Subsidiary Faults Research Articles

Lithospheric Contraction on Mars: A 3D Model of the Amenthes Thrust Fault System

AbstractAmenthes Rupes is the topographic expression of a main fault belonging to a thrust fault system located parallel to the martian dichotomy boundary. A 3D forward model has been applied to the Amenthes thrust fault system, constraining fault geometries at depth, variations of slip along strike, and structural parameters controlling the formation of fault propagation folds. Our results provide a complex 3D view of the tectonic framework of the area, with implications for tectonic evolution, regional shortening distribution, and the main mechanical discontinuities in the lithosphere. The modeled fault surfaces show planar morphologies combined with listric geometries at depth. The obtained depths of faulting for the major faults of this fault system suggest a depth of the brittle‐ductile transition (at the time of formation) of 20–24 km, somewhat shallower than previous estimates for this area. A possible mechanical discontinuity located at 9.5–13 km deep can be deduced from the faulting depths of the secondary faults. The listric geometries at depth imply that slip is transmitted from the decollement, which, together with the inclusion in the model of secondary and subsidiary faults, allow us to estimate the horizontal shortening recorded in this area ranging from 2–3 km up to ~5.5 km in the southeastern part of the fault system. This range increases the previous shortening estimates in this area between ~60% and ~200%. Consequently, global shortening estimates based on global fault maps are biased by the detail of mapping, and shortening would substantially increase if secondary faults were included.

Damage zone heterogeneity on seismogenic faults in crystalline rock; a field study of the Borrego Fault, Baja California

Complex fracture damage around large faults is often simplified to fit exponential or power law decay in fracture density with distance from the fault. Noise in these datasets is attributed to large subsidiary faults or random natural variation. Through a field study of the Borrego Fault (Baja California) damage zone, combining mm-resolution structural mapping and point sampling, we show that such variations are the expression of systematic damage heterogeneity. The oblique-slip Borrego Fault comprises NW and SE segments that ruptured during the Mw7.2 2010 El Mayor-Cucapah earthquake. Measurements of fracture density along eight linear fault-perpendicular transects and from a high-resolution 68 m2 structural map display a power law decay and define footwall damage zone widths of ~85 m and ~120 m for the NW and SE segments respectively. Variance in fracture density decays with distance following an inverse exponential relationship, to background variance at ~16 m. Spatial analysis of the high-resolution fracture map reveals a patchy distribution of high- and low-intensity clusters at metre- and decimetre-scales. We attribute high-intensity clusters at these scales to local complexity caused by interactions between minor subsidiary faults (101 m length and 10−2-10−1 m displacement). Fracture density differences between high- and low-intensity clusters decrease with distance from the fault, demonstrating a systematic change in outcrop-scale damage heterogeneity. Based on these observations we present a revised model for damage zone growth including growth of heterogeneity.

Geology, geochronology, and S-Pb-Os geochemistry of the Alastuo gold deposit, West Tianshan, NW China

The Alastuo gold deposit, located in the Narati region of the Chinese West Tianshan, is a newly discovered gold deposit within the world-class Tianshan gold belt. The gold mineralization is hosted by an Early Carboniferous granodiorite intrusion and controlled structurally by subsidiary faults of the transcrustal North Narati Fault. The deposit consists of auriferous alteration assemblages and minor vein ores. Field relations and petrographic data suggest four stages of mineralization with gold precipitated coevally with quartz, chlorite, epidote, pyrite, and galena during the second and third stages that are characterized by tectonic transformation from ductile shearing to brittle deformation. The temperature range of gold precipitation is estimated to be 280–330 °C using chlorite geothermometry. Ten auriferous pyrite samples yielded a well-defined Re-Os isochron age of 325 ± 3 Ma and two sericite samples yielded plateau-like 40Ar/39Ar ages of 321.7 ± 3.0 Ma and 321.2 ± 2.8 Ma. These consistent ages suggest that the gold mineralization in the Narati region was emplaced in a transitional regime from subduction to continental collision between the Tarim block and Middle Tianshan terrane (325–310 Ma), rather than during the post-collisional stage as in most part of the Tianshan gold belt. Gold-bearing pyrite has an initial Osi value of 0.7 ± 0.1 and lead isotopes ratios of 17.897 to 18.723 for 206Pb/204Pb, 15.474 to 15.643 for 207Pb/204Pb, and 37.522 to 38.299 for 208Pb/204Pb, while samples of pyrite, galena, and sphalerite have 34SV-CDT of +1.9 to +7.8‰. The isotope data suggest that the sulfur and metals are most likely derived from mixed mantle and crustal sources including metamorphic devolatilization of subducted oceanic slab and overlying sedimentary rocks. The mineralization styles, structural controls, metal and sulfur sources, and the timing of gold mineralization with respect to the orogenic event all suggest that the Alastuo gold deposit represents an orogenic type deposit. This newly recognized Carboniferous orogenic gold mineralization event calls for a re-evaluation of the gold metallogeny in the Middle Tianshan.

Coseismic Motion on the Delta Fault within the Sherashevo–Inkino Site (Baikal Area): GPR Evidence

Abstract —Ground Penetrating Radar (GPR) surveys in the area between Sherashevo and Inkino villages provide insights into the structure of the Delta Fault and allow estimating the amount of vertical slip caused by the M = 7.5 Tsagan earthquake of 12 January 1862. The surveys with shielded AB-90 and AB 250-M antennas of an OKO-2 georadar along five profiles spaced at 25 m reveal normal slip from 2.6 to 4.5 m in different segments of the main seismogenic fault. The surface rupture caused by the 1862 event is traceable in interpreted radar images together with subsidiary faults; some possibly resulted from the 1959 Middle Baikal earthquake (M = 6.8). The GPR data are used to construct a 3D model of the area, which illustrates the evolution of the Delta Fault scarp since the Tsagan earthquake. Much of surface rupture during the Tsagan event is due to gravity sliding, judging by the amount of displacement estimated from GPR, structural, and field data of different years. Comprehensive understanding of the displacement pattern along the seismogenic fault requires further study extended to other segments of the fault zone.

Genesis of gold deposits in the Wulong orefield, Liaodong Peninsula, North China Craton: Constraints from ore deposit geology, REE, and C–H–O–S–Pb isotopes

The Wulong gold orefield is located in the eastern part of the Liaodong Peninsula, and contains the large Sidaogou and Wulong gold deposits, as well as several small gold deposits with a total of >100 tonnes of gold reserves. These deposits are hosted within Paleoproterozoic metamorphic rocks of the Liaohe Group and Mesozoic granitoids, and are structurally controlled by NNE–SSW‐ and NNW–SSE‐trending faults and interlayer fractures. The Sidaogou deposit is characterized by altered‐rock‐type mineralization occurring within interlayer fracture zones, whereas the mineralization in the Wulong deposit occurs as auriferous quartz veins along subsidiary faults. These two types of gold mineralization are representative of the mineralization in the Wulong orefield. The REE contents of quartz and calcite, together with previous studies of fluid inclusions, indicate that the ore‐forming fluids of the Sidaogou deposit were reduced NaCl–H2O–CO2 fluids. Quartz yielded δ18OH2O‐SMOW, δDSMOW, and δ13CCO2 values of +4.7‰ to +6.6‰, −82.2‰ to −79.0‰, and − 13.1‰ to −6.6‰, respectively, and calcite yielded δ13CCO2 and δ18OV‐SMOW values of −2.3‰ to −2.0‰ and + 7.4‰ to +7.9‰, respectively. These results indicate that the ore‐forming fluids of the Sidaogou deposit were derived primarily from magmatic water, and the C was sourced mainly from deep‐seated magma with a minor contribution from sedimentary organic matter. Pyrite crystals have δ34SCDT values of +4.9‰ to +12.9‰, indicating the S source was a combination of the magmatic system and ore‐bearing metamorphic rocks. Pyrite has 208Pb/204Pb ratios of 38.226–38.336, 207Pb/204Pb ratios of 15.606–15.683, and 206Pb/204Pb ratios of 18.345–18.492, suggesting that the Pb in the Sidaogou deposit was derived primarily from the upper crust, with minor input from the mantle. Taking into account previous studies of the Wulong deposit, we propose a metallogenic model for the Wulong gold orefield. Deep‐seated ore‐forming fluids were released from different intrusions hidden below the Wulong orefield for each of the deposits. The ore‐forming fluids moved through the metamorphic rocks of the Gaixian Formation along interlayer fracture zones, where intensive water–wallrock reactions, and episodic temperature and pressure drops resulted in the formation of altered‐rock‐type Sidaogou deposit. The fluids ascended to the Wulong and Sanguliu intrusions along NNE–SSW‐ and NNW–SSE‐trending faults, where temperature and pressure dropping, water–wallrock reactions, and fluid immiscibility generated the vein‐type Wulong deposit. The gold deposits in the Wulong orefield share many similarities with the Linglong‐ and Jiaojia‐type gold deposits in the Jiaodong Peninsula, including geological setting, mineralization style and age, and sources of ore‐forming fluids and metal.

Fractures and Subsidiary Faults Developed in the Active Strike‐Slip Nojima Fault Zone, Japan, and Tectonic Implications

AbstractThe Nojima Fault Zone, on which the 1995 Mw 6.9 Kobe (Japan) earthquake occurred, is characterized by pulverized cataclastic rocks with numerous fractures and subsidiary faults. The fractures and subsidiary faults were observed in trench walls and using borehole televiewer (BHTV) images from boreholes NFD‐1 (~1,000‐m depth) and AFD‐1 (~700‐m depth), which were drilled through the Nojima Fault and its branch, the Asano Fault, respectively. Measurements show that the orientations of fractures and subsidiary faults in the two boreholes are concentrated at N10–70°E, averaging N40°E. In contrast, the orientations of fractures and subsidiary faults in the walls of a trench across the main surface trace of the Asano Fault are constrained to N0–40°E, averaging N20°E, parallel to the general trend of the Asano Fault but different with that observed in the deep holes at the Ogura site. Structural analysis shows that the fractures and subsidiary faults are mainly concentrated in Riedel Y and R shears of the right‐lateral strike‐slip Nojima Fault and its branch Asano Fault. Our findings demonstrate that the fractures and subsidiary faults within the Nojima Fault Zone were formed by dextral strike‐slip shearing under the current ENE‐WSW to E‐W compressive regional tectonic stress field, related to ongoing subduction of the Pacific and Philippine Sea plates beneath the Japanese Islands.

Geology and fluid geochemistry of the Na-metasomatism U deposits in the Longshoushan uranium metallogenic belt, NW China: Constraints on the ore-forming process

The Longshoushan belt in Gansu Province is a very important uranium metallogenic belt in Northwest China and contains a series of different types (magmatic, Na-metasomatism, supergenetic) of uranium deposits and occurrences. Of all these mineralization types, the Na-metasomatism type is the most predominant with great potential, represented by two Na-metasomatism uranium deposits (Xinshuijing and Jiling). Different from the fluorite-, illite-, quartz-dominated alteration minerals in other hydrothermal uranium mineralization system, the Na-metasomatism type uranium deposits are characterized by the unique albite-dominated (up to 90 vol%) alteration mineral assemblage, which is referred to albitite in many literatures. All the uranium orebodies in both the Xinshuijing and Jiling deposits are hosted within the metasomatic albitites, and controlled by the subsidiary faults of the regional Malugou Fault. The mineralization process in both deposits can be divided into four stages, namely: the Na-metasomatism stage, U mineralization stage, post-mineralization stage and supergene mineralization stage. The Na-metasomatism stage is predominantly composed of albite (with hematite staining), with minor chlorite and anatase, while the U mineralization stage is characterized by the occurrence of uranium minerals (mostly pitchblende), intergrown with some tiny albite, calcite, chlorite, pyrite, apatite, REE-minerals and sometimes hydrothermal zircons. The mineral assemblages of the Na-metasomatism stage and U mineralization stage are too intimately related to each other and might be the products of the evolution from the same ore-forming fluids. The post-mineralization stage is mainly the calcite-quartz veinlets or occur as aggregate clusters, while the supergene mineralization stage includes mineral assemblage of secondary uranium mineralization-clay minerals (e.g. kaolinite)-limonite. Mineralogical studies and comparison of major and trace elements assay results between fresh biotite granite (predominate host rocks), albitite and uranium ores suggest that the ore-forming fluids resulting in the formation of the Na-metasomatism alteration are oxidized and rich in Na, Sc, V, Cr, Co, Ni, U, Th, while the ore-forming fluids of the U mineralization stage are relatively reduced and rich in heavy rare earth elements, U, Th, PO34-. Fluid inclusion and calcite C-O isotope studies indicate that the ore-forming fluids are meteoric water-dominated, which circulated at depth (~8 km, by fluid inclusion geobarometer) and scavenged uranium from the host biotite granites and metamorphic rocks of the Longshoushan Group. The ore precipitation is mainly attributed to physicochemical changes (fO2, temperature, fluid composition) from fluid-rock interaction and further temperature decrease of the ore-forming fluids, while fluid boiling and fluid mixing processes are limited. The proposed ore-forming process is not only important for the understanding of the ore genesis for the Na-metasomatism mineralization systems in Longshoushan, but also highlights the significance of albite alteration, which could be used as an important indicator during further uranium exploration.

The first Precambrian gold deposit in North Xinjiang, NW China: Geochronology, metallogenic character, and ore genesis of the Dajingou gold deposit

The Dajingou gold deposit is one of the typical gold deposits in the Quruqtagh metamorphic terrane, north of the Tarim Craton. The deposit mainly comprises gold-bearing quartz veins hosted in Precambrian metamorphic rocks and structurally controlled by the subsidiary faults and ductile shear zone. To determine the age of gold mineralization in the Dajingou deposit, a single sericite 40Ar/39Ar age of 829.4 ± 4.8 Ma (MSWD = 0.57) and hydrothermal zircon weighted average U–Pb ages (812 ± 15 Ma, MSWD = 3.2) from auriferous quartz veins were obtained in this study, indicating that mineralization occurred in the mid-Neoproterozoic (ca. 0.83 Ga), corresponding to regional metamorphism of the Tarim Craton, termed the Tarim orogeny. The Dajingou deposit is the first Precambrian gold deposit identified in North Xinjiang, NW China. The δ18O and δD of quartz ranges from 14.7‰ to 15.9‰ and −104‰ to −75‰, respectively; the calculated δ18OH2O of the fluids ranges from 0.9‰ to 5.0‰. This suggests that the ore-forming fluids in the Dajingou gold deposit were derived from dehydration of metamorphic rocks during regional metamorphism, with a subsequent input of meteoric water. Based on its similarities with typical orogenic gold deposits, the Dajingou deposit is proposed to be an orogenic gold deposit formed during the mid-Neoproterozoic Tarim orogeny in the Tarim Craton.

Physical and chemical strain-hardening during faulting in poorly lithified sandstone: The role of kinematic stress field and selective cementation

Abstract In this work, we report the results of a multidisciplinary study describing the structural architecture and diagenetic evolution of the Rocca di Neto extensional fault zone developed in poorly lithified sandstones of the Crotone Basin, Southern Italy. The studied fault zone has an estimated displacement of ∼90 m and consists of: (1) a low-deformation zone with subsidiary faults and widely spaced deformation bands; (2) an ∼10-m-wide damage zone, characterized by a dense network of conjugate deformation bands; (3) an ∼3-m-wide mixed zone produced by tectonic mixing of sediments with different grain size; (4) an ∼1-m-wide fault core with bedding transposed into foliation and ultra-comminute black gouge layers. Microstructural investigations indicate that particulate flow was the dominant early-stage deformation mechanism, while cataclasis became predominant after porosity loss, shallow burial, and selective calcite cementation. The combination of tectonic compaction and preferential cementation led to a strain-hardening behavior inducing the formation of “inclined conjugate deformation band sets” inside the damage zone, caused by the kinematic stress field associated with fault activity. Conversely, conjugate deformation band sets with a vertical bisector formed outside the damage zone in response to the regional extensional stress field. Stable isotope analysis helped in constraining the diagenetic environment of deformation, which is characterized by mixed marine-meteoric signature for cements hosted inside the damage zone, while it progressively becomes more meteoric moving outside the fault zone. This evidence supports the outward propagation of fault-related deformation structures in the footwall damage zone.

Complex geometry and kinematics of subsidiary faults within a carbonate-hosted relay ramp

Minor fault geometry and kinematics within relay ramps is strongly related to the stress field perturbations that can be produced when two major fault segments overlap and interact. Here we integrate classical fieldwork and interpretation of a virtual outcrop to investigate the geometry and kinematics of subsidiary faults within a relay ramp along the Tre Monti normal fault in the Central Apennines. Although the Tre Monti fault strikes parallel to the regional extension (NE-SW) it shows predominant dip-slip kinematics, suggesting a NW-SE oriented extension acting at sub-regional scale (1–10 km). Conversely, the slickenlines collected on the front segment of the relay ramp highlight right-lateral kinematics. The subsidiary faults in the relay ramp show a complex geometry (variable attitudes) and slickenlines describe multiple kinematics (left-lateral, dip-slip, right-lateral), independently of their orientation. Our fault slip analysis indicates that a local stress field retrieved from the kinematic inversion of the slickenlines collected on the front segment, and likely promoted by the interaction between the overlapping fault segments that bound the relay zone, can explain most of the geometry and kinematics of the subsidiary faults. Further complexity is added by the temporal interaction with both the regional and sub-regional stress fields.

Analysis of fault damage zones using three-dimensional seismic coherence in the Anadarko Basin, Oklahoma

Fault damage zones may significantly affect subsurface fluid migration and the development of unconventional resources. Most analyses of fault damage zones are based on direct field observations, and we expand these analyses to the subsurface by investigating the damage zone structure of an approximately 32-km (∼105-ft)-long right-lateral strike-slip fault in Oklahoma. We used the three-dimensional (3-D) seismic attribute of coherence to first define its regional and background levels, and then we evaluated the damage zone dimensions at multiple sites. We found damage zone thickness of approximately 1600 m (∼5300 ft) at a segment that is dominated by subsidiary faults, and it is slightly thicker at a segment with a pull-apart basin. The damage zone intensity decays exponentially with distance from the fault core, in agreement with field observations and distribution of seismic events. The coherence map displays a strong asymmetry of the damage zone between the two sides of the 3-D fault, which is related to the subsidiary structures of the fault zone. We discuss the effects of heterogeneous stress field on damage zone evolution through the detected subsidiary structures. It appears that seismic coherence is an effective tool for subsurface characterization of fault damage zones.

Mixed deformation styles observed on a shallow subduction thrust, Hikurangi margin, New Zealand

AbstractGeophysical observations show spatial and temporal variations in fault slip style on shallow subduction thrust faults, but geological signatures and underlying deformation processes remain poorly understood. International Ocean Discovery Program (IODP) Expeditions 372 and 375 investigated New Zealand’s Hikurangi margin in a region that has experienced both tsunami earthquakes and repeated slow-slip events. We report direct observations from cores that sampled the active Pāpaku splay fault at 304 m below the seafloor. This fault roots into the plate interface and comprises an 18-m-thick main fault underlain by ∼30 m of less intensely deformed footwall and an ∼10-m-thick subsidiary fault above undeformed footwall. Fault zone structures include breccias, folds, and asymmetric clasts within transposed and/or dismembered, relatively homogeneous, silty hemipelagic sediments. The data demonstrate that the fault has experienced both ductile and brittle deformation. This structural variation indicates that a range of local slip speeds can occur along shallow faults, and they are controlled by temporal, potentially far-field, changes in strain rate or effective stress.

Structural to stratigraphic framework of “Banghan” field, onshore Niger Delta, southern Nigeria

ABSTRACTBanghan field shows growth fault structures with dips ranging from 32° to 71° at the top and it decreases towards the base of the fault. The syn-depositional growth fault throw is 10 m to 91 m. The synthetic growth faults trend NW-SE while antithetic and other subsidiary faults trend NE-SW. Fault assisted closures correspond to the crest of rollover structures that are possible trapping mechanism for hydrocarbon. The lithologic units are Burdigalian to Tortonian in age. The lithofacies include bioturbated heteroliths, shale’s, and laminated sandstones. The field had undergone at least six cycles of deposition. The sequence of deposition and various chrono surfaces identified using well logs, biofacies, seismic and core data shows that the sediments deposited obey Walther’s law.

Multi-segment rupture of the 2016 Amatrice-Visso-Norcia seismic sequence (central Italy) constrained by the first high-quality catalog of Early Aftershocks

We present the first high-quality catalog of early aftershocks of the three mainshocks of the 2016 central Italy Amatrice-Visso-Norcia normal faulting sequence. We located 10,574 manually picked aftershocks with a robust probabilistic, non-linear method achieving a significant improvement in the solution accuracy and magnitude completeness with respect to previous studies. Aftershock distribution and relocated mainshocks give insight into the complex architecture of major causative and subsidiary faults, thus providing crucial constraints on multi-segment rupture models. We document reactivation and kinematic inversion of a WNW-dipping listric structure, referable to the inherited Mts Sibillini Thrust (MST) that controlled segmentation of the causative normal faults. Spatial partitioning of aftershocks evidences that the MST lateral ramp had a dual control on rupture propagation, behaving as a barrier for the Amatrice and Visso mainshocks, and later as an asperity for the Norcia mainshock. We hypothesize that the Visso mainshock re-activated also the deep part of an optimally oriented preexisting thrust. Aftershock patterns reveal that the Amatrice Mw5.4 aftershock and the Norcia mainshock ruptured two distinct antithetic faults 3–4 km apart. Therefore, our results suggest to consider both the MST cross structure and the subsidiary antithetic fault in the finite-fault source modelling of the Norcia earthquake.

Neotectonics and tree mortality in a forest ecosystem of the Negro basin: Geomorphic evidence of contemporary seismicity in the intracratonic Brazilian Amazonia

Neotectonic activity was increasingly recorded over the Amazonian lowlands, even during historical times of a few tens of centuries. However, detailed analyses linking structures and styles of deformation are still few considering the large dimension of the region. Such events in a relatively recent time are expected to have caused conspicuous impacts on Amazonian ecosystems and forest disturbance. Changes in topography and hydrology by fracturing, faulting and folding during either subsidence or uplifting are direct effects on landscape that leave marks on the structure of the current forest, such as tree mortality and community succession. Two rectangular elbow-like shaped wetlands from the left margin of the middle Negro River valley in northwestern Amazonia were attributed to neotectonics. Remote sensing imagery was applied to test this hypothesis and also provide a tectonic model that can explain deformation dynamics in this area. The studied wetlands and adjacent river systems display various morphostructural anomalies compatible with a tectonic control, as well as lineaments paralleling main NW and NE-trending regional structures. The geomorphic analysis led to suggest that the wetlands are depressions formed by NW and NE-trending master boundary faults of horizontal displacements intercepted by various subsidiary faults. Transtensional strike slip regime is recorded by both left and right-lateral faults, with the wetlands corresponding to subsiding areas owned to conjugate strike slip faults. Habitat fragmentation and other ecological processes promoted by tectonic deformation would have impacted the overlying forest canopy by changing its structure due to tree mortality. The complex compartmentalization imposed by active tectonics would have exposed the ground to contrasting hydrological conditions, which controlled the rate of tree mortality within the wetlands. We pose that the tectonic disturbance and associated tree mortality documented in the middle Negro River evidence contemporary seismicity within the intracratonic Brazilian Amazonia.

Structural characteristics, bulk porosity and evolution of an exhumed long-lived hydrothermal system

The geometry and spatial variability of fracture networks and matrix porosity of fault rocks are key parameters controlling the permeability and ultimately the fluid flux along fault zones. Detailed understanding of evolution and long-term sustainability of naturally porous and permeable fault rocks is thus of prime importance for predicting the occurrence and the successful exploration of natural fault-bound hydrothermal systems. This study presents continuous structural data and matrix porosity measurements collected from a cored drillhole across a long-lived and still active fault-bound hydrothermal system in the crystalline basement of the Aar Massif (Swiss Alps). Image analysis and He-pycnometry analysis for quantification of matrix porosity of tectonites showing variable ductile and brittle deformation intensity is combined with fracture porosity calculations to develop a bulk porosity profile across this hydrothermally active fault zone. In the investigated example, a central fault core that shows a several meter wide fault breccia with consolidated gouge material of increased porosity with maximum values of 9% (He-pycnometry) and >20% (image analysis) is adjoined by several large subsidiary faults and interconnected by a intensly fractured damage zone embedded in granitic to ultramylonitic host rock showing 0.1–6% porosity. The variable degree of ductile precursors forms a succession of subparallel sealing and high-porosity structures parallel to the fault zone bridged by a dense fracture network. Fluid flow is therefore directly related to the combined effect of fractures and enhanced fault-related matrix porosity, possibly dynamically changing with time due to fracturing and precipitation cycles. This suggests a key importance of matrix porosity within fault core rocks (breccia & fault gouge) for the transport of hydrothermal fluids.

Temporal and Spatial Variations of the M<sub>L</sub> 5.8 Gyeongju Earthquake on September 12, 2016

An earthquake of ML 5.8 hit the Gyeongju area on September 12, 2016. A sequence of foreshock-mainshockaftershock of 588 events with equal to or greater than magnitude 1.5 occurred for six months in this area. Around ninetynine percentage (98.8%) of the total energy was released intensively within a day, and about 80% of the total events took place within a month after the Gyeongju earthquake. The epicentral distribution of aftershocks of major events (ML 5.1, 5.8, 4.5, and 3.5) were elongated in the direction of N30 o E. They correlate well with the focal mechanism solution. These facts support the inference that the Gyeongju earthquakes occurred on a sub-parallel subsidiary fault of the Yangsan fault zone or on the linking damage zones between Deokcheon and Yangsan fault. During the last six years before the Gyeongju earthquake, there were few events within 10-km radius from the epicenter. This seismic gap area was filled with a sequence of the Gyeongju earthquakes. The b value for aftershock of the Gyeongju earthquakes is 1.09.

Dual control of fault intersections on stop-start rupture in the 2016 Central Italy seismic sequence

Large continental earthquakes necessarily involve failure of multiple faults or segments. But these same critically-stressed systems sometimes fail in drawn-out sequences of smaller earthquakes over days or years instead. These two modes of failure have vastly different implications for seismic hazard and it is not known why fault systems sometimes fail in one mode or the other, or what controls the termination and reinitiation of slip in protracted seismic sequences. A paucity of modern observations of seismic sequences has hampered our understanding to-date, but a series of three Mw>6 earthquakes from August to November 2016 in Central Italy represents a uniquely well-observed example. Here we exploit a wealth of geodetic, seismological and field data to understand the spatio-temporal evolution of the sequence. Our results suggest that intersections between major and subsidiary faults controlled the extent and termination of rupture in each event in the sequence, and that fluid diffusion, channelled along these same fault intersections, may have also determined the timing of rupture reinitiation. This dual control of subsurface structure on the stop-start rupture in seismic sequences may be common; future efforts should focus on investigating its prevalence.

Investigating the source of the c. AD 1620 West Coast earthquake: implications for seismic hazards

ABSTRACTA c. AD 1620 West Coast earthquake has been attributed to the northern and central segments of the Alpine Fault but does not match well with paleoseismic indicators or the currently understood recurrence interval of Alpine Fault ruptures. We model the landsliding and resulting river aggradation from a Mw 7.6 earthquake on a modelled reverse fault located between the central Alpine Fault and the main divide of the Southern Alps. The Waiho–Callery and Whataroa catchments are significantly more impacted by coseismic landsliding from this Southern Alps earthquake than from an Alpine Fault earthquake. The modelling suggests that the c. AD 1620 earthquake was not an Alpine Fault event, and that large earthquakes on subsidiary faults (or fault systems) can have larger localised impacts on river behaviour west of the Southern Alps than an Alpine Fault earthquake.

Upward migration of gas in an active tectonic basin: An example from the sea of Marmara

Gases of various sources were collected at the seafloor of the Marmara basin suggesting that the gases expelled have experienced multiple sequences of upward migration, from multiple sources. The pathways of upward migration of gas can be reconstructed by considering the distribution of gas seeps with respect to the near-surface geomorphostructure and the regional stratigraphic architecture of the Marmara Basin. Gas seeps appear to be more favourably localized within a 1–2 km swath around active faults where sediment permeability is probably enhanced by deformation. In the fault zones, fault intersections between sets of transtensive and transpressive subsidiary faults, or between subsidiary faults and main faults, are the preferred gas pathways. These subsidiary structures localize methane seeps, observed as elongated black patches with bacterial mats, gas bubble emission sites, and chemoherms associated with buried mud volcanoes. Gas seeps are, however, rare along active faults segments crossing basin depocenters but focus along basin edges and along topographic highs. Considering the role of sedimentary layers as gas migration pathways can explain this characteristic of the distribution of gas seeps. Fault zones that cross updip gas migration pathways will vent comparatively more gas than fault zones on the downdip side. Moreover, gas accumulation and resulting overpressuring along the western fault segment crossing the Western High may be associated with aseismic fault creep and intense gas emissions at the seafloor. In contrast, the poorly focused seepage along the fault segment crossing the Central High may be linked to the locked state of this fault segment.