Whether the stress-loading of faults to failure in earthquakes appears to be random or to an extent explainable, given constraints on fault/shear-zone interaction and the build-up and release of stress over many earthquake cycles, is a key question for seismic hazard assessment. Here we investigate earthquake recurrence for a system of 25 active normal faults arranged predominantly along strike from each other, allowing us to isolate the effects of stress-loading due to regional strain versus across-and along-strike fault interaction. We calculate stress changes over 6 centuries due to interseismic loading and 25 > Mw 5.5 earthquakes. Where only one fault exists across strike, stressloading is dominated by the regional tectonics through slip on underlying shear zones and fault planes have spatially smooth stress with predominantly time-dependent stress increase. Conversely, where faults are stress-loaded by across-strike fault interactions, fault planes have more irregular stress patterns and interaction-influenced stress loading histories. Stress-loading to failure in earthquakes is not the same for all faults and is dependent on the geometry of the fault/shear-zone system. The stress-loading of faults to failure in earthquakes is driven by regional tectonics, but is also influenced by fault interaction during earthquakes, evidenced by calculations of Coulomb stress transfer (CST) and corresponding changes in the rate of seismicity 1-3. Faults also interact over longer time periods evidenced by fault displacement profiles that show steep displacement gradients and enhanced displacements on adjacent fault tips 4,5 , and observations of finite fault displacement profiles that adhere to global scaling relationships between fault length and fault displacement, both for isolated faults and for closely-spaced fault networks (d = γL, where γ = 0.03 for both isolated faults and summed across the strike of fault networks 6,7). However, despite the above evidence for organisation of both the stress-loading to failure process and long-term displacement accumulation, earthquake recurrence is often considered to be a random Poisson process for some seismic hazard purposes 8,9. The link between fault interaction during single earthquakes and over multiple seismic cycles described above argues that the recurrence of earthquakes must sum to produce the long-term displacement profiles. Hence displacement accumulation during earthquakes over the long-term is not random, and must be influenced by interaction, which is in turn governed by fault system geometry. However, study of geometry-controlled interaction is complicated by the fact that fault systems are complex, commonly exhibiting multiple overlapping faults, and it is difficult to isolate the effects of earthquake-inducing stress accumulation caused by tectonic loading as opposed to loading due to rupture of neighbouring faults both along and across strike 10. Without isolation of these effects it is challenging to assess what aspects of the stress-loading to failure, and h...

The geometry, rates and kinematics of active faulting in the region close to the tip of a major crustal-scale normal fault in the Gulf of Corinth, Greece, are investigated using detailed fault mapping and new absolute dating. Fault offsets have been dated using a combination of 234 U/ 230 Th coral dates and in situ 36 Cl cosmogenic exposure ages for sediments and wave-cut platforms deformed by the faults. Our results show that deformation in the tip zone is distributed across as many as eight faults arranged within ~700 m across strike, each of which deforms deposits and landforms associated with the 125 ka marine terrace of Marine Isotope Stage 5e. Summed throw-rates across strike achieve values as high as 0.3-1.6 mm/yr, values that are comparable to those at the centre of the crustal-scale fault (2-3 mm/yr from Holocene palaeoseismology and 3-4 mm/yr from GPS geodesy). The relatively high deformation rate and distributed deformation in the tip zone are discussed in terms of stress enhancement from rupture of neighbouring crustal-scale faults and in terms of how this should be considered during fault-based seismic hazard assessment. IntroductionUnderstanding the deformation that occurs at the tips of normal faults is important because (a) it contributes to knowledge on fault growth and linkage (e.g. Cowie and Shipton, 1998;Peacock and Sanderson, 1991;McLeod et al., 2000;Peacock, 2002), (b) has the potential to inform fault-based seismic hazard analysis about fault connectivity and maximum rupture extent (Scholz and Gupta, 2000), and (c) influences our understanding of fluid connectivity or otherwise of faulted hydrocarbon reservoirs (Yielding et al., 1996). One of the key observations from studies on tip-zone deformation is that the shape of the displacement gradients differs between isolated and interacting faults as a result of perturbation to the surrounding stress field (

In order to investigate the geometry, rates and kinematics of active faulting in the region close to the tip of a major crustal-scale normal fault in the Gulf of Corinth, Greece, we have mapped faults and dated their offsets using a combination of 234U/230Th coral dates and in situ 36Cl cosmogenic exposure ages for sediments and wave-cut platforms deformed by the faults. Our results show that deformation in the tip zone is distributed across as many as eight faults arranged within ~700 m across strike, each of which deforms deposits and landforms associated with the 125 ka marine terrace of Marine Isotope Stage 5e. Summed throw-rates across strike achieve values as high as 0.3-1.6 mm/yr, values that are relatively high compared to that at the centre of the crustal-scale fault (2-3 mm/yr from Holocene palaeoseismology and 3-4 mm/yr from GPS geodesy). The relatively high deformation rate and distributed deformation rate in the tip zone are discussed in terms of stress enhancement from rupture of neighbouring crustal-scale faults and in terms of how this should be considered during fault-based seismic hazard assessment.

Multiple measurements of the geometry, kinematics and rates of slip across the Auletta fault (Campania, Italy) are presented, and we use these to determine: (1) the spatial resolution of field measurements needed to accurately calculate a representative strain-rate; (2) what aspects of the geometry and kinematics would introduce uncertainty with regard to the strainrate if not measured in the field. We find that the magnitude of the post last-glacial maximum throw across the fault varies along strike. If such variations are unnoticed, different values for a representative strain-rate, hence different results in seismic hazard calculations, would be produced. To demonstrate this, we progressively degrade our dataset, calculating the implied strain-rate at each step. Excluding measurements can alter strain-rate results beyond 1σ uncertainty, thus we urge caution when using only one measurement of slip-rate for calculating hazard. We investigate the effect of approximating the throw profile along the fault with boxcar and triangular distributions and show that this can underestimate or overestimate the strain-rate, with results in the range of 72-237% of our most detailed strainrate calculation. We discuss how improved understanding of the potential implied errors in strain-rate calculations from field structural data should be implemented in seismic hazard calculations.

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.

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<p>Crustal-scale active normal faults dominate seismic hazard in some regions and have been intensely studied. However, the lateral tips of these structures have received relatively little attention in the literature so their geometries are poorly known. This is an important omission because locating the tips of normal faults is vital in order to define fault lengths and calculate maximum expected earthquake magnitudes. Identifying tips will be challenging if their geometries, kinematics and rates of deformation are poorly known. Consequently, incorrectly identified tips and hence fault lengths may contribute to uncertainty in Probabilistic Seismic Hazard Assessment.</p><p>We investigate the geometry, rates and kinematics of active normal faulting in the western tip zone of the South Alkyonides Fault System (SAFS) (Gulf of Corinth, Greece) by detailed fault mapping and fault offset dating using a combination of new <sup>234</sup>U/<sup>230</sup>Th coral ages and in situ <sup>36</sup>Cl cosmogenic exposure ages on wave-cut platforms deformed by faults.</p><p>Our results reveal that there is no clear singular fault tip and that distributed deformation in the tip zone of the SAFS occurs across as many as eight faults arranged within ~700 m across strike, each of which deforms deposits and landforms associated with the 125 ka marine terrace of Marine Isotope Stage 5e. Summed throw-rates across strike achieve values as high as 1.6 mm/yr, values that approach those close to the centre of the crustal-scale fault of 2-3 mm/yr from Holocene palaeoseismology and 3-4 mm/yr from GPS geodesy. Considering the uncertainty in the location of the western tip induced by distributed faulting, the SAFS fault length is uncertain by up to ± 6%, which equates to a total maximum magnitude uncertainty of Mw 0.1.</p><p>The calculated tip displacement gradient summed across parallel faults since 125 ka for the western tip zone of the SAFS is within the upper range compared to data from other normal crustal-scale faults. We discuss stress interaction between the SAFS and a neighbouring along-strike crustal-scale fault as a potential cause of the observed fault complexity and anomalously high throw and investigate this by undertaking Coulomb stress transfer modelling. The results from the study are discussed within the context of fault-based seismic hazard assessment.</p><p> We conclude that identifying the locations of fault tips is challenging. While the results of this study may or may not be typical of other tip zones owing to the interaction, there is a need for further studies that explore the geometry of both non-interacting and interacting fault tip zones.</p>