Fault Angle Research Articles

A nonlinear inversion of InSAR-observed coseismic surface deformation for estimating variable fault dips in the 2008 Wenchuan earthquake

Satellite Interferometric Synthetic Aperture Radar (InSAR) is playing an increasingly important role in the observation of coseismic surface deformation caused by earthquakes, and has been used to invert for subsurface fault structure and reveal earthquake source mechanisms. However, the mapping of complex non-planar or curved (e.g., listric-shaped) faults still remains a challenging task due to variable dips along the underground depth and the impenetrability of the deep crust. Here, we develop a set of new inversion algorithms to determine the listric fault geometry with InSAR- and GPS-observed surface deformation as the significant constraints. The fault surface with variable dip angles is discretized into consecutive sub-fault layers along the down-dip direction. A nonlinear iteration algorithm is used to minimize the objective function to determine the dip angle for each sub-fault layer. The proposed method is first tested using synthetic data to show its effectiveness for retrieval of varying fault geometry dips, and then applied to the 2008 Mw 7.9 Wenchuan earthquake that ruptured the Yingxiu-Beichuan fault for over 320 km along the southwest-northeast strike. The inversion shows that the dip angle of the seismogenic fault is up to 76° near the surface layer, and gradually decreases along the down-dip direction. A significant decrease in dip occurs within the depths of 6–15 km with a dip of 32° at a depth of 15 km. The dip angle decreases to 2° at a depth of 20 km, and finally merges with the subparallel PengGuan fault, which is basically consistent with geological investigations and seismic waveform data inversion. Using the inferred fault geometry, the slip model associated with the event is estimated. Five high-slip concentrations along the strike of the Yingxiu-Beichuan fault are recognized. The inversion misfit of InSAR data is reduced to 7.1 cm with a significant improvement compared to previous studies.

Evaluation of the Effect of Fault on the Stability of Underground Workings of Coal Mine through DEM and Statistical Analysis

The rock mass around an excavation is generally traversed by different geological discontinuities such as faults, folds, slips, joints, etc. Fault is one of the major geological discontinuities which creates lot of difficulties during underground winning of coal. Entire stress regime and ground conditions in the formation are altered in and around the faults. Faults also impose detrimental effects by introducing impurities, including clay and various forms of mineral matter into the coal seams; opening of pathways for the influx of water and gas into the underground workings; displacing the coal seams upward/downwards making the coal seams difficult or sometimes impractical to mine. Appropriate evaluation of the effect of the fault on the stability of the underground workings is a requisite for safe design of the underground mining structures. In this paper, a study has been carried out to assess the effect of the fault on the stability of underground coal mines by numerical simulation with distinct element method (DEM). On the calibrated DEM model, parametric study has been performed by varying the selected parameters, the dip and the friction angles of the fault. The analysis of variance (ANOVA) shows that both the factors have statistically significant effect on the strength of the coal pillar. Similarly, the displacement of the immediate roof and the height of the disturbed strata are evaluated by the DEM modelling and statistical analysis when the fault passes through the middle of the gallery. The results of ANOVA for both cases indicate that the both factors have significant effect on the displacement of the immediate roof and the height of the disturbed strata. It is obtained from the study that the low angle fault causes high instability in the immediate roof. The paper has been supplemented with the field observations where instability in underground roadways of a coal mine in India is caused by the fault. It was observed in VK-7 incline mine of Singareni Collieries Company Limited, India that there was sudden failure of immediate roof of a roadway where a low angle fault crosses the middle of the roadway. The findings of the paper help to understand the behaviour of the coal pillar and the surrounding rock mass in the presence of the fault. The study would also help to take appropriate decisions about the unstable regions of the working safeguarding safety in underground coal mines.

HVSRMS localization formula and localization law: Localization diagnosis of a ball bearing outer ring fault

Abstract The angle position of the outer ring fault has a significant influence on the residual life and the operation performance of ball bearing. Therefore, the realization of the localization diagnosis of the outer ring fault is of great importance to the bearing performance degradation assessment and the life prediction. In this paper, a novel localization diagnosis method, named horizontal–vertical synchronized root mean square (HVSRMS) localization law and localization formula, is developed to diagnose the angle position of outer ring fault accurately. Firstly, based on the established nonlinear contact model of the bearing system, the detailed change law of static contact force when the angle position of the outer ring fault is on both sides of 270° is studied, which lays a theoretical foundation for the discovery of localization law. Subsequently, the vibration acceleration signal and dynamic contact force of the outer ring fault are synchronously analyzed through theoretical research, and the new feature for localization diagnosis is presented as: the prime impact direction of horizontal vibration acceleration (IPDoHVA). Therefore, the outer ring fault localization law of ball bearing is proposed. On this basis, the HVSRMS localization formula is improved by combining it with the localization law. Thus the one-to-one mapping relationship between HVSRMS and outer ring fault angle position is determined. Finally, experiments are carried out to validate the accuracy and feasibility of the proposed localization law and localization formula. The research results provide a new approach and method for localization diagnosis and the residual life prediction of a defective bearing.

Controls of faulting on synrift infill patterns in the Eocene PY4 Sag, Pearl River Mouth Basin, South China Sea

Tectono-stratigraphic analysis of the Eocene PY4 Sag, Pearl River Mouth Basin, reveals that the evolution of normal faulting exerted an important control on the basin infill patterns. Seismic, well log and core data jointly indicate the Eocene Wenchang Formation can be subdivided into four third-order sequences that are related to early rift (WSQ1), rift climax (WSQ2–3) and late rift (WSQ4) stages. During the early rift stage, the PY4 Sag was defined by three, small-sized depocentres; it was related to low tectonic subsidence and likely overfilled by footwall-derived fan-delta deposits. The isolated depocentres coalesced and formed a larger and more uniform depocentre during the rift climax stage. Significantly increased tectonic subsidence during the rift climax WSQ2 sub-stage gave rise to prominent deep lake deposition including nearshore subaqueous fans, turbidite fan deposits and good source rocks. Two sediment infill patterns adjacent to boundary fault are developed during the rift climax WSQ3 sub-stage: (i) a more likely sediment balance-filled pattern controlled by the low-angled fault and (ii) a sediment under-filled pattern adjacent to the relatively high-angle boundary fault. The late rift stage was characterised by a waned displacement on the NE-striking fault but an increase in displacement on E-striking rift-related faults; this non-synchronous faulting has led to the sediment overfilled pattern observed in the South Sub-sag but a more sediment balance-fill on the South-western Slope. The variability in the basin infill patterns is suggested to have resulted from the different fault displacement patterns, which are closely linked to the boundary fault angles and possible stress field transitions during the basin evolution. This study may provide potential implications for basin infill evaluations in the Eocene Pearl River Mouth Basin and other similar rift systems.

Fault Angle Control on Potential Seismic Slip in the Illinois Basin Region

Fault Angle Control on Potential Seismic Slip in the Illinois Basin Region

A study on stress of buried oil and gas pipeline crossing a fault based on thin shell FEM model

With the large deformation of the thin shell and the coupling interaction between pipe and soil considered, a FEM model based on shell elements is established for different conditions. These conditions include whether the site located on each side of the seismic fault is consistent or different and whether the fault zone exists at the intermediate areas of the fault. The strain response of large diameter oil and gas pipeline of grade X80 steel passing through seismic fault is studied using the shell elements of large deformation in finite element method (FEM). Considering the nonlinearity of pipeline and soil interaction, the influences on the strain response from the parameters including the pipe wall thickness, buried depth, width of the fault zone, fault displacement, intersection angle of pipe and fault under the corresponding site conditions, as well as the tendency curves, are systematically analyzed. The seismic strain responses of pipeline are compared under different conditions and the sensitivity of each influencing factor are discussed. When the width of fault zone is wider, the axial deformation of the pipeline is smaller. It means pipeline crossing a wider fault zone can resist against the earthquake better. Simultaneously, the pipeline in a large diameter is more likely to become deformed with different soil properties on either side of the fault than that with same soil properties on either side of the fault. The maximum strain response occurs on the side with harder soil, where the damage is more serious.

Propagation and termination of a strike slip fault in an extensional domain: The westward growth of the North Anatolian Fault into the Aegean Sea

Geometry and kinematics of the active submarine faults over the North Aegean Trough, a complex and highly deforming boundary zone between the Eurasian and the Anatolian/Aegean micro-plates, were examined to investigate how a strike slip motion propagates through an extensional domain. The bathymetric and seismic reflection data analysis show, that the investigated area is affected from east to west, by three sets of active faults striking ENE-WSW, NE-SW and WNW-ESE. The ENE-WSW and NE-SW striking faults are right lateral strike slip faults and are considered strands of the northern branch of the North Anatolian Fault into the north Aegean Sea. The former set bounds the Saros basin and the latter set occupies the eastern and central part of the Sporades Basin in the eastern and western parts of the North Aegean Trough, respectively. The WNW-ESE striking faults are normal low angle faults and develop at the western parts of the Sporades Basin. It is suggested that the strike-slip fault system advances westwards and when it reaches the 23.8°E meridian, its right lateral motion is transferred to the WNW-ESE normal faults system, forming a 70 km wide normal faulting zone through which the shear is distributed. The interaction between the strike slip and normal faults probably started with the arrival of the northern branch of the North Anatolian Fault into the western part of the Sporades Basin since 430 ka BP (Marine Isotopic Stage, 12). The described fault architecture and their spatial distribution in the North Aegean Trough are consistent with: (i) the earthquake fault plane solutions and (ii) the stress and strain pattern prevailing across the area. Furthermore, all these three provide evidence that the northern branch of the North Anatolian Fault terminates at the western end of the NAT and does not continue to central Greece.

THE MORPHONEOTECTONIC STRUCTURE OF THE TRANSITIONAL ZONE BETWEEN THE GORTYNIA MT. HORST AND THE PYRGOS-OLYMPIA BASIN (CENTRAL -WESTERN PELOPONNESE, GREECE)

The present paper aims to the understanding of the neotectonic deformation in areas where no post-alpine sediment occurs. The study area is located at the transitional zone between the horst ofGortynia Mt. (Arcadia) and the Pyrgos-Olympia basin in the central-western Péloponnèse and is tectonically and seismically active. The studied neotectonic faults can be distinguished in low to mid angle faults and high angle faults. The majority of them present striation sets with significant horizontal component that causes the change in the direction and plunge of the fold axes of Pindos unit. The younger and with more active characteristics fault zones are the Lefkohori and Ohthia ones

Volcanic Geoheritage of Landslides and Rockfalls on a Tropical Ocean Island (Western Samoa, SW Pacific)

Landslides and rockfalls on volcanic islands in tropical climate are characteristic landscape shaping features. Their common formation poses potential hazard for island communities; hence, understanding their formation and recognition on modern landscapes are vital element of educating local communities and providing mitigation strategies for future events. As landslides and rockfalls are continuously shaping tropical islands’ landscapes, they contribute significantly to the volcanic geoheritage of those islands. Rockfall and landslide hazards are commonly associated with the Fagaloa Formation and Salani Formations of Western Samoa in the SW Pacific. Four case studies (Mauga-o-Fao, Mauga-o-Vaea, Fagaloa Bay, and Leagi’agi Hill) are reported here, based on highly populated areas which are predicted to be vulnerable in generating rockfalls and landslides in the future. Field observations and interpretation of aerial photographs and satellite images were used to identify landslide and rockfall hazards in the region. The lack of records and previous studies related to rockfalls and landslides in the region is the major challenge for this investigation. Commonly, rockfall and landslide scenarios in Western Samoa are associated with the presence of high angle faults, highly weathered, extensively jointed, and fractured rocks. Surface and groundwater could expand the size of the joints, and existing fault scarps trigger the rock face to become more unstable by losing its support. There are two networks of jointing patterns commonly occurring in both old (3 Ma to 800 ka) and young (200 ka to 3 ka) volcanic rock formations: parallel and perpendicular with lava flow axis. These joint networks would increase the instability of tabular lava flows especially nearby to fault scarps and thick columnar jointed lavas. It is suggested that the major faults on the main islands almost running perpendicular to the central volcanic rift (elongated north east to south west) could be other main drivers of the rockfall and landslide hazards in the region.

Peak Current Limiting Characteristics of Transformer Type Superconducting Fault Current Limiters With Two Non-Isolated Secondary Windings

In this paper, the transformer type superconducting fault current limiter (SFCL) with two nonisolated secondary windings was suggested and its peak fault current limiting characteristics were analyzed. The suggested transformer type SFCL largely consists of two high-TC superconducting elements, one primary winding, and two nonisolated secondary windings, which are wound on the same iron core. Except for the structure with nonisolated secondary winding, the suggested transformer type SFCL has the similar operation to the previously reported transformer type SFCL with two triggering current levels. However, its peak fault current limiting characteristics are different from the previous transformer type SFCL. To confirm its peak fault current limiting operation with two limiting operational currents, the fault current limiting tests of the suggested SFCL according to the fault angle, which affects the amplitude of the initial transient fault current, were performed and its peak fault current limiting characteristics were discussed.

Impacts of small-scale faults on continental to coastal plain deposition: Evidence from the Realgrunnen Subgroup in the Goliat field, southwest Barents Sea, Norway

Abstract In this study we synthesise sedimentological, fault, and Amplitude Versus Angle (AVA) analysis and propose that the Fruholmen and Tubaen formations (Realgrunnen Subgroup) are syn-kinematic deposits that record a previously undocumented early phase of Mesozoic rifting on the Troms-Finnmark fault Complex and within the Hammerfest Basin. The Realgrunnen Subgroup hosts one of two Triassic reservoirs currently being produced in the Goliat field. Here, the subgroup sits unconformably on top of the Storfjorden Subgroup (Carnian Snadd Formation). Away from the Goliat field, which is characterised by a periclinal anticline, the Realgrunnen Subgroup also comprises the Lower–Middle Jurassic Nordmela and Sto formations. Sedimentological analysis of six exploration wells reveals that the Fruholmen Formation was deposited in a prodelta to delta plain environment where tide-influenced and fluvial-dominated distributary channels are represented by clay/siltstones and very fine grained sandstones. The overlying Tubaen Formation is characterised by medium to very coarse-grained deposits (locally conglomeratic) and represents a widespread braid plain with localised alluvial fans. Displacement profiles of faults and along-fault thickness variations demonstrate that an immature fault system was active during deposition of the Realgrunnen Subgroup. A series of unconnected fault segments hosted isolated sub-basins and erosional catchment areas in their hanging and footwalls, respectively. An AVA attribute map generated from a 10 ms interval of the uppermost part of the subgroup reveals gross sand-prone depositional bodies, i.e., individual and amalgamated channels, (some of which show meandering geometries), ox-bow lakes and alluvial fans. Sand bodies frequently show elongate geometries parallel to faults indicative of syn-depositional fault-related subsidence. Driving mechanisms responsible for the Norian to Rhaetian event may relate to contemporaneous rejuvenation of the Fennoscandian hinterland, development of the Novaya Zemlya fold-and-thrust belt and/or the early Cimmerian tectonic phase in northern Europe.

Rift–drift transition in the Dangerous Grounds, South China Sea

The South China Sea (SCS) has a long record of rifting before and after subsequent seafloor spreading, affecting the wide continent of the Dangerous Grounds, and its scissor-shape opening manner results in the rifting structures that vary along this margin. Some 2000 km of regional multichannel seismic data combined with borehole and dredge data are interpreted to analyze the multistage rifting process, structural architecture and dynamic evolution across the entire Dangerous Grounds. Key sequence boundaries above the Cenozoic basement are identified and classified into the breakup unconformity and the rift end unconformity, which consist of the rift-related unconformities. Reflector T70 in the east of the Dangerous Grounds represents the breakup unconformity, which is likely corresponding to the spreading of the East Subbasin. T60 formed on the top of carbonate platform is time equivalent to the spreading of the Southwest Subbasin, marking the breakup unconformity of the central Dangerous Grounds. The termination of the spreading of the SCS is manifested by the rift end unconformity of T50 in the southwest and the final rift occurring in the northwest of the Dangerous Grounds is postponed to the rift end unconformity of T40. On the basis of the stratigraphic and structural analysis, distinct segments in the structural architecture of the syn-rift units and the ages of rift–drift transition show obvious change from the proximal zone to the distal zone. Three domains, which are the Reed Bank-Palawan Rift domain, the Dangerous Grounds Central Detachment domain and Nam Con Son Exhumation domain, reflect the propagation of the margin rifting developed initially by grabens formed by high angle faults, then large half-grabens controlled by listric faults and detachments and finally rotated fault blocks in the hyper-extended upper crust associated with missing lower crust or exhumed mantle revealing a migration and stepwise rifting process in the south margin of the SCS.

3D geometries of normal faults in a brittle-ductile sedimentary cover: Analogue modelling

It is well known that ductile layers play a major role in the style and location of deformation. However, at the scale of a single normal fault, the impact of rheological layering is poorly constrained and badly understood, and there is a lack of information regarding the influence of several décollement levels within a sedimentary cover on the single fault geometry under purely extensive deformation. We present small-scale experiments that were built with interbedded layers of brittle and ductile materials and with minimum initial constraints (only a velocity discontinuity at the base of the experiment) on the normal fault geometry in order to investigate the influence of controlled parameters such as extension velocity, rate of extension, ductile thickness and varying stratigraphy on the 3D fault geometry. These experiments showed a broad-spectrum of tectonic features such as grabens, ramp-flat-ramp normal faults and reverse faults. Forced folds are associated with fault flats that develop in the décollement levels (refraction of the fault angle). One of the key points is that the normal fault geometry displays large variations in both direction and dip, despite the imposed homogeneous extension. This result is exclusively related to the presence of décollement levels, and is not associated with any global/regional variation in extension direction and/or inversion.

Sentinel-1 observation of the 2017 Sangsefid earthquake, northeastern Iran: Rupture of a blind reserve-slip fault near the Eastern Kopeh Dagh

New satellites are now revealing InSAR-based surface deformation within a week after natural hazard events. Quick hazard responses will be more publically accessible and provide information to responding agencies. Here we used Sentinel-1 interferometric synthetic aperture radar (InSAR) data to investigate coseismic deformation associated with the 2017 Sangsefid earthquake, which occurred in the southeast margin of the Kopeh Dagh fault system. The ascending and descending interferograms indicate thrust-dominated slip, with the maximum line-of-sight displacement of 10.5 and 13.7 cm, respectively. The detailed slip-distribution of the 2017 Sangsefid Mw6.1 earthquake inferred from geodetic data is presented here for the first time. Although the InSAR interferograms themselves do not uniquely constrain what the primary slip surface is, we infer that the source fault dips to southwest by analyzing the 2.5 D displacement field decomposed from the InSAR observations. The determined uniform slip fault model shows that the dip angle of the seimogenic fault is approximately 40°, with a strike of 120° except for a narrower fault width than that predicted by the empirical scaling law. We suggest that geometric complexities near the Kopeh Dagh fault system obstruct the rupture propagation, resulting in high slip occurred within a small area and much higher stress drop than global estimates. The InSAR-determined moment is 1.71 × 1018 Nm with a shear modulus of 3.32 × 1010 N/m2, equivalent to Mw 6.12, which is consistent with seismological results. The finite fault model (the west-dipping fault plane) reveals that the peak slip of 0.90 m occurred at a depth of 6.3 km, with substantial slip at a depth of 4–10 km and a near-uniform slip of 0.1 m at a depth of 0–2.5 km. We suggest that the Sangsefid earthquake occurred on an unknown blind reverse fault dipping southwest, which can also be recognised through observing the long-term surface effects due to the existence of the blind fault.

Relocation of mainshock and aftershock sequence of the <italic>M</italic><sub>s</sub>7.0 Sichuan Jiuzhaigou earthquake

An earthquake of M s7.0 struck Jiuzhaigou Country in Sichuan Province on 8 August 2017. It occurred at the northeastern boundary of the Bayan Har block. The Jiuzhaigou earthquake was one of the largest earthquakes in continental China since the 2013 M s7.0 Lushan earthquake, and it was widely felt across Sichuan and adjacent provinces. The earthquake left 25 dead and up to 500 injured. Because the Jiuzhaigou earthquake did not produce obvious surface ruptures, the causative fault of this event was much debated. The shape of the seismogenic fault, the focal depth of the mainshock and the characteristics of the aftershock sequence were also not very clear. The Sichuan earthquake administration deployed 6 temporary seismic stations around the source region within 4 d of the mainshock. In this study, we collected seismic waveform data and phase arrival data recorded by permanent and temporary stations. We present a systematic investigation about the location of the Jiuzhaigou earthquake sequence with high precision. These results provide new constraints for refining earthquake rupture process, constructing seismogenic models, and evaluating regional earthquake risks. The mainshock of the Jiuzhaigou M s7.0 earthquake was relocated using a 3D velocity model. The procedure takes station elevation, topography and the Earth’s ellipticity into consideration. The location of the mainshock was found to be 103.806°E, 33.201°N, and the focal depth to be 20.4 km. The depth of the initial rupture point of the mainshock was estimated to be below 14 km with S-P arrival time difference recorded at the nearest strong motion station. The locations of early aftershocks were corrected with latter aftershocks recorded using dense temporary seismic stations. A double difference algorithm was used to relocate 1-month aftershock sequence of the Jiuzhaigou earthquake. The locations of 3030 aftershocks were here determined. The average location error was found to be 0.16, 0.15, and 0.18 km in the E-W, N-S, and U-D directions, respectively. The relocation results showed that the aftershocks spread approximately 42 km, trending NWW. The aftershock zone was found to connect the Tazang fault to the north and to the Huya fault to the south. The mainshock is located at the center of the aftershock zone, with similar aftershock length at the both sides. There is a ~5 km sparse aftershock segment to the NW of the mainshock which is consistent with large coseismic slip area. This phenomenon can be explained due to the large amount of stress released during the mainshock rupture. The depth of the aftershock in the northwest end was relatively shallow, and the aftershock zone was about 6 km wide. The depth of aftershock in the southeast section was deeper and the area was narrower, about 4 km. The predominant distribution of the focal depth is 4–20 km. The dip angle of the seismogenic fault is steep, with an average value of about 84°, and changes obviously along the strike direction. The seismogenic fault inclined to the southwest in the shallow part and to the northeast in the deep part. The complex fault shape should be considered when constructing seismogenic model and inverting earthquake rupture process. The depth of the initial rupture point of the mainshock is deeper than the centroid depth and average depth of the aftershock sequence, indicating that the earthquake rupture spreads from deep space to shallow. We find that the aftershocks migrated in the along-strike direction with logarithmic time since the mainshock. The length of the aftershock area expanded from 25 to 42 km in 2 d. The rate of spread was approximately 1.3 km/log(s), consistent with aftershock expansion caused by propagating afterslip. There may be afterslip in the source area of the Jiuzhaigou earthquake.

3D seismic analysis investigating the relationship between stratigraphic architecture and structural activity in the intra-cratonic Cooper and Eromanga basins, Australia

This research uses four three-dimensional (3D) seismic surveys located in Australia's largest onshore hydrocarbon province, the intra-cratonic Cooper and Eromanga basins, to present an approach that extracts important structural and stratigraphic information from geophysical data that can then be used to define the tectonostratigraphic evolution of subsurface provinces. The methodology consists of: (1) analysing isopach maps; (2) cross-section interpretation of stratigraphic features, erosional surfaces, and faults; and (3) constraining the evolution of fault activity. Most faults within this province are basement-involved with high dip angles. The primary fault set is NE-SW striking, with secondary sets striking N-S, E-W and NW-SE. These high angle faults most likely developed as normal faults before being reactivated by five of the six major tectonic events. Field scale NW-SE strike-slip faults are prolific and can often be overlooked due to the low seismic resolution. A close relationship between on-lapping features and present-day structural highs was found during each of the major structural events, particularly within hydrocarbon-rich Permian stratigraphy, inferring that present-day structures were present throughout basin development and intermittently reactivated. Significant stratal-package thinning, and a high presence of on-lapping features, were associated with regional basement-involved faults, particularly along the Gidgealpa-Merrimelia-Innamincka and Murteree-Nappacoongee ridges. Initial structural trap development occurred during the early Permian, but was most significant during the Late Triassic. Hydrocarbon accumulations were unaffected by structural growth after the critical moment in the petroleum system (90 Ma), as the final period of fault activity was during the Late Cretaceous. This research constrains the tectonostratigraphic evolution of the intra-cratonic Cooper and Eromanga basins, while detailing an approach that extracts and analyses important structural and stratigraphic information from geophysical data, where outcrop is not accessible.

Petroleum geology controlled by extensive detachment thinning of continental margin crust: A case study of Baiyun sag in the deep-water area of northern South China Sea

Abstract The relationships between crustal stretching and thinning, basin structure and petroleum geology in Baiyun deep-water area were analyzed using large area 3D seismic, gravity, magnetic, ocean bottom seismic (OBS), deep-water exploration wells and integrated ocean drilling program (IODP). During the early syn-rifting period, deep-water area was a half-graben controlled by high angle faults influenced by the brittle extension of upper crust. In the mid syn-rifting period, this area was a broad-deep fault depression controlled by detachment faults undergone brittle-ductile deformation and differentiated extension in the crust. In the late syn-rifting period, this area experienced fault-sag transition due to saucer-shaped rheology change dominated by crustal ductile deformation. A broad-deep fault depression controlled by the large detachment faults penetrating through the crust is an important feature of deep-water basin. The study suggests that the broad-deep Baiyun sag provides great accommodation space for the development of massive deltaic-lacustrine deposition system and hydrocarbon source rocks. The differentiated lithospheric thinning also resulted in the different thermal subsidence during post-rifting period, and then controlled the development of continental shelf break and deep-water reservoir sedimentary environment. The high heat flow background caused by the strong thinning of lithosphere and the rise of mantle source resulted in particularities in the reservoir diagenesis, hydrocarbon generation process and accumulation of deep-water area in northern South China Sea.

Preliminary analysis on the source properties and seismogenic structure of the 2017 Ms7.0 Jiuzhaigou earthquake

At GMT time 13:19, August 8, 2017, an M s7.0 earthquake struck the Jiuzhaigou region in Sichuan Province, China, causing severe damages and casualties. To investigate the source properties, seismogenic structures, and seismic hazards, we systematically analyzed the tectonic environment, crustal velocity structure in the source region, source parameters and rupture process, Coulomb failure stress changes, and 3-D features of the rupture plane of the Jiuzhaigou earthquake. Our results indicate the following: (1) The Jiuzhaigou earthquake occurred on an unmarked fault belonging to the transition zone of the east Kunlun fault system and is located northwest of the Huya fault. (2) Both the mainshock and aftershock rupture zones are located in a region where crustal seismic velocity changes dramatically. Southeast to the source region, shear wave velocity at the middle to lower crust is significantly low, but it rapidly increases northeastward and lies close to the background velocity across the rupture fault. (3) The aftershock zone is narrow and distributes along the northwest-southeast trend, and most aftershocks occur within a depth range of 5–20 km. (4) The focal mechanism of the Jiuzhaigou earthquake indicates a left-lateral strike-slip fault, with strike, dip, and rake angles of 152°, 74° and 8°, respectively. The hypocenter depth measures 20 km, whereas the centroid depth is about 6 km. The co-seismic rupture mainly concentrates at depths of 3–13 km, with a moment magnitude ( M w) of 6.5. (5) The co-seismic rupture also strengthens the Coulomb failure stress at the two ends of the rupture fault and the east segment of the Tazang fault. Aftershocks relocation results together with geological surveys indicate that the causative fault is a near vertical fault with notable spatial variations: dip angle varies within 66°–89° from northwest to southeast and the average dip angle measures ~84°. The results of this work are of fundamental importance for further studies on the source characteristics, tectonic environment, and seismic hazard evaluation of the Jiuzhaigou earthquake.

On the Reactivation of the Pre-Existing Normal Fault

The reactivation of pre-existing faults is a common phenomenon in a basin. This paper discusses the relationship between the pre-existing faults and the newly formed Coulomb shear fractures regarding pore fluid pressures. Based on the Coulomb fracture criterion and Byerlee frictional sliding criterion, an equation relating pore pressure coefficient (λe), minimum dip angle (αe) of the reactive pre-existing fault and the intersection point depth (z) between the pre-existing fault and a newly formed Coulomb shear fault in an extensional basin, is established in this paper. This equation enhanced the understanding on the reactivation of pre-existing faults and can be used to calculate paleo-pore fluid pressures. The bigger the pore fluid pressure in a pre-existing fault is, the less the minimum dip angle for a reactive pre-existing fault will be. The minimum dip angle is less in shallow area than that in deep area. This will be of significance in petroleum exploration and development.

Impact of near-surface fault geometry on secular slip rate assessment derived from uplifted river terraces: implications for convergence accommodation across the frontal thrust in southern Central Bhutan

Vertical velocities obtained from uplifted river terrace dating near mountain fronts are commonly converted into overthrusting slip rates assuming simple geometry of the fault at depth. However, the lack of information on the dip angle of these shallow structures can lead to misinterpretation in the accommodation of convergence, and thus to erroneous conclusions on the transfer of shortening to the emergent thrust faults. Here, to assess the impact of fault geometry, we focus on the eastern Himalayan region in the south Central Bhutan, where the topographic frontal thrust (TFT) has been already documented by GPS, paleoseismic, geomorphic and geological studies. This study is based on high-resolution near-surface geophysical investigations, including electrical resistivity, seismic and gravity measurements. Using a similar stochastic inversion approach for all data sets, new quantitative constraints on both fault geometry and petrophysical parameters are obtained to image shallow depths, in the upper ca. 80 m. The combined results from both surface observations and geophysical measurement provide a TFT geometry that is dipping northwards with a shallow angle at the top (0–5 m), steeply dipping in the middle (5–40 m) and flattening at deeper depths (>40 m). Together, our new constraints on the fault geometry allow us to estimate a minimum overthrusting slip rate of 10 ± 2 mm yr−1, which is only a part of the ca. 17 mm yr−1 GPS convergence. This suggests that, in the study area, significant deformation partitioning on several faults including TFT and the Main Boundary Thrust cannot be ruled out. More importantly, assuming constant slip rate, the obtained dip angle variations lead to uplift rate changes with distance to the TFT. This underlines that taking into account uplift rate from terrace dating only at the front location and assuming a constant dip angle fault geometry based on surface observations may significantly bias the slip rate estimates.