Conjugate Fault Research Articles

The 2019 Mw 6.4 and Mw 7.1 Ridgecrest earthquake sequence in Eastern California: rupture on a conjugate fault structure revealed by GPS and InSAR measurements

SUMMARYOn 4 and 6 July 2019, an Mw 6.4 foreshock and an Mw 7.1 main shock successively struck the city of Ridgecrest in eastern California. These two events are the most significant earthquake sequences to strike in this part of California for the past 20 yr. We used both continuous global positioning system (GPS) measurements and interferometric synthetic aperture radar (InSAR) images taken by the Sentinel-1 and ALOS-2 satellites in four different viewing geometries to fully map the coseismic surface displacements associated with these two earthquakes. Using these GPS and InSAR measurements both separately and jointly, we inverted data to find the coseismic slip distributions and fault dips caused by the two earthquakes. The GPS-constrained slip models indicate that the Mw 7.1 main shock was predominately controlled by right-lateral motions on a series of northwest-trending faults, while the Mw 6.4 foreshock involved both right-lateral slipping on a northwest-trending fault and left-lateral slipping on a northeast-trending fault. The two earthquakes both generate significant surface slip, with the maximum inferred slip of 5.54 m at the surface. We estimate the cumulative geodetic moment of the two earthquakes to have been 4.93 × 1019 Nm, equivalent to Mw 7.1. Furthermore, our calculations of the changes in static Coulomb stress suggest that the Mw 7.1 main shock was promoted significantly by the Mw 6.4 foreshock. This latest Ridgecrest earthquake sequence ruptured only the northern part of the seismic gap between the 1992 Mw 7.3 Landers earthquake and the 1872 M 7.4–7.9 Owens Valley earthquake. The earthquake risk in this area, therefore, remains very high, considering the significant accumulation of strain in the Eastern California Shear Zone, especially in the southern part of the seismic gap.

Co- and Early Postseismic Deformation Due to the 2019 Ridgecrest Earthquake Sequence Constrained by Sentinel-1 and COSMO-SkyMed SAR Data

Abstract The 2019 Ridgecrest earthquake sequence ruptured a series of conjugate faults in the broad eastern California shear zone, north of the Mojave Desert in southern California. The average spacing between Global Navigation Satellite System (GNSS) stations around the earthquakes is 20–30 km, insufficient to constrain the rupture details of the earthquakes. Here, we use Sentinel-1 and COSMO-SkyMed (CSK) Synthetic Aperture Radar data to derive the high-resolution coseismic and early postseismic surface deformation related to the Ridgecrest earthquake sequence. Line of sight (LoS) Interferometric Synthetic Aperture Radar displacements derived from both Sentinel-1 and CSK data are in good agreement with GNSS measurements. The maximum coseismic displacement occurs near the Mw 7.1 epicenter, with an estimated fault offset of ∼4.5 m on a northwest-striking rupture. Pixel tracking analysis of CSK data also reveals a sharp surface offset of ∼1 m on a second northwest-striking fault strand on which the Mw 6.4 foreshock likely nucleated, which is located ∼2–3 km east of the major rupture. The lack of clear surface displacement across this fault segment during the Mw 6.4 event suggests this fault might have ruptured twice, with more pronounced and shallow slip during the Mw 7.1 mainshock. Both Sentinel-1 and CSK data reveal clear postseismic deformation following the 2019 Ridgecrest earthquake sequence. Cumulative postseismic deformation near the Mw 7.1 epicenter ∼2 months after the mainshock reaches ∼5 cm along the satellites’ LoSs. The observed postseismic deformation near the fault is indicative of both afterslip and poroelastic rebound. We provide data derived in this study in various data formats, which will be useful for the broad community studying this earthquake sequence.

Orogen-transverse structures in the eastern Himalaya: Dextral Riedel shear along the Main Boundary Thrust in the Garu-Gensi area (Arunachal Pradesh, India), implication in hydrocarbon geoscience

Abstract Structural mapping and analyses reveal deformation in the rocks of the Lesser Himalayan Miri and Gondwana, and Lower Siwalik Dafla successions across the Miri Thrust (MT) and the Main Boundary Thrust (MBT) in the Garu-Gensi area (Arunachal Himalaya, India). The-north-dipping Gondwana sequence is sandwiched between the Miri Group of rocks in the north and the Lower Siwalik Dafla sandstones in the south. The Gondwana rocks folded four times (D1-D4). Detail outcrop mapping and structural analyses substantiated by satellite images elucidate that the Miri, the Gondwana and the Lower Siwalik rock sequences in the area are traversed by two sets of NW and NE trending cross-fault zones with~70–100 m thickness mappable for ~70 km. The cross-faults trend NW around the locality Daring, while in the western part around Gensi, they trend NE. The westernmost fault slipped dextrally and the easternmost one at Daring sinistrally. These transverse faults offset the MBT: 0.52–0.59 km for the western- and 0.58–1.41 km for the eastern faults. The deformation in the Gondwana sequence may be constrained as syn-Himalayan (i.e., India-Asia collisional event). This was followed by a compression from E/ESE due to the post-Eocene Indo-Burmese collision. The culminating phase of combined India-Asia and Indo-Burmese compression is manifested in field with a strong NE-directed compression with dextral Reidal shear developing in the transverse conjugate fault zones in the area with maximum displacement towards SE. Cluster of the earthquake data indicates record of the events (M ≥ 6) within 30–80 km depth-both N and S of the MBT in the study area. Further, the southerly continuation of the transverse faults below the alluvium can provide vital clue in studying the potential hydrocarbon trap in the northern fringe of the Brahmaputra river.

Present-day interseismic deformation characteristics of the Beng Co-Dongqiao conjugate fault system in central Tibet: implications from InSAR observations

SUMMARYNumerous V-shaped conjugate strike-slip fault systems distributed between the Lhasa block and the Qiangtang block serve as some of the main structures accommodating the eastward motion of the Tibetan Plateau. The Beng Co-Dongqiao conjugate fault system is a representative section, and determining its tectonic environment is a fundamental issue for understanding the dynamic mechanism of the V-shaped conjugate strike-slip fault systems throughout central Tibet. In this paper, we investigate the deformation rates of the Beng Co-Dongqiao conjugate faults using 3 yr of SAR data from both ascending and descending tracks of Sentinel-1 satellites. Only interferograms with a long temporal baseline were used to increase the proportion of the deformation signals. The external atmospheric delay product and the InSAR stacking strategy were employed to reduce various errors in the large-spatial-coverage Sentinel-1 data. The InSAR results revealed that the fault-parallel deformation velocities along the eastern and western segments of the Beng Co fault are 5 ± 1 mm/yr and 2.5 ± 1 mm/yr, respectively. The second invariant of the horizontal strain rates shows that the accumulated strain is centered on the eastern segment of the Beng Co Fault and Gulu rift. The velocity and strain rate fields show that the Anduo-Peng Co faults may be paired with the Beng Co fault to form a new conjugate system and the tectonic transformation between the Beng Co fault and Gulu rift. These results can better explain the tectonic deformation environment of the Beng Co-Dongqiao conjugate fault system and provide insights on the crustal dynamics throughout the entire plateau interior.

The 2014 Mw 6.1 Ludian Earthquake: The Application of RADARSAT-2 SAR Interferometry and GPS for this Conjugated Ruptured Event

Although the Zhaotong–Ludian fault is a seismically active zone located in the boundary between the Sichuan–Yunnan block and the South China block, it has not experienced a large earthquake greater than Mw 7 since at least 1700. On 3 August, 2014, an Mw 6.1 earthquake (the Ludian earthquake) ruptured the Zhaotong active belt in Ludian County, Yunnan province, China. This earthquake was the largest earthquake recorded in the region since 2000, and it provides us with a unique opportunity to study the active tectonics in the region. The analysis of the aftershocks showed that two conjugate faults could have been involved in the event. We first used Global Positioning System (GPS) data and C-band RADARSAT-2 imagery to map the coseismic surface deformation. We then inverted the derived coseismic deformation for the slip distribution based on the constructed conjugate fault model. Finally, the coulomb failure stress due to the Ludian earthquake was estimated to investigate the potential seismic hazards in this region. Our investigations showed that the Ludian earthquake was mainly a bilateral rupture event. The major slip of the main shock was located at depths of 0–5 km, which is close but does not superpose with the aftershocks that are mostly located at depths of 5–20 km. Interestingly, the seismic moment released by the aftershocks (6.9 × 1018 N∙m) was greater than that of the main shock (2.6 × 1018 N∙m). This evidence suggests that the accumulated elastic strain at depths of 0–20 km could have been fully released by the Ludian earthquake and its subsequent aftershocks. Furthermore, our analysis of the coulomb failure stress changes due to the main shock showed that the aftershocks could be the result of dynamic triggering rather than static triggering.

Constraining the Oceanic Lithosphere Seismogenic Zone Using Teleseismic Relocations of the 2012 Wharton Basin Great Earthquake Sequence

AbstractThe great 2012 Mw 8.6 strike‐slip earthquake beneath the Wharton Basin generated a complex aftershock sequence that maps onto a system of conjugate faults. Analysis of high‐precision aftershock locations with improved depth constraint is used here to characterize the seismogenic limits of the oceanic lithosphere. The study presents teleseismic double‐difference earthquake relocation results for 695 events in and around the 2012 Wharton Basin intraplate earthquake sequence. We highlight seven major clusters of seismicity and show that the 2012 earthquake sequence ruptured in the oceanic crust and upper mantle. The refined aftershock locations projected onto available mainshock finite‐fault models show that aftershocks occur outside the largest coseismic slip region and tend to cluster in low slip areas, a pattern commonly seen for large continental and megathrust sequences. For events with depth phases, the relocated focal depths generally correspond to predicted depths from pP‐P time observations. Reported pP‐P observations for intraplate events correspond to depths ranging from ~5 to 35–40 km, such that the deepest events occur within the expected limit of brittle seismic failure at 600 °C, here defined by a half‐space cooling model of the region. The 74 low magnitude events in our catalog that locate below the 600 °C isotherm do not have consistent depth phase observations and cannot be interpreted as strong evidence of rupture into the ductile regime. The refined double‐difference catalog supports that, along with deep coseismic rupture, moderate‐sized earthquakes ruptured across the full extent of the elastic oceanic lithosphere in the Wharton Basin.

The kinematics of the Dasht-e Bayaz earthquake fault during Pliocene-Quaternary: Implications for the tectonics of eastern Central Iran

The Arabia - Eurasia convergence is accommodated across the Iranian plateau and surrounding mountain ranges. Strike-slip faults play significant roles in the current deformation of the Iranian plateau and were responsible for several moderate to large earthquakes, especially in east of Iran. The sinistral Dasht-e Bayaz fault shows conspicuous seismic and geological activities, while the role of the fault in the accommodation of active convergence is still debated. This paper focuses on the Pliocene-Quaternary to present-day states of stress in the region affected by the Dasht-e Bayaz fault. We applied the fault-slip inversion technique to both kinds of seismologic and geological fault slip data. The inversion results indicate a mean N045±5°E trending σ1 in the modern stress field, which agrees with the present-day tectonic regime (regional N050±05°E trending σ1) deduced from the inversion of earthquake focal mechanism data. The paleostress state is characterized by a N135±15°E trending regional mean σ1, with a transpressional stress regime; E-W faults of the region such as the Dasht-e Bayaz fault were reverse dextral in this paleostress field. We show that the shift in fault kinematics during the Pliocene-Quaternary times has not been restricted to northeastern parts of the Iranian plateau. The cross-cutting relationship between active N-S dextral and E-W sinistral faults indicates a crosswise to conjugate fault arrangement in the middle and the eastern end of the Dasht-e Bayaz fault. Integrating our results at a larger regional scale, we present a kinematic model in which the region between Lut and Kopeh Dagh is divided by the Doruneh fault into two tectonic domains. In the northern domain, active convergence is taken up by the extrusion of fault-bounded crustal blocks while, in the southern domain the convergence is accommodated through crosswise strike-slip faulting accompanied by reverse/thrust faulting in confining wedges.

Observing Oblique Slip During Rift Linkage in Northern Afar

AbstractExtensional plate boundaries are segmented by offsets that transfer extension between the ends of adjacent portions of the rift by linkage zones ranging in width from a few tens of kilometers to several hundreds of kilometers. However, the kinematics of linkage zones is poorly constrained as direct observations are difficult to make. Here we combine InSAR, seismicity, and structural geology data from the Afar rift to show that an active linkage zone currently connects the two offset Erta Ale and Tat Ali segments. The overall right‐lateral shear between the segments is accommodated primarily by oblique left‐lateral slip along faults subparallel to the rift segments but an active conjugate fault system with right‐lateral slip is also present. Our results provide the first direct observational evidence that offset rift segments during continental breakup can be linked by a shear zone composed of a conjugate set of oblique slip faults.

Seismic Characteristics of the Nootka Fault Zone: Results from the Seafloor Earthquake Array Japan–Canada Cascadia Experiment (SeaJade)

Abstract The Nootka fault zone (NFZ) divides the incoming Explorer and Juan de Fuca plates of the Cascadia subduction zone. Three months of seafloor monitoring using 33 ocean‐bottom seismometers off the west coast of Vancouver Island has allowed us to better understand the tectonic configuration and seismogenic characteristics of the NFZ. We have learned that the NFZ is comprised of northern and southern primary bounding faults, and several conjugate faults developed subperpendicular to the primary faults. Earthquakes typically occur over the depth ranges of 15–20 and 6–15 km along the primary bounding and conjugate faults, respectively. Focal mechanisms reveal that the most common modes of failure in this region are left‐lateral strike slip, with normal faulting occurring along the southwestern extent of the NFZ and thrust faulting to the northeast before the subduction front. Seismic tomography suggests that the oceanic Moho is at a depth of 12–14 km below sea level (10–12 km below seafloor) just seaward of the Cascadia deformation front, and that it deepens to 19 km (17 km below seafloor) approximately 20 km landward of the deformation front. Converted phase analysis illuminates four velocity‐contrasting interfaces with average depths below sea level deepening landward of the subduction front at ∼4–6, ∼6–9, ∼11–14, and ∼14–18 km. We interpret them as the sedimentary basement, upper–lower crust boundary, oceanic Moho, and the base of the highly fractured and seawater or mineral enriched veins within mantle. The precipitation of minerals such as quartz or the formation of talc, which is made possible by the intense degree of fracturing within the NFZ facilitating the infiltration of seawater, may reduce mantle velocities, as well as VP/VS ratios. The lack of seismicity observed along the interplate thrust zone in northern Cascadia may suggest that the megathrust fault is completely locked, consistent with prior studies.

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

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.

A super-critical stress model for polymodal faulting of rocks

A polymodal faulting phenomenon is further explored here that comprises penecontemporaneous fractures showing finite dispersion of strikes and dips with poles confined in one annular or two arc regions in stereographic projection. The fracture pattern represents three-dimensional strain of rocks with characteristics distinct from previously known bi-, four- or poly-modal faulting, and thus is unable to be interpreted in terms of the corresponding theories such as conjugate faulting, slip on preexisting weak planes and tensile crack interaction, respectively. Here, a super-critical stress model is advanced to decode stress significance of the specific fracture pattern with a particular emphasis on model prediction for orientations of failure surfaces. The model is an extension of the slip model of Reches (1983) that prescribes formation of four sets of faults in orthorhombic symmetry conforming to the Coulomb-Mohr criterion of τ=C+μσ (equivalent to C=F=τ-μσ where C is cohesion of rocks and F is function of shear stress τ, normal stress σ and friction coefficient μ). The mechanic boundary condition means a super-critical stress state (Fmax>C) because stress resolution allows only two sets of planes to coevally have maximum value of F and thus shear fail equally according to Andersonian (1905, 1951) conjugate faulting derivation that corresponds to a critical stress state, i.e., Fmax is just equal to C. This super-critical stress state further means that there are some planes on which F>C certainly on verge of faulting at least possessing a not even lower tendency than those planes on which C=F, and thus polymodal faulting results. The planes with F≥C tend to be areal distribution with arc or annular geometry in stereographic projection of poles. Given the Coulomb-Mohr friction and failure criteria are not fundamentally different, the super-critical model may also apply to initial failure of intact rocks. The new model represents a major reorganization in thinking about shear failure of rocks and fracturing patterns. Several natural examples are provided with fracture patterns fit to the super-critical model proposed here.

Complexity of the 2009 L'Aquila earthquake causative fault system (Abruzzi Apennines, Italy) and effects on the Middle Aterno Quaternary basin arrangement

An Mw 6.1, devastating earthquake, on April 6, 2009, struck the Middle Aterno Valley (Abruzzi Apennines, Italy) due to the activation of a poorly known normal fault system. Structural analysis of the fault population and investigation of the relationships with the Quaternary continental deposits through integrated field and laboratory techniques were conducted in order to reconstruct the long-term, tectono-sedimentary evolution of the basin and hypothesize the size of the fault segment.A polyphasic evolution of the Middle Aterno Valley is characterized by a conjugate, ∼E-W and ∼NS-striking fault system, during the early stage of basin development, and by a dip-slip, NW-striking fault system in a later phase.The old conjugate fault system controlled the generation of the largest sedimentary traps in the area and is responsible for the horst and graben structures within the basin. During the Early Pleistocene the E-W and NS system reactivated with dip-slip kinematics. This gave rise to intra-basin bedrock highs and a significant syn-tectonic deposition, causing variable thickness and hiatuses of the continental infill.Subsequently, since the end of the Early Pleistocene, with the inception of the NW-striking fault system, several NW-strands linked into longer splays and their activity migrated toward a leading segment affecting the Paganica-San Demetrio basin: the Paganica-San Demetrio fault alignment.The findings from this work constrain and are consistent with the subsurface basin geometry inferred from previous geophysical investigations. Notably, two major elements of the ∼E-W and ∼NS-striking faults likely act as transfer to the nearby stepping active fault systems or form the boundaries, as geometric complexities, that limit the Paganica-San Demetrio fault segment overall length to 19 ± 3 km. The resulting size of the leading fault segment is coherent with the extent of the 6 April 2009 L'Aquila earthquake causative fault.The positive match between the geologic long-term and coseismic images of the 2009 seismogenic fault highlights that the comprehensive reconstruction of the deformation history offers a unique contribution to the understanding faults seismic potential.

Simulating intersection angles between conjugate faults in sea ice with different viscous–plastic rheologies

Abstract. Recent high-resolution pan-Arctic sea ice simulations show fracture patterns (linear kinematic features or LKFs) that are typical of granular materials but with wider fracture angles than those observed in high-resolution satellite images. Motivated by this, ice fracture is investigated in a simple uni-axial loading test using two different viscous–plastic (VP) rheologies: one with an elliptical yield curve and a normal flow rule and one with a Coulombic yield curve and a normal flow rule that applies only to the elliptical cap. With the standard VP rheology, it is not possible to simulate fracture angles smaller than 30∘. Further, the standard VP model is not consistent with the behavior of granular material such as sea ice because (1) the fracture angle increases with ice shear strength; (2) the divergence along the fracture lines (or LKFs) is uniquely defined by the shear strength of the material with divergence for high shear strength and convergent with low shear strength; (3) the angle of fracture depends on the confining pressure with more convergence as the confining pressure increases. This behavior of the VP model is connected to the convexity of the yield curve together with use of a normal flow rule. In the Coulombic model, the angle of fracture is smaller (θ=23∘) and grossly consistent with observations. The solution, however, is unstable when the compressive stress is too large because of non-differentiable corners between the straight limbs of the Coulombic yield curve and the elliptical cap. The results suggest that, although at first sight the large-scale patterns of LKFs simulated with a VP sea ice model appear to be realistic, the elliptical yield curve with a normal flow rule is not consistent with the notion of sea ice as a pressure-sensitive and dilatant granular material.

Mini thief zones: Subcentimeter sedimentary features enhance fracture connectivity in shales

This study examines the influences on fluid flow within a shale outcrop where the networks of two distinct paleoflow episodes have been recorded by calcite-filled veins and green alteration halos. Such direct visualization of flow networks is relatively rare and provides valuable information of fluid-flow behavior between core and seismic scale. Detailed field mapping, fracture data, and sedimentary logging were used over a 270 m2 (2910 ft2) area to characterize the paleo–fluid-flow networks in the shale. Distal remnants of turbidite flow deposits are present within the shale as very thin (1–10 mm [0.04–0.4 in.]) fine-grained sandstone bands. The shale is cut by a series of conjugate faults and an associated fracture network, all at a scale smaller than seismic detection thresholds. The flow episodes used fluid-flow networks consisting of subgroups of both the fractures and the thin turbidites. The first fluid-flow episode network was mainly comprised of thin turbidites and shear fractures, whereas the network of the second fluid-flow episode was primarily small joints (opening mode fractures) connecting the turbidites. The distribution of turbidite thicknesses follows a negative exponential trend. which reflects the distribution of thicker turbidites recorded in previous studies. Fracture density varies on either side of faults and is highest in an area between closely spaced faults. Better predictions of hydraulic properties of sedimentary-structural networks for resource evaluation can be informed from such outcrop subseismic scale characterization. These relationships between the subseismic features could be applied when populating discrete fracture networks models, for example, to investigate such sedimentary-structural flow networks in exploration settings.

Hydrogeochemical and isotopic assessment and geothermometry applications in relation to the Karahayıt Geothermal Field (Denizli Basin, SW Anatolia, Turkey)

The Karahayit Geothermal Field (KGF) is located at the northern margin of the Denizli Basin in SW Anatolia (Turkey) where thermal waters discharge along the Quaternary normal fault segments locally displaced by conjugate transfer faults. Major and trace element contents and stable isotopes (δ18O, δ2H, δ3H and δ13C) of the KGF thermal and cold waters were analyzed in order to determine their origin and evolution and reservoir temperatures. Two main thermal waters, indicated as being fed by steam-heated aquifers, are recognized: (1) Ca-HCO3 and (2) Ca-SO4 types. All thermal waters have shown non-equilibrium chemical conditions, indicating mixing processes. According to the δ18O (−9.14 to −8.07‰) and δ2H (−59.50 to −51.80‰) data, the KGF thermal waters are meteoric in origin and originated from precipitation in the northern piedmont of Yenice Horst with elevation of 900 m asl. Various geothermometers yield the reservoir temperatures of 80–130 °C. A conceptual flow model for the KGF was suggested as follows: the thermal waters were derived from a regional flow system with high recharge areas and deep circulation depth. The NW- and NE-trending conjugate fault segments (the Pamukkale Fault Zone) serve as features of hydraulic channelling, magmatic heat source and fluid convection in the extensional settings. This fault system is characterized by migration of a large amount of CO2-rich gas from the deep geothermal reservoir. Consequently, the KGF is characterized by a fault-hosted geothermal system affected by magmatism and active extensional tectonics, the same as other geothermal fields in this crustal extensional setting.

Seismic Activity of the Khambinskii Fault (Southwestern Transbaikalia)

Abstract —The paper presents results of a seismogeological study based on analysis of seismic data and historical facts about the seismic activity of the Khambinskii fault zone. According to the data obtained, a genetic type of dislocations on conjugate faults (Gusinoe Ozero and Orongoi paleoseismogenic structures) is related to reverse faults with a strike-slip component. Geophysical studies of the Gusinoe Ozero structure have determined the dip of the fault plane toward the mountain framing of the depression and its outcrop at the bottom of the seismic scarp. The significant seismic potential of the Khambinskii fault is responsible for the maximum intensity of shocks in the nearby cities and settlements of southeastern Transbaikalia. The seismic fault activity has been confirmed by the historical earthquakes of 1856 and 1885, the M = 5 earthquake that occurred on 2 October 1980, and at least two prehistoric earthquakes. The latest of the latter occurred no earlier than ~4 ka and had M = 7.0–7.3, while the earliest was even more intense and took place in the first half of the Holocene, no later than ~6 ka.

Structural characterization of intracratonic strike-slip faults in the central Tarim Basin

The strike-slip fault systems in the central Tarim Basin, China, afford an exceptional opportunity to document the structural characteristics and evolution process of small displacement intracratonic strike-slip faults using three-dimensional seismic reflection data. These strike-slip faults display subvertical segments at depth and en echelon normal fault zones where relatively shallow. Fault segmentation and flower structures can be commonly observed in plan view and cross-section view, respectively. Consistent with the notion that segment coalescence is the fundamental process for fault evolution, the mean segment length of representative strike-slip faults examined in this study is positively correlated to the measured fault offset. The width of the en echelon normal fault zone is positively correlated with the estimated maximum overburden thickness. The integrated data sets suggest that the evolution of the conjugate fault array followed a sequential evolution process instead of forming simultaneously. The switch in slip direction of the master fault of the conjugate fault array is attributed to the change of stress orientation. Regarding individual strike-slip faults, increase in displacement induces the formation of faults with lower fault-array angles linking initially formed en echelon normal faults. In cross sections, throughgoing fault surfaces can also form, connecting the lower subvertical fault segment and the upper en echelon normal faults. The presented data sets and evolution models established in this study can be used as tools to better predict the structural attributes of subsurface strike-slip fault systems with important consequences for reservoir formation and hydrocarbon accumulation in the Tarim Basin in particular, and in ancient marine basins in general.

The 20 May 2016 Petermann Ranges earthquake: centroid location, magnitude and focal mechanism from full waveform modelling

Ground velocity records of the 20 May 2016 Petermann Ranges earthquake are used to calculate its centroid-moment-tensor in the 3 D heterogeneous Earth model AuSREM. The global-centroid-moment-tensor reported a depth of 12 km, which is the shallowest allowed depth in the algorithm. Solutions from other global and local agencies indicate that the event occurred within the top 12 km of the crust, but the locations vary laterally by up to 100 km. We perform a centroid-moment-tensor inversion through a spatiotemporal grid search in 3 D allowing for time shifts around the origin time. Our 3 D grid encompasses the locations of all proposed global solutions. The inversion produces an ensemble of solutions that constrain the depth, lateral location of the centroid, and strike, dip and rake of the fault. The centroid location stands out with a clear peak in the correlation between real and synthetic data for a depth of 1 km at longitude 129.8° and latitude –25.6°. A collection of acceptable solutions at this centroid location, produced by different time shifts, constrain the fault strike to be 304 ± 4° or 138 ± 1°. The two nodal planes have dip angles of 64 ± 5° and 26 ± 4° and rake angles of 96 ± 2° and 77 ± 5°, respectively. The southwest-dipping nodal plane with the dip angle of 64° could be seen as part of a near vertical splay fault system at the end of the Woodroffe Thrust. The other nodal plane could be interpreted as a conjugate fault rupturing perpendicular to the splay structure. We speculate that the latter is more likely, since the hypocentres reported by several agencies, including the Geoscience Australia, as well as the majority of aftershocks are all located to the northeast of our preferred centroid location. Our best estimate for the moment magnitude of this event is 5.9. The optimum centroid is located on the 20 km surface rupture caused by the earthquake. Given the estimated magnitude, the long surface rupture requires only ∼4 km of rupture down dip, which is in agreement with the shallow centroid depth we obtained.

An Overview of Tensors, Gradient and Invariant Products in Imaging and Qualitative Interpretation

Potential Field Gradient Tensors are a multichannel dataset combining 5 independent components in a matrix array. As such, the data can be used and combined in many ways. A very common problem right across the world of geoscience is that even standard potential field transforms are not actually well understood by users. How does one expand grid transform concepts into the realm of tensors, where so many new combinations and concepts such as Invariants and Phase exist, and create lasting basis for industry interpretation?It is important that all images used in potential field analysis carry some sort of physical meaning which is understood by the interpreter. True understanding arises from geophysically modelling a known 3D geological model, creating the grid transforms from the forward response of the model, and comparing these to the geology.3D forward gravitational responses of a 3D model of a simple two-body basin-basement system with conjugate faulting and a dome-basin shape are used to generate the examples. Depths to the Basin-Basement interface were computed from the model and are presented as grids and contours draped on the gravity gradient imaging products to illustrate their responsiveness to the basement architecture.Various combinations of traditional gravity and its gradient transforms, as well as tensor invariants and phase products, are assessed against the model. It is shown that certain imaging products show more responsiveness to physical property variations, whilst others are more sensitive to geometry, but combining these in novel ways can approach understanding of subsurface mapping possibly not explored previously using potential fields.

AcehSeis project provides insights into the detailed seismicity distribution and relation to fault structures in Central Aceh, Northern Sumatra

For the first time a dense seismic network (AcehSeis) consisting of 30 short-period seismic stations was installed for 10 months from July 2014 in order to record crustal earthquakes in Central Aceh, Northern Sumatra, following the hazardous earthquake with magnitude Mw. 6.1, July 2nd, 2013. The objective of this research is to investigate the active fault characteristics derived from local earthquake data combined with a digital elevation model. The network recorded more than 1790 local events occurring along the main Great Sumatran Fault (GSF) and its secondary faults. Local earthquakes with azimuthal gap angles greater than 180° were excluded to avoid non-unique earthquake locations. Therefore from all recorded earthquakes, 1127 local earthquakes were used for further seismological analysis. A method of simultaneous earthquake relocation and 1D velocity determination was applied to obtain better location of earthquakes. The output of the first relocated seismicity was then relocated again by using a double difference relocation method. After relocation, earthquakes are concentrated in a narrow zone along the active fault, indicating the reliability of the earthquake location. We then determined the focal mechanism of earthquakes, based on the first polarity picks.The seismicity distribution correlates with the well-known GSF and its main secondary faults. Earthquake locations also clearly revealed several active segments which include the Batee, Beutong, Tangse-Geumpang, Lampahan, Aceh and Seulimeum, and Nisam faults. Also less-known faults as the Pamue and Atu Lintang-Peusangan faults are imaged. In Northern Aceh an unknown earthquake cluster associated with an active fault, the Nisam Fault was detected, which is also visible in the topography. The seismicity along GSF is offset at around 96.7 E, constrained by the nearly N-S striking Atu Lintang-Peusangan Fault confirming the segmentation of GSF at this longitude. The Aceh segment and the Seulimum Fault in the northernmost region of the network are both active and clearly imaged by the seismicity. The Pamue and the Lampahan Faults seem to be the splay of GSF as indicated by identical strike direction as the GSF. The Lampahan fault is characterized by NW-SE seismicity lineation, which is in agreement with the focal mechanisms. The Laut Tawar Lake could be a part of the Lampahan Fault that is also offset at ∼96.7 E at the line of the Atu Lintang-Peusangan fault. The Burni Geureudong Fault on the other hand strikes ENE-WSW which suggests a conjugate fault of the Lampahan Fault.