Showing papers on "Fault (geology) published in 2019"

Hierarchical interlocked orthogonal faulting in the 2019 Ridgecrest earthquake sequence.

Abstract: A nearly 20-year hiatus in major seismic activity in southern California ended on 4 July 2019 with a sequence of intersecting earthquakes near the city of Ridgecrest, California. This sequence included a foreshock with a moment magnitude (Mw) of 6.4 followed by a Mw 7.1 mainshock nearly 34 hours later. Geodetic, seismic, and seismicity data provided an integrative view of this sequence, which ruptured an unmapped multiscale network of interlaced orthogonal faults. This complex fault geometry persists over the entire seismogenic depth range. The rupture of the mainshock terminated only a few kilometers from the major regional Garlock fault, triggering shallow creep and a substantial earthquake swarm. The repeated occurrence of multifault ruptures, as revealed by modern instrumentation and analysis techniques, poses a formidable challenge in quantifying regional seismic hazards.

Evidence of supershear during the 2018 magnitude 7.5 Palu earthquake from space geodesy

Abstract: A magnitude 7.5 earthquake hit the city of Palu in Sulawesi, Indonesia on 28 September 2018 at 10:02:43 (coordinated universal time). It was followed a few minutes later by a 4–7-m-high tsunami. Palu is situated in a narrow pull-apart basin surrounded by high mountains of up to 2,000 m altitude. This morphology has been created by a releasing bend in the Palu-Koro fault, a rapidly moving left-lateral strike-slip fault. Here we present observations derived from optical and radar satellite imagery that constrain the ground surface displacements associated with the earthquake in great detail. Mapping of the main rupture and associated secondary structures shows that the slip initiated on a structurally complex and previously unknown fault to the north, extended southwards over 180 km and passed through two major releasing bends. The 30 km section of the rupture south of Palu city is extremely linear, and slightly offset from the mapped geological fault at the surface. This part of the rupture accommodates a large and smooth surface slip of 4–7 m, with no shallow slip deficit. Almost no aftershock seismicity was recorded from this section of the fault. As these characteristics are similar to those from known supershear segments, we conclude that the Palu earthquake probably ruptured this segment at supershear velocities. The devastating 2018 magnitude 7.5 earthquake in Palu, Indonesia, ruptured at supershear speeds according to evidence from space geodesy.

The role of aseismic slip in hydraulic fracturing-induced seismicity.

Abstract: Models for hydraulic fracturing-induced earthquakes in shales typically ascribe fault activation to elevated pore pressure or increased shear stress; however, these mechanisms are incompatible with experiments and rate-state frictional models, which predict stable sliding (aseismic slip) on faults that penetrate rocks with high clay or total organic carbon. Recent studies further indicate that the earthquakes tend to nucleate over relatively short injection time scales and sufficiently far from the injection zone that triggering by either poroelastic stress changes or pore pressure diffusion is unlikely. Here, we invoke an alternative model based on recent laboratory and in situ experiments, wherein distal, unstable regions of a fault are progressively loaded by aseismic slip on proximal, stable regions stimulated by hydraulic fracturing. This model predicts that dynamic rupture initiates when the creep front impinges on a fault region where rock composition favors dynamic and slip rate weakening behavior.

Slip on a mapped normal fault for the 28th December 1908 Messina earthquake (Mw 7.1) in Italy

Abstract: The 28th December 1908 Messina earthquake (Mw 7.1), Italy, caused >80,000 deaths and transformed earthquake science by triggering the study of earthquake environmental effects worldwide, yet its source is still a matter of debate. To constrain the geometry and kinematics of the earthquake we use elastic half-space modelling on non-planar faults, constrained by the geology and geomorphology of the Messina Strait, to replicate levelling data from 1907–1909. The novelty of our approach is that we (a) recognise the similarity between the pattern of vertical motions and that of other normal faulting earthquakes, and (b) for the first time model the levelling data using the location and geometry of a well-known offshore capable fault. Our results indicate slip on the capable fault with a dip to the east of 70° and 5 m dip-slip at depth, with slip propagating to the surface on the sea bed. Our work emphasises that geological and geomorphological observations supporting maps of capable non-planar faults should not be ignored when attempting to identify the sources of major earthquakes.

Pore-pressure diffusion, enhanced by poroelastic stresses, controls induced seismicity in Oklahoma.

Abstract: Induced seismicity linked to geothermal resource exploitation, hydraulic fracturing, and wastewater disposal is evolving into a global issue because of the increasing energy demand. Moderate to large induced earthquakes, causing widespread hazards, are often related to fluid injection into deep permeable formations that are hydraulically connected to the underlying crystalline basement. Using injection data combined with a physics-based linear poroelastic model and rate-and-state friction law, we compute the changes in crustal stress and seismicity rate in Oklahoma. This model can be used to assess earthquake potential on specific fault segments. The regional magnitude-time distribution of the observed magnitude (M) 3+ earthquakes during 2008-2017 is reproducible and is the same for the 2 optimal, conjugate fault orientations suggested for Oklahoma. At the regional scale, the timing of predicted seismicity rate, as opposed to its pattern and amplitude, is insensitive to hydrogeological and nucleation parameters in Oklahoma. Poroelastic stress changes alone have a small effect on the seismic hazard. However, their addition to pore-pressure changes can increase the seismicity rate by 6-fold and 2-fold for central and western Oklahoma, respectively. The injection-rate reduction in 2016 mitigates the exceedance probability of M5.0 by 22% in western Oklahoma, while that of central Oklahoma remains unchanged. A hypothetical injection shut-in in April 2017 causes the earthquake probability to approach its background level by ∼2025. We conclude that stress perturbation on prestressed faults due to pore-pressure diffusion, enhanced by poroelastic effects, is the primary driver of the induced earthquakes in Oklahoma.

Train Traffic as a Powerful Noise Source for Monitoring Active Faults With Seismic Interferometry.

Abstract: Laboratory experiments report that detectable seismic velocity changes should occur in the vicinity of fault zones prior to earthquakes. However, operating permanent active seismic sources to monitor natural faults at seismogenic depth is found to be nearly impossible to achieve. We show that seismic noise generated by vehicle traffic, and especially heavy freight trains, can be turned into a powerful repetitive seismic source to continuously probe the Earth's crust at a few kilometers depth. Results of an exploratory seismic experiment in Southern California demonstrate that correlations of train-generated seismic signals allow daily reconstruction of direct P body waves probing the San Jacinto Fault down to 4-km depth. This new approach may facilitate monitoring most of the San Andreas Fault system using the railway and highway network of California.

Inducing mode analysis of rock burst in fault-affected zone with a hard–thick stratum occurrence

Abstract: Study of movement characteristics of a hard–thick stratum (HTS) affected by a fault in a coalmine is significant to predict the dynamic hazards (i.e., rock bursts and shock bumps) because of the particular structural and mechanical properties of the HTS and the fault. Hence, using UDEC numerical simulation, the movement characteristic of HTS and fault-slipping law with different mining directions towards the fault were studied. Then, two different inducing modes and corresponding mechanisms of rock burst were obtained. The results show that the structure of overlying strata on two fault walls is different because of fault cutting and fault dip; it results in the HTS of two fault walls presenting different movement stage characteristics. From analysis of fault plane stress and fault slipping, we obtain that footwall mining has higher risk of rock burst than hanging wall mining. Finally, summarizing two different inducing modes of rock burst affected by the HTS and the fault: one that mainly resulted from the strain energy release caused by the HTS obvious bending and failure (i.e., hanging wall mining) and one that notably affected by fault slipping and HTS failure subsidence (i.e., footwall mining). A field case regarding microseismic monitoring is used to verify the numerical simulation results. Study results can serve as a reference for predicting of rock bursts and their classification into hazardous areas under similar conditions.

Shallow seismic activity and young thrust faults on the Moon

Abstract: The discovery of young thrust faults on the Moon is evidence of recent tectonic activity, but how recent is unknown. Seismometers at four Apollo landing sites recorded 28 shallow moonquakes between 1969 and 1977. Some of these shallow quakes could be associated with activity on the young faults. However, the epicentre locations of these quakes are poorly constrained. Here we present more-accurate estimates of the epicentre locations, based on an algorithm for sparse seismic networks. We found that the epicentres of eight near-surface quakes fall within 30 km of a fault scarp, the distance of the expected strong ground shaking. From an analysis of the timing of these eight events, we found that six occurred when the Moon was less than 15,000 km from the apogee distance. Analytical modelling of tidal forces that contribute to the current lunar stress state indicates that seven near-apogee events within 60 km of a fault scarp occur at or near the time of peak compressional stresses, when fault slip events are most likely. We conclude that the proximity of moonquakes to the young thrust faults together with evidence of regolith disturbance and boulder movements on and near the fault scarps strongly suggest the Moon is tectonically active. Shallow moonquakes detected at four Apollo landing sites between 1969 and 1977 occurred during maximum stress and in close proximity to young faults, suggesting that the Moon is tectonically active, according to reanalyses of the seismic data and tidal force modelling.

Focal Mechanism of Mining-Induced Seismicity in Fault Zones: A Case Study of Yongshaba Mine in China

Abstract: It is essential to investigate focal mechanisms of induced seismicity for understanding the rock fracturing, the failure mode, and the hazard evolution in underground mines. But the conventional methods using empiricism to infer the source mechanisms usually lead to ambiguous results for individual events. An optimized moment tensor inversion method using full waveforms was employed to quantitatively determine the rock fracturing orientation and the type of rupture process. Source parameters including the scalar moment, the moment magnitude, the full moment tensor, and the fault plane solutions were resolved of a seismic sequence in fault zones. Results show that the shear failure events in the fault F1 vicinities have similar focal mechanisms and suggest that the fault F1 is a reverse fault. The resolved strikes and dips are basically constant with the orientation of the fault. The events in the fault F2 vicinities are mainly dominated by shear-tensional failure. But the shear events experienced shear rupture and crack opening simultaneously, resulting in slippages not along the actual fault plane. There are more events in the fault F3 area which are characterized by complicated focal mechanisms. Three of the non-shear events are dominated by compressional failure and related to rock collapse, while the other non-shear events are dominated by tensional failure and related to crack opening. The shear dominated events experienced dual effects of shear failure and compression failure. The resolved fault plane solutions cannot reflect the actual geometry of the fault. It is proved that the moment tensor inversion is able to quantitatively analyze the focal mechanism of mining-induced seismicity in fault zones and it provides beneficial understandings of mining-induced fault slips.

Fluid overpressure from chemical reactions in serpentinite within the source region of deep episodic tremor

Abstract: Slow fault slip includes a range of transient phenomena that occur over timescales longer than those of standard earthquakes. Slow slip events are often closely associated with swarms of tectonic tremor. Deep episodic tremor and slip close to the slab–mantle interface in subduction zones has been linked to high fluid pressures produced by dehydration of the subducting slab at greater depths. The slab–mantle interface is a fundamental chemical boundary, where mantle rocks are sheared and mixed with oceanic slab lithologies in a highly reactive environment to form serpentinite. Here we present field and microstructural observations from the plate boundary-scale crustal Livingstone Fault in New Zealand that suggest chemical reactions involving serpentinite can promote rock hardening and generate in situ fluid overpressures. We infer that these processes collectively can result in hydrofracturing and a transition from distributed creep to localized brittle failure and faulting. Serpentinite-related reactions occur over a wide range of pressure and temperature conditions that overlap with those in many forearc mantle wedges. We conclude that the release of fluids derived from such reactions may be an additional and widespread mechanism to generate high fluid pressure patches and brittle failure in the source region of deep tremor along the slab–mantle interface. Chemical reactions between slab and mantle rocks may lead to brittle failure where deep episodic tremor occurs in subduction zones, according to field and microstructural observations of a shear zone in New Zealand.

The 12 November 2017 Mw 7.3 Ezgeleh‐Sarpolzahab (Iran) Earthquake and Active Tectonics of the Lurestan Arc

Abstract: The 12 November 2017 Mw 7.3 Ezgeleh‐Sarpolzahab earthquake is the largest instrumentally recorded earthquake in the Zagros Simply Folded Belt by a factor of ∼10 in seismic moment. Exploiting local, regional, and teleseismic data and synthetic aperture radar interferometry imagery, we characterize the rupture, its aftershock sequence, background seismicity, and regional tectonics. The mainshock ruptured slowly (∼2 km/s), unilaterally southward, for ∼40 km along an oblique (dextral‐thrust) fault that dips ∼14°E beneath the northwestern Lurestan arc. Slip is confined to basement depths of ∼12–18 km, resolvably beneath the sedimentary cover which is ∼8 km thick in this area. The gentle dip angle and basement location allow for a broad slip area, explaining the large magnitude relative to earthquakes in the main Fars arc of the Zagros, where shallower, steeper faults are limited in rupture extent by weak sedimentary layers. Early aftershocks concentrate around the southern and western edges of the mainshock slip area and therefore cluster in the direction of rupture propagation, implying a contribution from dynamic triggering. A cluster of events ∼100 km to the south near Mandali (Iraq) reactivated the ∼50° dipping Zagros Foredeep Fault. The basement fault responsible for the Ezgeleh‐Sarpolzahab earthquake probably accounts for the ∼1 km elevation contrast between the Lurestan arc and the Kirkuk embayment but is distinct from sections of the Mountain Front Fault that define frontal escarpments elsewhere in the Zagros. It may be related to a seismic interface underlying the central and southern Lurestan arc, and a key concern is whether or not the more extensive regional structure is also seismogenic.

Rupture Energetics in Crustal Rock From Laboratory-Scale Seismic Tomography

Abstract: The energy released during earthquake rupture is partly radiated as seismic waves and mostly dissipated by frictional heating on the fault interface and by off‐fault fracturing of surrounding host rock. Quantification of these individual components is crucial to understand the physics of rupture. We use a quasi‐static rock fracture experiment combined with a novel seismic tomography method to quantify the contribution of off‐fault fracturing to the energy budget of a rupture and find that this contribution is around 3% of the total energy budget and 10% of the fracture energy Gc. The off‐fault dissipated energy changes the physical properties of the rock at the early stages of rupture, illustrated by the 50% drop in elastic moduli of the rock near the fault, and thus is expected to greatly influence later stages of rupture and slip. These constraints are a unique benchmark for calibration of dynamic rupture models.

Illuminating subduction zone rheological properties in the wake of a giant earthquake.

Abstract: Deformation associated with plate convergence at subduction zones is accommodated by a complex system involving fault slip and viscoelastic flow. These processes have proven difficult to disentangle. The 2010 Mw 8.8 Maule earthquake occurred close to the Chilean coast within a dense network of continuously recording Global Positioning System stations, which provide a comprehensive history of surface strain. We use these data to assemble a detailed picture of a structurally controlled megathrust fault frictional patchwork and the three-dimensional rheological and time-dependent viscosity structure of the lower crust and upper mantle, all of which control the relative importance of afterslip and viscoelastic relaxation during postseismic deformation. These results enhance our understanding of subduction dynamics including the interplay of localized and distributed deformation during the subduction zone earthquake cycle.

Possible link between long-term and short-term water injections and earthquakes in salt mine and shale gas site in Changning, south Sichuan Basin, China

Abstract: Late at night on 17 June 2019, a magnitude 6.0 earthquake struck Shuanghe Town and its surrounding area in Changning County, Sichuan, China, becoming the largest earthquake recorded within the southern Sichuan Basin. A series of earthquakes with magnitudes up to 5.6 occurred during a short period after the mainshock, and we thus refer to these earthquakes as the Changning M6 earthquake sequence (or swarm). The mainshock was located very close to a salt mine, into which for ~3 decades fresh water had been extensively injected through several wells at a depth of 2.7–3 km. It was also near (within ~15 km) the epicenter of the 18 December 2018 M5.7 Xingwen earthquake, which is thought to have been induced by shale gas hydraulic fracturing (HF), prompting questions about the possible involvement of industrial activities in the M6 sequence. Following previous studies, this paper focuses on the relationship between injection and seismicity in the Shuanghe salt field and its adjacent Shangluo shale gas block. Except for a period of serious water loss after the start of cross-well injection in 2005–2006, the frequency of earthquakes shows a slightly increasing tendency. Overall, there is a good correlation between the event rate in the Shuanghe area and the loss of injected water. More than 400 M ≥ 3 earthquakes, including 40 M ≥ 4 and 5 M ≥ 5 events, had been observed by the end of August 2019. Meanwhile, in the Shangluo area, seismicity has increased during drilling and HF operations (mostly in vertical wells) since about 2009, and dramatically since the end of 2014, coincident with the start of systematic HF in the area. The event rate shows a progressively increasing background with some fluctuations, paralleling the increase in HF operations. More than 700 M ≥ 3 earthquakes, including 10 M ≥ 4 and 3 M ≥ 5 in spatially and temporally clustered seismic events, are correlated closely with active fracturing platforms. Well-resolved centroid moment tensor results for M ≥ 4 earthquakes were shown to occur at very shallow depths around shale formations with active HF, in agreement with some of the clusters, which occurred within the coverage area of temporary or new permanent monitoring stations and thus have been precisely located. After the Xingwen M5.7 earthquake, seismic activity in the salt well area increased significantly. The Xingwen earthquake may have created a unidirectional rupture to the NNW, with an end point close to the NW-trending fault of the Shuanghe earthquake. Thus, a fault in the Changning anticline might have terminated the fault rupture of the Xingwen earthquake, possibly giving the Xingwen earthquake a role in promoting the Changning M6 event.

Mechanical behaviour of fluid-lubricated faults

Abstract: Fluids are pervasive in fault zones cutting the Earth's crust; however, the effect of fluid viscosity on fault mechanics is mainly conjectured by theoretical models. We present friction experiments performed on both dry and fluid-permeated silicate and carbonate bearing-rocks, at normal effective stresses up to 20 MPa, with a slip-rate ranging between 10 μm/s and 1 m/s. Four different fluid viscosities were tested. We show that both static and dynamic friction coefficients decrease with viscosity and that dynamic friction depends on the dimensionless Sommerfeld number (S) as predicted by the elastohydrodynamic-lubrication theory (EHD).Under favourable conditions (depending on the fluid viscosity (η), co-seismic slip-rate (V), fault geometry (L/H02) and earthquake nucleation depth (∝σeff)), EHD might be an effective weakening mechanism during natural and induced earthquakes. However, at seismic slip-rate, the slip weakening distance (Dc) increases markedly for a range of fluid viscosities expected in the Earth, potentially favouring slow-slip rather than rupture propagation for small to moderate earthquakes.

The susceptibility of Oklahoma’s basement to seismic reactivation

Abstract: Recent widespread seismicity in Oklahoma is attributed to the reactivation of pre-existing, critically stressed and seismically unstable faults due to decades of wastewater injection. However, the structure and properties of the reactivated faults remain concealed by the sedimentary cover. Here, we explore the major ingredients needed to induce earthquakes in Oklahoma by characterizing basement faults in the field, in seismic surveys and via rock-mechanics experiments. Outcrop and satellite mapping reveal widespread fault and fracture systems with trends that display a marked similarity to the trends of recent earthquake lineaments. Our three-dimensional seismic analyses show steeply dipping basement-rooted faults that penetrate the overlying sedimentary sequences, representing pathways for wastewater migration. Experimental stability analysis indicates that Oklahoma’s basement rocks become seismically unstable at conditions relevant to the dominant hypocentral depths of the recent earthquakes. These analyses demonstrate that the geometry, structure and mechanical stability of Oklahoma’s basement make it critically susceptible to seismic reactivation. Seismicity induced by wastewater injections is widespread in Oklahoma, probably because its basement is susceptible to the reactivation of basement-rooted faults, according to three-dimensional seismic analyses, rock-mechanics experiments and field surveys.

Depth Extent and Kinematics of Faulting in the Southern Tanganyika Rift, Africa

Abstract: Unusually deep earthquakes occur beneath rift segments with and without surface expressions of magmatism in the East African Rift system (EARS). The Tanganyika rift is part of the Western rift and has no surface evidence of magmatism. The TANG14 array was deployed in the southern Tanganyika rift, where earthquakes of magnitude up to 7.4 have occurred, to probe crust and upper mantle structure and evaluate fault kinematics. 474 earthquakes detected between June 2014 and September 2015 are located using a new regional velocity model. The precise locations, magnitudes and source mechanisms of local and teleseismic earthquakes are used to determine seismogenic layer thickness, delineate active faults, evaluate regional extension direction, and evaluate kinematics of border faults. The active faults span more than 350 km with deep normal faults transecting the thick Bangweulu craton, indicating a wide plate boundary zone. The seismogenic layer thickness is 42 km, spanning the entire crust beneath the rift basins and their uplifted flanks. Earthquakes in the upper mantle are also detected. Deep earthquakes with steep nodal planes occur along subsurface projections of Tanganyika and Rukwa border faults, indicating that large offset (≥ 5 km) faults penetrate to the base of the crust, and are the current locus of strain. The focal mechanisms, continuous depth distribution, and correlation with mapped structures indicate that steep, deep border faults maintain a half‐graben morphology over at least 12 My of basin evolution. Fault scaling based on our results suggests M > 7 earthquakes along Tanganyika border faults are possible.

Surface-Rupturing Historical Earthquakes in Australia and Their Environmental Effects: New Insights from Re-Analyses of Observational Data

Abstract: We digitize surface rupture maps and compile observational data from 67 publications on ten of eleven historical, surface-rupturing earthquakes in Australia in order to analyze the prevailing characteristics of surface ruptures and other environmental effects in this crystalline basement-dominated intraplate environment. The studied earthquakes occurred between 1968 and 2018, and range in moment magnitude (Mw) from 4.7 to 6.6. All earthquakes involved co-seismic reverse faulting (with varying amounts of strike-slip) on single or multiple (1–6) discrete faults of ≥ 1 km length that are distinguished by orientation and kinematic criteria. Nine of ten earthquakes have surface-rupturing fault orientations that align with prevailing linear anomalies in geophysical (gravity and magnetic) data and bedrock structure (foliations and/or quartz veins and/or intrusive boundaries and/or pre-existing faults), indicating strong control of inherited crustal structure on contemporary faulting. Rupture kinematics are consistent with horizontal shortening driven by regional trajectories of horizontal compressive stress. The lack of precision in seismological data prohibits the assessment of whether surface ruptures project to hypocentral locations via contiguous, planar principal slip zones or whether rupture segmentation occurs between seismogenic depths and the surface. Rupture centroids of 1–4 km in depth indicate predominantly shallow seismic moment release. No studied earthquakes have unambiguous geological evidence for preceding surface-rupturing earthquakes on the same faults and five earthquakes contain evidence of absence of preceding ruptures since the late Pleistocene, collectively highlighting the challenge of using mapped active faults to predict future seismic hazards. Estimated maximum fault slip rates are 0.2–9.1 m Myr−1 with at least one order of uncertainty. New estimates for rupture length, fault dip, and coseismic net slip can be used to improve future iterations of earthquake magnitude—source size—displacement scaling equations. Observed environmental effects include primary surface rupture, secondary fracture/cracks, fissures, rock falls, ground-water anomalies, vegetation damage, sand-blows/liquefaction, displaced rock fragments, and holes from collapsible soil failure, at maximum estimated epicentral distances ranging from 0 to ~250 km. ESI-07 intensity-scale estimates range by ± 3 classes in each earthquake, depending on the effect considered. Comparing Mw-ESI relationships across geologically diverse environments is a fruitful avenue for future research.

Intra-Arc Crustal Seismicity: Seismotectonic Implications for the Southern Andes Volcanic Zone, Chile

Abstract: We examine the intra‐arc crustal seismicity of the Southern Andes Volcanic Zone (SVZ). Our aim is to resolve inter‐seismic deformation in an active magmatic arc dominated by both margin‐parallel (Liquine‐Ofqui fault system, (LOFS)) and Andean transverse faults (ATF). Crustal seismicity provides information about the schizosphere tectonic state, delineating the geometry and kinematics of high strain domains driven by oblique‐subduction. Here, we present local seismicity based on 16‐months data collected from 34 seismometers monitoring a ~200 km long section of the Southern Volcanic Zone, including the Lonquimay and Villarrica volcanoes. We located 356 crustal events with magnitudes between Mw 0.6 and Mw 3.6. Local seismicity occurs at depths down to 40 km in the forearc and consistently shallower than 12 km beneath the volcanic chain, suggesting a convex shape of the crustal seismogenic layer bottom. Focal mechanisms indicate strike‐slip faulting consistent with ENE‐WSW shortening in line with the long‐term deformation history revealed by structural geology studies. However, we find regional to local‐scale variations in the shortening axes orientation as revealed by the nature and spatial distribution of microseismicity, within three distinctive latitudinal domains. In the northernmost domain, seismicity is consistent with splay faulting at the northern termination of the LOFS; in the central domain, seismicity distributes along ENE‐ and WNW‐striking discrete faults, spatially associated with, hitherto seismic ATF. The southernmost domain, in turn, is characterized by activity focused along a N15°E striking master branch of the LOFS. These observations indicate a complex strain compartmentalization pattern within the intra‐arc crust, where variable strike‐slip faulting dominates over dip‐slip movements.

Coulomb pre-stress and fault bends are ignored yet vital factors for earthquake triggering and hazard

Abstract: Successive locations of individual large earthquakes (Mw > 5.5) over years to centuries can be difficult to explain with simple Coulomb stress transfer (CST) because it is common for seismicity to circumvent nearest-neighbour along-strike faults where coseismic CST is greatest. We demonstrate that Coulomb pre-stress (the cumulative CST from multiple earthquakes and interseismic loading on non-planar faults) may explain this, evidenced by study of a 667-year historical record of earthquakes in central Italy. Heterogeneity in Coulomb pre-stresses across the fault system is >±50 bars, whereas coseismic CST is <±2 bars, so the latter will rarely overwhelm the former, explaining why historical earthquakes rarely rupture nearest neighbor faults. However, earthquakes do tend to occur where the cumulative coseismic and interseismic CST is positive, although there are notable examples where earthquake propagate across negatively stressed portions of faults. Hence Coulomb pre-stress calculated for non-planar faults is an ignored yet vital factor for earthquake triggering. Scattered earthquake locations in the same region cannot be explained solely by coseismic Coulomb stress on planar faults. Instead, the authors suggest Coulomb pre-stress to influence earthquake locations. Pre-stress was modelled on strike-variable faults and consists of coseismic and interseismic Coulomb stress.