear zones beneath the brittle faults. This behaviour is strongly modulated by along-strike fault spacing and strike variations. In settings with closely spaced fault pairs and limited strike variations, earthquake clusters induce positive differential stress variations on neighbouring shear-zones of sufficient magnitude to induce positive slip-rate variations on their overlying brittle faults. This produces positive feedback mechanism that sustains the occurrence of earthquake clusters that will continue to positively load the neighbouring shear zones. These findings provide new insights into fault system dynamics across multiple timescales and have important implications for seismic hazard evaluation.

To undertake fault-based seismic hazard assessment, we need to accurately identify source faults and assess their slip-rate and kinematics through pertinent data collection. For example, converging slip vectors may be used to deduce whether isolated fault strands are connected at depth. Kinematic (slip vector) data can be collected through offset piercing points or from striations preserved on fault scarps. However, striations are surficial features and may therefore be readily eroded and not preserved or visible on degraded scarps. Tensional fracture networks are ubiquitous on bedrock fault scarps and extend deeper into the scarp, and therefore have a greater preservation potential when compared to striations. In this work we characterise fracture-scarp (F-S) lineation patterns across eight faults in Italy (Central Apennines) and Greece (Perachora Peninsula) to explore how these patterns relate to fault plane geometry and slip-vector.Various fracture-scarp (F-S) lineation patterns (including sinistral/dextral en-echelon arrays, slip-parallel/-perpendicular fractures, and conjugate sets) are recognised. These patterns show evidence of progressive growth during exhumation. This suggests F-S lineations formed near the surface as the footwall uplifts, with larger features becoming more connected and smaller ones remaining ‘isolated’. The orientations of F-S lineations align within a pure or Riedel shear geometry where the shear sense is related to the rake of the slip vector. We propose that the observed patterns are controlled by fault plane orientation relative to a 3D strain ellipsoid and the progressive reduction of effective normal stress during footwall exhumation. As fractures form under the same stress regime as striations, they can serve as a kinematic indicator even on highly degraded active fault scarps.

Tectonically raised paleoshorelines have been recently identified along the southern fault scarps of the Mt. Fellino and Roccarainola horst blocks, which are part of the northeastern border of the Campania Plain coastal basin (southern Apennines, Italy). Such horst blocks are bounded to the south by the Polvica Fault, a roughly E-W trending normal fault. The sequence of uplifted paleoshorelines has been studied in detail by integrating geomorphological, structural and stratigraphical analyses to assess the Quaternary uplift of the Mt. Fellino and Roccarainola horst blocks. Yet, the staircase of paleoshorelines is still not chronologically well constrained. Aimed at constraining the uplift history of Mt. Fellino and Roccarainola horst blocks and the rate of activity of the Polvica fault, in this study, we integrate former knowledge on paleoshorelines with a geomorphological analysis to map erosional terraces, that we interpret as remnants of shore platforms. We apply the synchronous correlation method, driven by new and a former Th-230/U-234 dating of calcite veins cutting marine sands, to infer the age of the paleoshorelines and terraces. Based on the synchronous correlation, the mapped paleoshorelines and terraces are correlated with sea-level peaks of the late Early to Late Pleistocene. In particular, the paleoshorelines along the Mt. Fellino ridge are correlated with the Marine Isotope Stage (MIS) 7e and 9c or 11, while the oldest terrace is correlated with the sea-level peak of 980 ka. Using inferred paleoshorelines ages, we estimate the uplift rate of the Polvica Fault footwall. The uplift rate varies from c. 0.2 mm/yr close to the western fault tip up to c. 0.5-0.6 mm/yr in the East, in the Roccarainola block. We combine surface evidence with subsurface data from a shallow well to constrain the vertical throw of the Polvica Fault. A mean fault throw rate of c. 0.4 mm/yr in the last c. 1 Ma is estimated for the central part of the PF. Assuming that the Polvica Fault is still active, we estimate the maximum expected earthquake by means of empirical relationship and obtain a Mw similar to 6.2 value and recurrence interval value of c. 1,200 yr. Historical seismicity activity of the PF has not been acknowledged to date. However, our results raise the crucial question of an in-depth assessment of the seismic hazard for the densely populated Campania Plain.

We present slip versus time histories derived from in situ 36 Cl cosmogenic dating for three active normal faults in the southern Apennines, Italy. In this region the total extensional strain is accommodated by either a small number of faults located across strike from each other or, in places, a single fault where no other active faults exist across strike. We investigate how strain‐rates on individual faults vary through time in the context of the overall geometry of the fault system. The 36 Cl results confirm that the San Gregorio Magno, Auletta, and Vallo di Diano faults were active in the Holocene, with each fault exhibiting alternating periods of relatively rapid and slow, or even absence of, slip. During periods of rapid slip, lasting a few millennia, the faults accumulate up to ∼5 m of slip, which we interpret as earthquake clusters. At other times, the faults exhibit no slip for time periods lasting multiple millennia. The fluctuations in slip‐rates reveal the migration of activity between faults and out‐of‐phase behavior. Such fluctuations have important consequences for tectonic evolution and crustal rheology, and in particular for hazard estimation because they introduce considerable variability and hence uncertainty in earthquake probability calculations.

We report the first example where the timing of earthquake slip from in situ 36Cl cosmogenic exposure dating of an active normal fault scarp can be verified using independently 14C dated Holocene coastal notches which are deformed along the strike of the fault. We have remodelled 36Cl data from the active Pisia-Skinos normal fault, Greece, published by Mechernich et al. (2018), which indicates that the fault slip rate fluctuated through time. We model the expected coastal uplift and subsidence induced by slip on the fault using elastic half-space models and surface ruptures observed following the 1981 Pisia-Skinos earthquakes. Coastal uplift is constrained by elevation measurements of Holocene coastal notches that have previously been dated using 14C by Pirazzoli et al. (1994) and agree with time periods consistent with Holocene climate stability. We mapped the elevations and numbers of notches along the strike of the Pisia-Skinos fault, including measurements made underwater for locations where fault slip has submerged the notches below the present-day shoreline. We show that the spatial patterns and timing of uplift and subsidence from the notches agrees with the timing of periods of high slip associated with earthquake clusters and quiescence associated with anti-clusters from the slip histories derived from 36Cl data, and with the uplift and subsidence derived from elastic half-space modelling. In particular, where modelled subsidence is highest, Holocene notches that formed between 6-2 ka can be preserved but are submerged. Notches could form at this time because the 36Cl data show that the Pisia fault had entered a period of relative quiescence with a slip-rate of

We investigate the Coulomb stress changes due to 30 strong earthquakes occurring on normal faults since 1509 A.D. in Calabria, Italy, including the influence of both coseismic and interseismic loading in our modelling. We compare the results to existing studies of stress interaction from the Central and Southern Apennines, Italy. The three normal fault systems have different geometries and long-term slip-rates. The Central Apennines hosts a complex fault system, with many faults across strike, so that when an earthquake occurs, many of the surrounding faults experience a stress decrease. The Southern Apennines and Calabria have a simpler geometry, with fewer faults, and faults are located predominantly along strike, therefore when an earthquake occurs the dominant process on the neighbouring faults is stress increase. We investigate how stress transfer may influence the occurrence of future earthquakes and what factors may govern the variability in earthquake recurrence in different fault systems. Within the analysed time period, the Calabrian, Central Apennines, and Southern Apennines fault systems have 91%, 73% and 70% of faults with a mean positive cumulative Coulomb stress change, respectively; this is due to fewer faults across strike, more across strike stress reductions, and greater along-strike spacing in the three regions respectively. In regions with close along strike spacing or few faults across strike, such as Calabria and Southern Apennines, the stress loading history is mostly dominated by interseismic loading and most faults are positively stressed before an earthquake occur on them (96% of all faults that ruptured in Calabria; 94% of faults in the Southern Apennines), and some of the strongest earthquakes occur on faults with the highest mean cumulative stress of all faults prior to the earthquake. In the Central Apennines, where across strike interactions are the predominant process, 79% of the earthquakes occur on faults that are positively stressed. The results highlight that fault system geometry plays a central role in characterizing the stress evolution associated with earthquake recurrence, and can possibly influence the occurrence of propagating triggered earthquake sequences.

We present an in situ 36Cl dataset recording the exhumation of 27 active normal fault planes by earthquake slip for the central Apennines, Italy. We do this to constrain the characteristics of earthquake clustering and anticlustering across the entire extending orogen, and in an attempt to constrain the reasons why clustering and anticlustering occurs. We show that duration and magnitude of clustering and anticlustering, and their characteristics, can be explained by a model where the transfer of differential stress between faults and their underlying shear-zones, and between neighbouring fault/shear-zone structures, produces changes in strain-rates on underlying viscous shear zones which drive periods of rapid or reduced slip-rate on their overlying faults. We suggest that stress increase on an underlying shear zone produced by coseismic slip on its overlying fault could be the mechanism that initiates an earthquake cluster. We suggest that stress reductions on shear-zones from coseismic slip located across strike could be the mechanism that initiates an earthquake anticluster. The durations of anticlusters are controlled by the summed stress decreases through time on shear zones, because although these shear zones are slipping relatively slowly, eventually they will load their overlying fault to failure initiating a new cluster, with anticlusters induced across strike. Thus, there is dynamic feedback both up and down dip between faults and their underlying shear zones and crucially across strike between neighbouring fault/shear-zone structures. If the dynamics producing clustering and anticlustering can be constrained, it may be that observations of these phenomena should be included in probabilistic seismic hazard assessments (PSHA) and also interpretations of regional deformation rates and crustal rheologies based on geodetic data. Multi-millennial clustering and anticlustering should become a subject for discussion in these scientific communities.

We report agreement between the timing of slip on an active normal fault recovered from in situ Cl-36 cosmogenic fault scarp dating with independently C-14 dated Holocene coastal notches deformed along the strike of the fault, reinforcing the validity of slip-rate timing and magnitude fluctuations implied by Cl-36 fault scarp dating. The Cl-36-dated Pisia fault, central Greece, shows slip-rate fluctuations but the timing of slip derived from this cosmogenic isotope have not been confirmed with an independent dating approach. However, Holocene coastal notches dated with C-14 on fossils occupying the notches exist around the Pisia fault, these can only form when the interplay between eustatic sea-level and tectonics result in stable relative sea-level. The Cl-36 site close to the center of the Pisia fault records ongoing slip from similar to 9.6 to 5.2 (+/- 0.5) ka and 2.0 +/- 0.5 ka to the present day which was interrupted by a low slip-rate period. Holocene sea-level stabilized close to its current elevation after 7.0-6.5 ka, so the combination of low slip-rate and stable sea-level allowed notch formation. During this time, notches were uplifted by slip on the offshore Strava fault, indicated by elastic half-space modeling. Toward the center of the Pisia-Skinos fault, these notches were then submerged during the high slip period from 2.0 +/- 0.5 ka. Our findings reveal that spatial patterns of deformed radiocarbon-dated Holocene notches agree with the timing of high slip earthquake clusters/quiescent anti-clusters from Cl-36 slip histories and support use of Cl-36 to investigate normal faults, crustal rheologies and seismic hazard.

In this study, we present scaling relationships between fault lengths, fault slip-rates and historical seismicity for an active normal fault system, seismically accommodating crustal extension within the upper plate of the Ionian subduction zone (southern Italy). This crustal extension is confirmed by historical seismicity and instrumental geodesy, with GNSS-derived values of horizonal deformation within a range of 2-3 mm/yr throughout Calabria and the Messina Strait region. We collated data for fault slip-rates, fault lengths and historical earthquakes for a given fault to explore whether fault slip-rates are correlated with fault size and their geometric moment. We present new results showing a robust correlation between fault lengths and fault slip-rates, which supports the idea of a relationship for a given fault between fault slip-rates and the geometric moment. We discuss our results in terms of how these correlations should be used if regional deformation is accommodated by localised strain on faults mostly arranged along strike rather than distributed strain on multiple faults across-strike. For instance, we compare our empirical correlation between fault lengths and fault throw-rates over the Middle-Late Pleistocene in Calabria and the Messina Strait with those from Central and Southern Apennines over the Holocene, characterized by strain distributed on multiple faults across-strike and strain localised on faults mostly arranged along-strike, respectively. Tectonic and seismic hazard implications are discussed for future investigations based on fault slip-rates, fault size and historical seismicity.

Loading and deformation during the interseismic period of the earthquake cycle is often considered to be constant for continental faults, therefore assuming that the short-term (annual-decadal) deformation is representative of longer-term deformation. Based on this assumption, geodetically-derived deformation rates are sometimes used to infer the slip-rates and thus seismic hazard of faults. However geological observations indicate that deformation and slip rates are variable over a range of timescales, and we present an observation of variable deformation across an active normal fault occurring on an annual timescale. The Pisia-Skinos normal fault in the Gulf of Corinth, Greece, is a well-known fault which slipped most recently during a sequence of damaging earthquakes in 1981. Using vertical deformation data, available from the European Ground Motion Service (EGMS), we observe uplift/subsidence of the footwall/hangingwall of the Pisia fault between 2016-2021. Of particular interest is our observation that the deformation is not uniform over the 6 year time period, instead there is an up to 7-fold increase in the vertical deformation rate in mid-2019. We hypothesise that this deformation is aseismic as there is no temporally correlated increase in the earthquake activity (M>1). We explore four possible causative mechanisms for observed deformation; shallow slip, post-seismic after-slip, deep slip on an underlying shear zone, and post-seismic visco-elastic rebound. Our preferred hypothesis is that the transient deformation is caused by centimetre-scale slip in the upper 5km of the Pisia fault zone, based on the magnitude and spatial extent of the deformation. Our results suggest that continental normal faults can exhibit variable deformation over shorter timescales than previously observed, implying that the interseismic period of the earthquake cycle on continental faults may be more variable than previously hypothesised. This also highlights potential pitfalls of using slip rates measured over short-timescales to infer seismic hazard.

Geodetically-derived deformation rates are sometimes used to infer seismic hazard, implicitly assuming that short-term (annual-decadal) deformation is representative of longer-term deformation. This is despite geological observations indicating that deformation/slip rates are variable over a range of timescales. Using geodetic data from 2016-2021, we observe an up to 7-fold increase in vertical deformation rate in mid-2019 across the Pisia-Skinos normal fault in Greece. We hypothesise that this deformation is aseismic as there is no temporally correlated increase in the earthquake activity (M>1). We explore four possible physical mechanisms, and our preferred hypothesis is that the transient deformation is caused by centimetre-scale slip in the upper 5km of the Pisia fault zone. Our results suggest that continental normal faults can exhibit variable deformation over shorter timescales than previously observed, and thus care should be taken when utilising geodetic rates to quantify seismic hazard.

Uncertainty concerning the processes responsible for slip-rate fluctuations associated with temporal clustering of surface faulting earthquakes is a fundamental, unresolved issue in tectonics, because strain-rates accommodated by fault/shear-zone structures are the key to understanding the viscosity structure of the crust and seismic hazard. We constrain the timing and amplitude of slip-rate fluctuations that occurred on three active normal faults in central Italy over a time period of 20–30 kyrs, using in situ 36Cl cosmogenic dating of fault planes. We identify five periods of rapid slip on individual faults lasting a few millennia, separated time periods of up to 10 millennia with low or zero slip-rate. The rapid slip pulses migrated across the strike between the faults in two waves from SW to NE. We replicate this migration with a model where rapid slip induces changes in differential stress that drive changes in strain-rate on viscous shear zones that drive slip-rate variability on overlying brittle faults. Earthquakes increase the differential stress and strain-rate on underlying shear zones, which in turn accumulate strain, re-loading stress onto the overlying brittle fault. This positive feedback produces high strain-rate episodes containing several large magnitude surface faulting earthquakes (earthquake clusters), but also reduce the differential stress on the viscous portions of neighbouring fault/shear-zones slowing the occurrence of large-magnitude surface faulting earthquakes (earthquake anticlusters). Shear-zones on faults experiencing anticlusters continue to accumulate viscous strain at a lowered rate, and eventually this loads the overlying brittle fault to failure, initiating a period of rapid slip through the positive feedback process described above, and inducing lowered strain-rates onto neighbouring fault/shear-zones. We show that these patterns of differential stress change can replicate the measured earthquake clustering implied by the 36Cl data. The stress changes are related to the fault geometry in terms of distance and azimuth from the slipping structure, implying that (a) strain-rate and viscosity fluctuations for studies of continental rheology, and (b) slip-rates for seismic hazard purposes are to an extent predictable given knowledge of the fault system geometry.

The relationships between kinematics and fault geometry for the coseismic ruptures from the 24th and February 25, 1981 earthquake sequence in the eastern Gulf of Corinth (Ms 6.7 and 6.4) are analysed. The two earthquakes ruptured faults located across strike rather than along strike as typifies other earthquake sequences. In detail, surface ruptures formed on the sub -parallel Pisia and Skinos Faults, with an 8 km along -strike overlap zone, separated across strike by < 2 km. The largest coseismic offsets occurred in the overlap zone. The 41 -year -old ruptures are still well preserved as bedrock fault plane lichen -free stripes and colluvial ruptures, allowing detailed structural mapping at 213 rupture localities. A comparison between our measurements and Jackson et al. (1982) showed no overall consistent signal of post -seismic slip as some of our measurements were greater and some smaller than those recorded in 1981. The ruptures produced a single maximum asymmetric profile (Pisia: maximum throw of 223 cm) and a double maxima profile (Skinos: maximum throw of 109 cm and 130 cm). The shapes of the profiles differed in previous earthquakes on these faults, as evidenced by an older lichenfree stripe, implying non -characteristic earthquakes. Summing the two overlapping throw profiles across -strike reveals a single maximum symmetric bell -like profile. Using the above observations on coseismic offsets, kinematic information, and the geometry of faults, a rupture scenario has been proposed in terms of fault bends and corrugation orientations which suggests that parts of each fault may have ruptured in each earthquake.

QUIN database integrates and organizes structural-geological information from published and unpublished sources to constrain deformation in seismotectonic studies. The initial release, QUIN1.0, comprised 3,339 Fault Striation Pairs, mapped on 445 sites exposed along the Quaternary faults of central Italy. The present Data Descriptor introduces the QUIN 2.0 release, which includes 4,297 Fault Striation Pairs on 738 Structural Sites from southern Italy. The newly investigated faults span ~500 km along the Apennines chain, with strikes transitioning from ~SE to ~SW and comprehensively details Fault Striation Pairs’ location, attitude, kinematics, and deformation axes. Additionally, it offers a shapefile of the fault traces hosting the data. The QUIN 2.0 release offers a significant geographic extension to the QUIN 1.0, with comprehensive description of local geometric-kinematic complexities of the regional pattern. The QUIN data may be especially relevant for constraining intra-Apennine potential seismogenic deformation patterns, where earthquake data only offer scattered or incomplete information. QUIN’s data will support studies aimed at enhancing geological understanding, hazard assessment and comprehension of fault rupture propagation and barriers.

SUMMARY Mt. Etna is a perfect laboratory for testing new approaches and new technologies in a very active geodynamic environment. It offers, in fact, the opportunity for measuring active crustal deformation, related to volcanic activity as well as to seismic faulting on its flanks. In this work, a network of low-cost/low-power Global Navigation Satellite System stations has been installed and tested on Mt. Etna, across a very active fault, the Provenzana–Pernicana system, cutting its north-eastern flank. During the test period, a lateral eruption occurred (starting on 2018 December 24), with a forceful dyke intrusion that stressed all the flanks of the volcano, soliciting all the main faults dissecting the edifice. Also the Provenzana–Pernicana fault system, where this network was recording, was activated during the dyke intrusion, producing a significant seismic swarm. The low-cost/low-power network data analysis allowed the fault slip during the intrusion to be clearly traced in time and space at all the stations lying on the hangingwall mobile block of the fault. All the stations lying south of the fault trace showed an eastward displacement, in very good agreement with the usual kinematics of the fault and the temporal duration of the M 3.5 December 24 earthquake, related to the seaward dislocation of the eastern mobile flank of the volcano, promoted and accelerated by dyke emplacement on the upper part of the edifice.

Cross-cutting relationships and the incision history for multiple outflow channels have been mapped and studied to establish their relative chronology in Grjota Valles, Mars, in order to establish whether observed geomorphic channels were formed in a single event or multiple events. The relative chronology can be established by mapping cross-cutting relationships between channel margins and successive incisions, where later channels incise downward into older channels. We show that the source areas of five distinct channels can be established, with younger channels progressively sourced further to the east along the Grjota Valles fault system, and incising downwards into older channels. The channels resemble examples interpreted elsewhere as cut using catastrophic aqueous flow processes (diluvial) due to their regional morphology, the presence of streamlined islands surrounded by anabranching channels marked by incisions, recessional terraces and longitudinal erosional grooves; however, turbulent lava flows may have also been involved. Those five distinct flows occur progressively further to the east may indicate the progressive propagation from west to east of the processes at depth that released the fluid responsible for cutting the channels, such as dike propagation and associated seismicity. Our observation of multiple flows and channel formation episodes implies instantaneous volumes of fluid that are smaller than that for a single flow interpretation.

Surface faulting earthquakes are known to cluster in time from historical and palaeoseismic studies in multiple active tectonic settings, including central Greece, southern California and central Italy. However, the mechanism(s) responsible for clustering, such as fault interaction, strain-storage, and evolving dynamic topography, are poorly quantified and hence not well understood. We combine surface dating of active normal fault scarps in central Italy with stress modelling and quartz flow laws, to produce a quantified replication of observed earthquake clustering.We study six active normal faults (including the Mt Vettore fault which ruptured during the 2016 central Italy earthquake sequence) using 36Cl cosmogenic dating. This reveals periods of high and low slip rate, which we interpret to be earthquake clusters/anti-clusters. Interestingly, these changes in slip rate (or clustering) are out-of-phase between neighbouring faults, i.e. when one fault slows down, nearby faults speed up at the same time. To explore the underlying processes driving this out-of-phase clustering behaviour, we link stress transfer caused by slip over clusters/anti-clusters on coupled fault/shear-zone structures with viscous quartz flow laws derived from laboratory experiments.We show that differential stress fluctuates due to fault/shear-zone interactions, and that the magnitude of these fluctuations are sufficient to induce changes in strain-rate and associated slip-rate on neighbouring faults and shear zones. Our results suggest that fault/shear-zone interactions are a plausible and quantifiable explanation for earthquake clustering, thus opening possibilities for process-led and time-dependent seismic hazard assessments.

We undertake spatio-temporal analysis on sequences of Pleistocene palaeoshorelines in southwestern Crete where deformed Holocene marine notches have predominantly been suggested to be linked to coseismic uplift from the 365 CE Mw > 8 earthquake. Previous investigations into the Holocene notches have been used to infer that the dominant mechanism of uplift may be slip either on a reverse crustal fault or on the subduction interface. However, seismic reflection studies attest to the presence of numerous active offshore extensional faults whose role in the long-term deformation is unclear. The relative contributions of upper-plate extensional and compressional faults to the overall deformation can be assessed through the study of uplifted and deformed Late Quaternary palaeoshorelines. New Cl-36 exposure dating on wave-cut platforms and palaeoshoreline mapping are combined with existing age controls to facilitate investigation into the deformed Late Quaternary palaeoshorelines. We observe that the Late Quaternary uplift rates increase from west (0.61 mm/yr) to east (0.83 mm/yr) over similar to 20 km, a spatial uplift pattern that is inconsistent with published vertical deformation models of slip solely on the subduction interface or on a reverse crustal fault. Elastic half-space modelling suggests that an offshore extensional fault may also contribute to the uplift. We conclude that a combination of active extensional and compressional faults may be responsible for Late Quaternary uplift across southwestern Crete.

We model Coulomb stress transfer (CST) due to 30 strong earthquakes occurring on normal faults since 1509 CE in Calabria, Italy, including the influence of interseismic loading, and compare the results to existing studies of stress interaction from the Central and Southern Apennines, Italy. The three normal fault systems have different geometries and long‐term slip‐rates. We investigate the extent to which stress transfer can influence the occurrence of future earthquakes and what factors may govern the variability in earthquake recurrence in different fault systems. The Calabrian, Central Apennines, and Southern Apennines fault systems have 91%, 73%, and 70% of faults with mean positive cumulative CST in the time considered; this is due to fewer faults across strike, more across strike stress reductions, and greater along‐strike spacing in the three regions respectively. In regions with close along strike spacing or few faults across strike, such as Calabria and Southern Apennines, the stress loading history is mostly dominated by interseismic loading and most faults are positively stressed before an earthquake occur on them (96% of all faults that ruptured in Calabria; 94% of faults in Southern Apennines), and some of the strongest earthquakes occur on faults with the highest mean cumulative stress of all faults prior to the earthquake. In the Central Apennines, where across strike interactions are the predominant process, 79% of earthquakes occur on faults positively stressed. The results highlight that fault system geometry plays a central role in characterizing the stress evolution associated with earthquake recurrence.

Surface faulting earthquakes are known to cluster in time from historical and palaeoseismic studies, but the mechanism(s) responsible for clustering, such as fault interaction, strain-storage, and evolving dynamic topography, are poorly quantified, and hence not well understood. We present a quantified replication of observed earthquake clustering in central Italy. Six active normal faults are studied using 36Cl cosmogenic dating, revealing out-of-phase periods of high or low surface slip-rate on neighboring structures that we interpret as earthquake clusters and anticlusters. Our calculations link stress transfer caused by slip averaged over clusters and anti-clusters on coupled fault/shear-zone structures to viscous flow laws. We show that (1) differential stress fluctuates during fault/shear-zone interactions, and (2) these fluctuations are of sufficient magnitude to produce changes in strain-rate on viscous shear zones that explain slip-rate changes on their overlying brittle faults. These results suggest that fault/shear-zone interactions are a plausible explanation for clustering, opening the path towards process-led seismic hazard assessments.