Fault Zone Research Articles

Topographic metrics for unveiling fault segmentation and tectono-geomorphic evolution with insights into the impact of inherited topography, Ulsan Fault Zone, South Korea

Abstract. Quantifying today's topography can provide insights into landscape evolution and its controls, since present topography represents a cumulative expression of past and present surface processes. The Ulsan Fault Zone (UFZ) is an active fault zone on the southeastern Korean Peninsula that was reactivated as a reverse fault around 5 Ma. The UFZ strikes NNW–SSE and dips eastward. This study investigates the relative tectonic activity along the UFZ and the landscape evolution of the hanging-wall side of the UFZ, focusing on neotectonic perturbations using 10Be-derived catchment-averaged denudation rates and bedrock incision rates, topographic metrics, and a landscape evolution model. Five geological segments were identified along the fault, based on their relative tectonic activity and fault geometry. We simulated four cases of landscape evolution to investigate the geomorphic processes and accompanying topographic changes in the study area in response to fault movement. Model results reveal that the geomorphic processes and the patterns of topographic metrics (e.g., χ anomalies) depend on inherited topography (i.e., the topography that existed prior to reverse fault reactivation of the UFZ). On the basis of this important model finding and additional topographic metrics, we interpret the tectono-geomorphic history of the study area as follows: (1) the northern part of the UFZ has been in a transient state and is in topographic and geometric disequilibrium, so this segment underwent asymmetric uplift (westward tilting) prior to reverse faulting on the UFZ around 5 Ma, and (2) its southern part was negligibly influenced by the asymmetric uplift before reverse faulting. Our study demonstrates the utility of topographic metrics as reliable criteria for resolving fault segments. Together with landscape evolution modeling, topographic metrics provide powerful tools for examining the influence of inherited topography on present topography and for the elucidation of tectono-geomorphic histories.

Seismic structure beneath Ebeko Volcano and surrounding areas of Paramushir Island (Kuril Arc) inferred from local earthquake tomography

Paramushir is a large northernmost island of the Kuril Arc in the Russian Far East. Here, we study the northern part of Paramushir, which is dominated by the Vernadsky Ridge with an altitude of around 1000 m above sea level. This ridge is composed of a series of Pleistocene and Holocene volcanoes and includes the presently active Ebeko Volcano, which exhibits frequent phreatomagmatic eruptions and ongoing fumarolic activity. Studying the internal structure beneath Northern Paramushir is important to understand the interrelationship of magmatic, hydrothermal, hydrological and geological processes below Ebeko Volcano. We use seismic data of a portable network consisting of 20 broadband stations installed for one year from the summer 2021 to 2022 and one permanent station in the city of Severo-Kurilsk. We performed the local earthquake tomography inversion, which provided the distributions of Vp, Vs, Vp/Vs and local seismicity in an area of northern Paramushir with lateral extent of ∼20 km and depth of a ∼ 15 km. The results of several tests gave the information about the resolution limitations of the computed model, which were taken into account during its interpretation. In the resulting model, the coexistence of low Vp, high Vs, low Vp/Vs and a seismicity cluster down to ∼10 km below Ebeko indicates the presence of a large gas-saturated area that was formed due to the contacts of liquid fluids with hot magmatic intrusions. The western border of this area coincides with the location of Verkhne-Yurievsky hot water sources and possibly highlights the path of fluids traveling around the hot body below Ebeko. Below the upper part of the eastern slope, we observe a shallow anomaly of low Vs and high Vp/Vs indicating the presence of a near-surface aquifer, confirming the previous results of ambient noise tomography. Below the lower part of the eastern slope, a nearly vertical anomaly of low Vs and high Vp/Vs may represent a fault zone, which is also marked on the surface by a series of lineaments.

SEISMIC RESPONSE IN MUARA BANGKAHULU SUB-DISTRICT, BENGKULU CITY, USING THE CONCEPT OF WAVE PROPAGATION

The position of Bengkulu Province, which is flanked by the subduction zone between the Euro-Asian and Indian-Australian plates to the west, and the Sumatra Fault zone to the east, makes Bengkulu City one of the areas prone to earthquake disasters. Muara Bangkahulu District is one of the sub-districts in Bengkulu City. Muara Bangkahulu sub-district is an area that functions as a government centre, trade and service centre, and one of the education centres in Bengkulu City. This study aims to determine the ground response in the Muara Bangkahulu sub-district area during an earthquake. The wave propagation method used in this study is a one-dimensional equivalent of linear and nonlinear modelling that propagates earthquake waves from bedrock to surface. The results of this study are Peak Ground Acceleration (PGA), amplification factor, earthquake acceleration time history, and acceleration spectra response. The results of this analysis will be compared between equivalent linear and nonlinear values. The results of this study can also help us realise and further consider the value of seismic design in the Muara Bangkahulu sub-district area, mainly if a stronger earthquake occurs in the future.

Seismic mapping of the central and southern segments of the Tanlu fault zone using P-wave receiver functions

The central and southern segments of the Tanlu fault zone, located in the collision boundary between the Yangtze Plate and the North China Craton, underwent complex tectonic deformation associated with the Pacific Plate subduction. The crustal thickness and Vp/Vs ratio are important parameters for comprehending tectonic evolution and geodynamic processes. By integrating a newly dense seismic array, our results based on P-wave receiver function analyses reveal the crust thickness varies significantly across the Tanlu fault zone. The Yangtze Plate is characterized by a thinner crust, contrasting a thicker crust imaged in the North China Craton, indicating the Tanlu fault zone is a prominent structural block boundary. The Vp/Vs ratio within the Tanlu fault zone is visibly higher than its surroundings, probably correlating with abundant fluids activity along the fault zone, as a result of the subduction of the Pacific Plate. Additionally, we observe a gradually deepening Moho discontinuity beneath the Sulu orogenic belt with a low-angle dip, presenting direct seismic evidence for supporting the ancient Yangtze Plate underthrusting beneath the Sulu orogenic belt.

Shallow crustal structure of the northern Longmen Shan fault zone revealed by a dense seismic array with ambient noise analysis

The Longmen Shan (LMS) fault zone located in the southwest of China is not only the boundary tectonic zone of the Tibetan Plateau and South China block, but also an important part of the north–south seismic belt of China. To investigate the detailed crustal structure of the LMS fault zone, we deployed a dense seismic array of 151 short-period temporary stations in its northern section from March 16 to April 6, 2023. Continuous vertical component ambient noise waveforms recorded by these stations were cross-correlated for all station pairs, enabling the extraction of Rayleigh wave phase velocity dispersion curves across periods ranging from 0.5 s to 6 s. We then inverted these data to derive the crustal shear wave velocity using direct surface wave tomography. Meanwhile, a linear sub-array consisting of 54 stations and crossing the surface rupture of the 2008 Wenchuan earthquake was used to image the fault structure by applying the Transmitted Surface Wave Reverse Time Migration method (TSW-RTM). The ambient noise tomography results show that strong lateral heterogeneity exists in the shallow crust in the study area, with a high shear velocity in the northwest and a low shear velocity in the southeast. The low shear velocity is generally distributed between the Yingxiu-Beichuan fault (YBF) and the Guanxian-Jiangyou fault (GJF). The results of TSW-RTM indicate that the Wenchuan-Maoxian fault (WMF) and the YBF are both characterized with a steep dip to the northwest. Our results could serve as the important basis for the study of segmentation characteristics of the LMS fault and seismic hazard preparation and mitigation.

Research on Subsurface Electrical Structure Based on a Dense Geomagnetic Array in Southern Yunnan.

The electrical resistivity of subsurface rocks is one of the important sensitive parameters characterizing the internal physics of the Earth. Currently, research on subsurface electrical structures using geomagnetic sounding methods primarily focuses on two approaches: the first is based on observations from a few geomagnetic stations, which have low spatial resolution and cannot effectively describe the distribution of anomalies; the second is based on mobile geomagnetic observations, which have low temporal resolution and cannot promptly reflect anomalies. To address these issues, this study deployed a dense geomagnetic array for long-term observation in the southern segment of the Xiaojiang Fault Zone in the Yuxi area of southern Yunnan. This setup aims to promptly capture seismic magnetic anomalies, providing more data support and fundamental information for short-term earthquake prediction. Based on the long-term observation data from the dense array, the study of the subsurface electrical structure is carried out. The results indicate that during the observation period, which was seismically quiet, the regional subsurface electrical structure remained stable. A large-scale subsurface low-resistivity body was observed in the region, and the electrical structures at the two ends of the southern segment of the Xiaojiang Fault Zone were found to be completely different.

Interrogating an along-strike variation in the evolution and rheology of a large continental strike-slip fault zone

Along-strike variations in lithology, temperature, fluid activity, etc. can induce rheological changes in strike-slip faults that may be recorded by different fault rocks and fabrics. This article interrogates localized formation of granite-derived mylonite in a large strike-slip fault zone in which ultramylonite is the principle granite-derived fault rock to better understand this rock record of faulting. Microtextures show that rate-limiting deformation mechanisms in mylonite were dislocation creep in quartz and diffusion creep in very-fine-grained feldspar aggregates. Microtextures also show that mylonite formation is fully compatible with continuous viscous deformation whereas coeval ultramylonite along strike formed after whole-rock cataclasis at the brittle-viscous transition. Differential stresses determined from quartz aggregates in ultramylonites are 40–150% greater than stresses in mylonites. Hence, mylonites represent a local weak sector within this otherwise relatively strong fault zone. Mylonite formation is correlated with syndeformational chemical alteration as well as quartz microstructures and mineral assemblages indicating elevated deformation temperatures. Not all mylonite samples record significant chemical alteration. Therefore, mylonite formation likely records locally elevated temperatures. These results illustrate how a local shift in deformation conditions can affect the evolution and rheology of a large strike-slip fault zone, and how fault rocks record these processes.

Applied experimental research on in-situ online monitoring instrument for soil radon in fault zone

As an emerging method for seismic precursory signals in underground fluid, the reliability and stability of in-situ online monitoring devices for soil radon are crucial performance indicators that are directly related to the successful implementation of continuous monitoring of soil radon in seismic areas. This study conducted laboratory testing and field applications of a new in-situ online monitoring instrument for seismic soil radon, utilizing the experimental testing conditions provided by the radon monitoring instrument detection platform of the China Earthquake Administration and the natural experimental site at Xianshuihe Fault Zone in the China Seismic Experimental Site. The results indicate that laboratory measurement repeatability of the instrument is 5.53%, the relative intrinsic error of radon volume response activity is 1.53%, and the response capability of hourly sampling at high radon volume response activity (greater than 1.0 × 105 Bq/m3) lags behind the standard instrument by 2 h, essentially meeting the requirements for seismic observation. Under field conditions, the instruments exhibit good synchronization in response at different depths at the same location. However, further improvements are needed to enhance the consistency of long-term operation in the field, as well as the adaptability to outdoor self-powering, data transmission, and environmental conditions.

Conditional earthquake probabilities along the North Anatolian fault zone based on inverse Gaussian against lognormal distribution

Conditional earthquake probabilities along the North Anatolian fault zone based on inverse Gaussian against lognormal distribution

The correlation between the active/buried faults and aeromagnetic data in inner and inner-east Anatolia, Turkey

The paper focuses on investigating the active/buried faults in inner and inner-east Anatolia, Turkey based on aeromagnetic data and comparison with the active fault map. This region includes the major tectonic units of Turkey and the largest basins of Turkey (Tuzg.lü Basin (TB) and Sivas Basin (SB)). Thus, the region deserves a detail investigation to delineate the lineaments of the active/buried fault system or the causative sources located in the upper crust. To this end, the region has been divided into two parts as region-1 and 2 including TB and SB, respectively. In this context, the geophysical data processing techniques including power spectrum analysis, high‑pass filter, and second vertical derivative method (SVD) have been applied to the aeromagnetic data of the both regions. Firstly, by utilizing power spectrum analysis, the average depths of the deep and shallow causative sources in the region-1 have been calculated as 11.37 km and 5.01 km whereas those depths are 8.5 km and 3.24 km for region-2. Afterwards, to enhance the responses of the residual anomalies, the high‑pass filter and the SVD method have been applied to the data. Hereby, these potential lineaments produced from the SVD map have been compared with the active fault map to delineate the uncovering buried faults and their lineaments in the region. Contrary to the discussion in the literature, this study revealed for the first time that the Tuzgölü fault zone located in the eastern part of lake does not merge with the Ecemis fault (EF) zone. Furthermore, the study has shown that this fault zone extended from Kayseri city to Sivas city does not junction with North Anatolian Fault Zone in the north, as well.

Stress heterogeneity in the eastern Tibetan Plateau and implications for the present-day plateau expansion

The eastward expansion of the Tibetan Plateau has resulted in different earthquake types in the eastern Tibetan Plateau, but the mechanism remains unclear. Here, we construct a three-dimensional visco-elastoplastic finite element model considering the topography to investigate the influence of fault geometry and rheological heterogeneity on stress fields. In our best-fitting model, the minimum principal stress is nearly vertical around the southern Huya fault zone, which is adjacent to the Longmen Shan fault zone, due to the significant mid-lower crust lateral rheological heterogeneity, and the thrust stress regime accounts for the reverse fault and thrust-dominated earthquakes. In this scenario, the eastward horizontal motion of the mid-lower crust is obstructed and facilitates thrust faulting, suggesting the limited eastward expansion of the Tibetan Plateau. In contrast, the northern Huya fault zone, one of the terminal branches of the East Kunlun fault, accommodates the continuous eastward extrusion of the East Kunlun fault, where the stress regime under a more homogenized crust favors the strike-slip faulting process, along with the dominant strike-slip earthquakes. Moreover, the best-fitting of stress regime explains the thrust-dominated 2008 Ms. 8.0 Wenchuan and 2013 Ms. 7.0 Lushan earthquakes on the Longmen Shan fault zone. Combining geophysical and geodetic observations and model analyses, we propose that the hybrid deformation mode in the eastern Tibetan Plateau is accommodated by upper crustal shear and thrusting deformation and mid-lower crustal thickening driven by the gravitational potential energy gradient. Our results elucidate the mechanism for differences in strong historical earthquakes and, more importantly, isolate the effect of fault geometry from those of heterogeneous viscosity on crustal deformation and stress heterogeneity in the eastern Tibetan Plateau.

Normal fault architecture, evolution, and deformation mechanisms in basalts, Húsavik, Iceland: Impact on fluid flow in geothermal reservoirs and seismicity

Faults within layered basaltic sequences significantly influence hydrothermal fluid flow in shallow geothermal reservoirs and potentially during CO2 sequestration and storage. Nevertheless, their characterization regarding fault zone architecture, fluid flow, deformation mechanisms, and seismic potential remains underdeveloped. This study addresses this gap by integrating structural and microstructural observations with X-ray diffraction analyses of exposed normal-transtensional faults associated with the seismically active Húsavík-Flatey Fault in the Tjörnes Fracture Zone, Northern Iceland. Our findings demonstrate that the evolution of basalt-hosted normal-transtensional faults progresses through distinct stages: (1) low-displacement fault propagation from pre-existing cooling joints; (2) fault linkage via dilational jogs; (3) damage zone/fault core growth through brecciation and cataclastic processes; (4) shear localization along sharp slip surfaces; and (5) smearing of volcaniclastic interbeds along the principal fault plane. Evidence of shear localization, truncated clasts, and hydrothermal breccias/veins suggests repeated seismic slip events facilitated by overpressured fluids. Conversely, the presence of clay-rich foliated cataclasite indicates aseismic slips during interseismic periods. Slip along fault jogs, bends, geometric irregularities, and orientation changes causes the dilatant opening of the fault planes and extensional horsetail fractures at fault tips. These structures create main tabular zones for lateral movement of hydrothermal fluids parallel to the fault strike in shallow geothermal reservoirs situated in active extensional-transtensional tectonic settings. In addition, the dilational jogs and the intersection of horsetail veins with the hosting faults may define linear zones of high structural permeability and intense localized fluid flow parallel to the σ2 paleostress orientation and finally mineral precipitation. The results of this study can be utilized to improve models of geothermal fluid flow for enhanced recovery in basaltic reservoirs and assess seismic risk in basaltic faults.

Transverse faulting in the Paradox Basin, Colorado Plateau, USA: Active, intermittent faulting over a 300 million year history of strain accommodation and fluid flow

The Paradox Basin (Colorado Plateau, USA) is dominated by major, first-order northwest-trending structures commonly 40 km or more in trace length, including (1) regional salt wall corridors related to passive diapirism during Pennsylvanian to Jurassic time, (2) gentle upright folds produced by Laramide shortening during the Late Cretaceous through early Cenozoic, (3) late Laramide normal faults, and (4) normal faults representing Neogene salt dissolution collapse. Less obvious at a regional scale are the fault zones aligned perpendicular (northeast-trending) to the dominant structural grain. There are 16 such faults zones, marked by short trace lengths (3−12 km), small offsets (10−100 m), and predominantly extensional kinematics. Based on published geological maps, field observations of fault zone properties (including fluid flow indicators), and kinematic analysis, we interpret these structures as transverse accommodation faults. Co-spatial structural associations reveal the transverse fault zones were active intermittently, likely as a means of minimizing along-strike strain incompatibility that accrued along the first-order structures as they evolved during late Paleozoic to Late Jurassic halokinesis, (mild) Late Cretaceous to Eocene folding, late Laramide normal faulting, and Neogene to recent collapse faulting. Locally, transverse faulting was influenced by reactivation of northeast-striking faults that offset sub-salt formations, including basement. Active, intermittent transverse faulting over the past ∼300 m.y. is consistent with the synthesis of published interpretations and age determinations focusing on the timing of diverse fluids that exploited the permeability of the transverse fault zones. The Paradox Basin, with its enormous subsurface salt volume and enduring fluid flow, has been an ideal dynamic environment for producing intermittently active transverse accommodation faulting.

Clastic injectites and seismic-induced liquefaction in latest Quaternary travertine deposits (Serre di Rapolano, Italy)

ABSTRACT The liquefaction and consequent injection of clastic dykes (injectites) is often associated with the occurrence of large earthquakes. Injectites are most documented in thick unconsolidated sedimentary sequences and less commonly in rocks as a result of localized clastic mud overpressure due to earthquake-induced shaking. This paper documents near-surface clastic dykes injected into Middle-Late Pleistocene travertine deposits following repeated seismic events. The novelty is that there is a clear relationship between the development of the injectites and the activity of seismogenic faults that cut the entire body of the travertine, deposited at least at 45.5 ± 8.5 ka, and the topographic surface. The clastic slurry, derived from the fluidization of silty-sandy continental deposits at the base of, or intercalated with the travertine body, was in geometric connection with the faults. When this occurred, the over-pressured slurry was injected into the fault zone, filling the fractures and assisting their development by over-pressure hydrofracturing and counteracting the frictional forces on the fault. This evidence, in addition to documenting another case of liquefaction and associated emplacement of clastic dykes, has a twofold significance: i) it records the effects of clastic dykes intruded at very shallow depths, up to the surface, within a capable fault zone; ii) it documents a polyphase injection of clastic material into travertine deposits, never before described, induced by repeated liquefaction phenomena following major seismic events, thus highlighting and reinforcing the importance of travertine for the study of active tectonics and earthquake studies.

Substantiating of oil and gas prospects of Mesozoic sediments based on the geochemical model of mud volcanoes in the Shamakhi-Gobustan structural element of the South Caspian megadepression

There are up to 120 mud volcanoes in the Shamakhi-Gobustan region. During the process of mud volcano eruptions, a large amount of solid, liquid, and gaseous products are thrown to the Earth’s surface, which are widely used for the identification of oil and gas deposits and are considered the main indicator of the productivity of the basin. According to laboratory results from the analysis of solid, liquid, and gas products of mud volcanoes, it is possible to obtain detailed information about the oil-gas content of deep-lying sediments. Therefore, the study of the geochemical composition of the mud volcanoes in the Shamakhi-Gobustan trough plays a major role in the discovery of new oil-gas and gas-condensate deposits. To clarify the changes in the chemical and isotopic composition of gases from mud volcanoes in the Shamakhi-Gobustan trough and to evaluate the oil-bearing and gas-bearing potential of Mesozoic sediments, the characteristics of changes in the isotopic composition of methane gas, heavy hydrocarbons, carbon dioxide, ni- trogen, as well as CH4 and CO2, were studied. The amount of methane in the gas emissions released from mud volcanoes in the Azerbaijan region is more than 90%. The amount of CH4 in the mud volcanoes of the Shamakhi-Gobustan trough ranges from 78.5% to 97.56%. The minimum amount of methane is found in North Gobustan (Gizmeydan 86.8%, Malikchobanli 78.5%), while relatively high amounts are mainly noted in Central (Kushchu 97.55%) and South Gobustan (including Dashgil 97.57%, Goturdag 96.55%, Bahar 96.78%). Mud vol- canoes are less developed in the North Gobustan region and are small in size. In the Central zone, both the size and the number of volcanic cones increase significantly compared to North Gobustan. The largest and most active volcanoes have developed in the southern zone of the trough. Huge mud volcanoes such as Dashmerdan, Solakhay, Ayrantoken, Goturdag, Kirdag, and other volcanoes have developed along the Alat range. The mud volcanoes of North and Central Gobustan are mainly confined to Cretaceous and Paleogene sediments, and due to the low occurrence of plastic clay rocks in these sediment sections, mud volcanoes are rare and weakly active. On the contrary, to the south of these zones, that is, in South Gobustan, they are located in Pliocene sediments, and due to the presence of very thick plastic rocks in their section, mud volcanoes are widespread, and they are active volcanoes that are morphologically sharply reflected. It should be noted that in areas where plastic clay rocks with significant thickness and rheologically active properties are developed, large cones of mud volcanoes that erupt frequently are observed. The calculated gas production of mud volcanoes increases from the northern zone towards South Gobustan. Thus, while the calculated gas production of the Nabur volcano located in the Northern zone is 1500 m3/day, the calculated gas production of the Dashgil mud volcano located in South-Eastern Gobustan is 40,000 m3/day. In the central and south- ern zones of the Shamakhi-Gobustan trough, the presence of ancient rock fragments from the Oligocene-Miocene (Maykop) age rocks forming their roots in the solid waste of mud volcanoes (Cretaceous, Paleocene, Eocene), a large percentage of methane gas (97.57%), transverse elevations, and the presence of an intersection zone of longitudinal faults suggests that there is a tectonic rupture zone rich in natural gas below the base of the volcano. This is proven once again by the occurrence of gas flows in Anart (110,000 m3/day), Utalgi (50,000 m3/day), and the Touragay field (20,000 m3/day). The presence of oil films in the eruption products of mud volcanoes indicates the accumulation of oil alongside gas.

Segmentation characteristics of strike-slip fault zone and its reservoir control mechanisms in the southwestern Tarim Basin

Understanding how fault-related structures influence oil and gas accumulation is crucial for geological investigations and exploration planning. This study, based on 3D seismic data, analyzes the northeast-trending strike-slip fault zone in the eastern part of the Bachu Uplift. Automatic fault extraction techniques were employed to delineate the strike-slip fault zone, and the parallel bedding indicator was used to identify reservoirs and investigate the fault’s segmented features and reservoir-controlling characteristics. The results show that the northeast-trending strike-slip fault is primarily governed by simple shear stress and conforms to the Riedel shear model. Three distinct structural styles were developed: vertical, pull-apart, and push-up segments, each exhibiting varying profile characteristics and planar patterns. The segmentation of the strike-slip fault controls the distribution of Ordovician fault-karst reservoirs. An oil and gas enrichment model for the strike-slip fault zone has been established, characterized by external hydrocarbon supply, fault-mediated migration, segmented reservoir control, and high-elevation accumulation. This study offers valuable insights for the exploration of fault-karst reservoirs controlled by strike-slip faults.

The tectonic development of the Central African Plateau: evidence from shear-wave splitting

SUMMARY The Central African Plateau comprises a mosaic of numerous Archean terranes—the Congo, Bangweulu and Kalahari Cratons—sutured in a series of Proterozoic to early Cambrian orogenic events. Major upper-crustal deformation and complex craton margin fault zones reflect the region’s diverse tectonic history: rifting during the Neoproterozoic, collision during the Pan-African orogeny, and more recently, Permo-Triassic Karoo rifting and the Pliocene development of the Southwestern branch of the East African Rift. The tectonic evolution and extent to which the lithospheric mantle has been re-worked by each tectonic event is poorly understood. New seismograph networks across the Plateau provide fresh opportunity to place constraints on the plate-scale Precambrian-to-Phanerozoic processes that have acted across the region. Utilizing data from seismograph deployments across the Central African Plateau, including the new Copper Basin Exploration Science network—a NW–SE-trending, 750-km-long profile of 35 broad-band stations—we explore lithospheric deformation fabrics associated with past and present tectonic events via a shear-wave splitting study of mantle seismic anisotropy. Results reveal short length-scale variations in splitting parameters (fast direction: $\phi$, delay time: $\delta$t), suggestive of a fossil lithospheric fabric cause for the observed anisotropy. A lack of fault-parallel $\phi$ across the Mwembeshi shear zone, suggests it may be too narrow at mantle depths, a thin-skinned, crustal-scale feature, and/or did not experience sufficient fault parallel shear-strain during its last active phase to form a lithospheric deformation fabric discernible via teleseismic shear-wave splitting. In the heart of the Lufilian Arc, we observe abrupt changes in splitting parameters with NE–SW, N–S and NW–SE $\phi$ and 0.5 s $< $$\delta$t$< $ 1.2 s evident at short length-scales: no single, uniform, anisotropic lattice preferred orientation (LPO) fabric defines the entire region. This is consistent with the view that multiple episodes of deformation shaped the Lufilian Arc, or perhaps that pre-existing fabrics, relating to Neoproterozoic Katangan Basin development, have failed to be completely overprinted by the Pan-African orogeny. Near the Domes, where most intense crustal re-working is thought to have taken place during the Pan-African orogeny, there is a cluster of null and low $\delta$t splits which likely reflects the lack of organized LPO fabrics, perhaps due to the presence of depth-dependent anisotropy. The neighbouring Congo Craton margin is marked by consistently weak anisotropy ($\delta$t$\lt $ 0.7 s) indicating a weak horizontal alignment of olivine at mantle lithospheric depths, typical of several Archean terranes worldwide.

Оценка рисков реактивации разломов при бурении горизонтальной скважины по результатам одномерного геомеханического моделирования

The article presents main factors affecting the fault stability. Concepts of critically stressed faults and critical pore pressure are discussed. The methodologies for evaluation of stressed-deformed state of rocks in the vicinity of fault zones during reservoir depletion and injection, well drilling are analyzed. 1D geomechanical models of fishbone sidetracks and main borehole of the well N are constructed. The fault stability evaluation is made using Mohr – Coulomb failurecriterion from data of insitustresses and near-wellborestressescalculated from Kirsch wellbore solution. Optimal mud weight ranges are determined for safe drilling of fishbone boreholes accounting fault reactivation risks. Basic recommendations for optimal drilling through the interval of unstable shales are made.

A Pn-wave spectral inversion technique based on trust region reflective algorithm

This paper suggests utilizing the trust region reflective (TRR) algorithm to address the inverse problem related to seismic spectrum analysis of Pn waves. By applying this approach, it becomes feasible to simultaneously deduce the seismic moment, corner frequency, tomographic model of Pn-wave attenuation, and site responses while incorporating appropriate constraints based on prior knowledge to ensure solution accuracy. The efficacy of this method has been validated using synthetic data. Specifically, the recorded Pn waves from four underground explosions in North Korea were employed to evaluate the performance of the proposed technique. The derived seismic moment and corner frequency for these events were found to align with the characteristics of the seismic data, earthquake magnitude calculations, and empirical data. The resulting tomographic model of Pn-wave attenuation revealed a spatial distribution of high and low attenuation, indicating significant lithospheric heterogeneity in northeastern China. Areas with low Q0 and low η, such as the Xialiaohe Basin and southwestern Changbai Mountain, suggest a high-temperature environment in the upper mantle cap layer or the presence of molten substances. Conversely, regions with high Q0 and high η, like the northern Changbai Mountain and the vicinity of the eastern Tanlu Fault Zone, characterized by high Pn-wave velocity and thick crust, indicate minimal modification or destruction in the lithosphere. Furthermore, site responses were determined for 75 seismic stations in northeastern China, with their characteristics preliminarily analyzed and interpreted in conjunction with geological context. The iterative process based on the TRR algorithm for Pn-wave spectral inversion proposed in this study demonstrates robust convergence and enhances the fitting accuracy of the observed spectrum. The inversion outcomes from this spectral technique yielded synthetic Pn-wave spectra that closely matched the observed spectra of the four underground explosions in North Korea across various frequency bands for over 90 % of the stations.

Fault slip and seismic source response characteristics under induced stress wave

As coal mining depth and structural complexity increase, the failure severity of mining-induced fault activation leading to rock bursts escalates. Fault fracture zones, consisting of various secondary structures, are essential for understanding tectonic evolution and seismic wave propagation. The interaction between mining-induced stress waves and in-situ stress complicates the dynamics and activation markers of fault fracture zones. This paper aims to clarify the patterns of stick–slip instability and stress evolution in these zones under coal mining-induced stress waves. Using theoretical models and continuous-discrete coupling simulations, the study explores the dynamic response of fault fracture zones in the meta-instability stage, the micro-scale fracture force chain structure, and the distribution and synergistic instability of fault plane seismic sources. The research shows that fault fracture zones changes stress wave propagation path and energy redistribution due to its heterogeneity leads to multiple times decomposition of wave field. Minor delamination between the hanging wall and footwall can trigger unstable slip. Tensional seismic sources with extensive failure drive fault slip. Dynamic stress waves disrupt force chains near seismic sources, increasing hazard levels. Static in-situ stress affects shear seismic sources by influencing porosity and crack angles. Seismic sources along the fault strike transition from divergent to concentrated, trending horizontally. In the meta-instability stage, the initial fracture area locks, with strong structures causing cohesive fractures in the middle of the fault zone, releasing strain energy and forming a sudden increase in slip intensity. These findings provide crucial insights into fault fracture-development-activation markers, fault region stress deformation evolution mechanisms, and risk control of fault-induced rock bursts.