Surface Geological Structures Research Articles

Constraining Ocean and Ice Shell Thickness on Miranda from Surface Geological Structures and Stress Modeling

Images from the Voyager 2 mission revealed the small Uranian satellite Miranda to be a complex, dynamic world. This is exemplified by signs of recent geological activity, including an extensive fault system and the mysterious coronae. This has led to speculation that Miranda may have been tectonically active within the geologically recent past and could have hosted a subsurface liquid water ocean at the time. In this work, we aim to constrain the thickness ranges for the ice shell and potential subsurface ocean on Miranda. Here, we present the results for our geological mapping of craters, ridges, and furrows on the surface. We also present the results for our comparison of the geographic distribution of these features to the predicted geographic distribution of maximum tidal stress based on stress models. We model eccentricity tidal stress, ice shell thickening stress, true polar wander stress, and obliquity tidal stress and compare the predicted surface stress pattern for each to what pattern can be inferred from the surface geology. Our results show that a thin crust (≤30 km) is most likely to result in sufficient stress magnitude to cause brittle failure of ice on Miranda’s surface. Our results also suggest the plausible existence of a ≥100 km thick ocean on Miranda within the last 100–500 million yr. This has implications for the dynamical history of Miranda and its status as a potential ocean world.

LITHO-STRUCTURAL MAPPING USING REMOTE SENSING AND FIELD WORK TECHNIQUES: A CASE STUDY FROM CENTRAL SALT RANGE ARA, RAWAL AND SIDHANDI VILLAGE, PUNJAB PAKISTAN

In the Central Salt Range lithological structural mapping has been a great challenge and of great interest to geologists as it is in rugged and inaccessible mountain terrain. In this paper a comprehensive lithological mapping conducted in the villages of Ara, Rawal and Sidhandi which are located in Central Salt Range region of Punjab, Pakistan. This study uses a combination of remote sensing techniques and field work to elucidate the geological and structural characteristics of the study area. Due to complex tectonic history and mineral resources Central Salt Range is of great geological interest. In this research we combined data from geological field observations and satellite images to produce detailed lithological and structural maps. Remote sensing data help to identify surface features and geological structures. These ground truth observations are complemented by extensive field work including geological mapping and structural analysis. Our work represent a diverse rock composition in the study area, including sedimentary rocks and various fault systems. We classify fault zones on basis of their characteristics, and our structural analysis provides insight into the fault history of the region. Moreover, the study discusses the implications of these geological features for understanding regional tectonics, potential mineral deposits, and land resources in the area. This work demonstrates the synergy between remote sensing techniques and fieldwork in understanding the geological complexities of the Central Salt Range. This work provides valuable information for geological mapping and risk assessment in the area. Additionally, this research demonstrates the importance of a multidisciplinary approach in geological investigations, emphasizing the importance of integrating remote sensing and field data to enhance our understanding of complex geological environments. Keywords: Litho-structural mapping, Remote sensing, Fieldwork techniques, Central Salt Range, Ara, Rawal, and Sidhandi villages, Punjab, Pakistan, Geological analysis, Satellite imagery, Structural mapping, Fault systems, Geological features, Tectonic history, Mineral resources, Sedimentary rocks, Deformation history

Mapping of surface hydrothermal mineral alterations and geological structures related to geothermal systems in the Songwe region, SW Tanzania

ABSTRACT Geothermal energy is one of the most valuable and preferable renewable energy sources, given that it produces sustainable, dependable, and clean energy. Understanding the geothermal systems, parameters such as heat, fluids, and structures help to identify potential geothermal systems. These parameters are manifested as hot springs, fumaroles, and alteration minerals. Hydrothermal alteration minerals are often used in geothermal system exploration to evaluate factors such as temperature, fluid, and structures. This study employed GIS and remote sensing in the investigation of hydrothermal mineral alterations and delineating geological structures which provided invaluable information for identifying prospective geothermal resource zones. ASTER was utilised to map phyllic, argillic, and propylitic minerals using SPCA, whereas ISO cluster classification was used to detect kaolinite, smectite, calcite, chlorite, and muscovite/illite-rich areas. DEM was used to extract and map geological lineaments. Results have revealed that the regions that were mapped with indicator hydrothermal alteration minerals were also associated with geological structures. The field work, XRD, and ASD laboratory analysis results indicated a good match with the findings from the remote sensing data. The approach applied in this study may be used along with other techniques during the early phases of geothermal exploration to define geothermal potential targets.

Preliminary design of lunar high-resolution remote sensing micro-nano satellite

The development and utilization of lunar resources and the construction of lunar scientific research stations are important parts of China’s future lunar exploration plan. Aiming at the problems of China’s micro-nano satellites with low radiation resistance, lack of autonomous Earth-Moon transfer capability, and limited optical system volume and quality, a preliminary design scheme of a lunar remote sensing micro-nano satellite with autonomous Earth-Moon transfer capability is proposed. Using the analytical orbit average technology and transcription and collocation method to calculate the satellite’s Earth-Moon transfer orbit, the satellite can use the Hall thruster with large specific impulse and low thrust to realize the autonomous Earth-Moon transfer. The passive + active method is used to optimize the radiation resistance of the system, and the total radiation dose of some single machines can reach more than 25krad. A compact high-resolution multispectral camera is designed, and imaging simulation and verification are carried out. In the end, the satellite can achieve high-resolution imaging of the moon with a resolution of 0.2 m at an orbital height of 100 km to achieve the purpose of fine exploration of information such as lunar surface topography and geological structure.

Inferring geological structural features from geophysical and geological mapping data using machine learning algorithms

AbstractWe present an automated approach for inferring surface geological structures from geophysical survey data. Our method employs machine learning, using mapped geological structures as labels and filtered geophysical surveys as reference maps. We compared the performance of the eight main machine learning algorithms and their 32 branches. Applied to the Geological Survey of Victoria's database for the Bendigo Zone, following an appropriate choice of geological features, the 3‐class classification model using subspace K‐nearest neighbour methods achieves a stable and validated 92% accuracy in around 1 min. The fault‐only classification model achieves a stable and validated 97% accuracy in around 6 min. This shows that geological structural features on the surface may be inferred from between one and three of the following geophysical data types: gravity, airborne total magnetic intensity and first vertical derivative of total magnetic intensity. It shows the prospect of machine learning in geological research and suggests that geophysical data combined with machine learning may be useful and efficient in determining the existence of geological structural features.

Shallow seismic structure around the Vrancea Seismic Zone from joint inversion of ambient noise H/V ratios and surface wave dispersion

The South-East Carpathians in Romania host the Vrancea Seismic Zone (VSZ), an intraplate seismic nest of frequent high magnitude earthquakes that cause unusually high damage over extended areas. Estimating the amplification of seismic signals strongly depends on the depth to the geophysical bedrock and the complex near surface geological structures. To improve seismic hazard estimates and ultimately mitigate seismic risk, we determine the frequency of resonance at 81 broadband seismic stations in southeast Romania using the horizontal-to-vertical spectral ratio (HVSR) of ambient seismic noise. We jointly invert HVSR functions with Rayleigh wave phase velocity dispersion curves using a Markov-chain Monte Carlo algorithm under the assumption of a diffuse field. Our results show excellent correlation between the fundamental frequency of resonance and local surface geology: stations located on foreland sedimentary basins show HVSR peaks in the range 0–1 Hz, while those located in orogenic nappes reach values as high as 10 Hz. Seismic bedrock topography mirrors the crystalline bedrock depth inferred from gravity modeling, but is consistently ∼2000 m shallower, corresponding to the interface between Paleozoic-Triassic to Jurassic-Cretaceous sedimentary levels on the Moesian Platform constrained by borehole data. The near surface seismic model shows anomalously low velocities in the Focsani forearc basin, adjacent to the VSZ, consistent with extremely low resonance frequencies and deep sedimentary layer successions (>5 km). In the back-arc of the Carpathian Orogen, at the contact with the Transylvanian Basin, average resonance frequencies are <2 Hz and inferred seismic bedrock depths vary laterally, likely affected by buried volcanic structures and diapiric salt domes. Our study presents the largest endeavor to study anomalous intensity patterns and local site effects in and around the VSZ and has fundamental implications for seismic risk evaluation and future civil engineering design planning.

Geological Structure Control of The Distribution Quartz Vein as Mineralization Pathway In Kawatuna Area, Palu City Indonesia

The Kawatuna area has a complex geological structure condition and is estimated to be a weak zone for hydrothermal solutions to rise and then accumulate into deposits in the form of veins. Based on this, it makes this area interesting to study. This study aims to determine the condition of the geological structure and its influence on the condition of the distribution of veins in the study area. The method used is surface geological mapping and geological structures analysis. Based on surface geological mapping, it was found that the study area was composed of schist rock units, gneiss, granodiorite, and molasses units. The geological structure that developed in the study area consisted of shear joints in the form of systematic and non-systematic shear joints and shear faults. The distribution of quartz veins in the study area has a general direction of northwest-southeast and northeast-southwest. The presence of mineralization in the form of quartz veins in the study area is strongly controlled by the presence of geological structures that form an oblique convergence fault model that shows a dilatational movement force. The deposit opening system formed in the study area is a jog deposit system in which pull-apart basin fracture arrays develop which are characterized by the presence of tension fractures/veins.

Orogenic Gold Deposit in Metamorphic Rocks: Minerals and Structural Control at Rarowatu Area, Southeastern Arm of Sulawesi, Indonesia

This research aims to examine the relationship between hydrothermal alteration and mineralization (ore mineralogy) in the study area and geological structures in the deformation mechanism. The hydrothermal alteration was determined based on petrographic analysis, and ore mineralogy which was determined based on the ore microscopic analysis. The deformation mechanism is determined by paleo stress analysis using win-tensor, and the direction of principal stress on joints/veins and faults is calculated by the right-dihedron method. Hydrothermal alteration includes silicification, argillic, propylitic, and phyllic alterations; and ore mineralogy consists of stibnite, cinnabar, pyrite, chalcopyrite, sphalerite, covellite, hematite, and arsenopyrite at metamorphic rocks. The geological structure developed in the study area consists of shear fractures and Wumbubangka right Normal-slip Fault, which is accompanied by Wumbubangka right-slip lag fault and Wumbubangka reverse-slip fault formed in a simple shear mechanism. Crenulation, micro-folds, and porphyroblastic are also recognized in thin sections. The Wumbubangka right normal-slip fault, considered a syn-mineralization structure, formed the transpressional and transtensional geologic structures, forming the quartz vein parallel to and crosscutting the foliation. The correlation between surface geological structures and microstructures indicates that tectonic regimes have controlled the alteration and gold mineralization in the study area.

Large-scale geological structures of the Egyptian Nubian Shield

Integration of potential field- and structural data make it possible to trace surface and subsurface large-scale geological structures of the Egyptian Nubian Shield (ENS). Obtained results indicate that the Northern Eastern Desert (NED) of the ENS is dominated by relatively younger (c. 580 Ma) E–W and NE–SW trending extensional structures that were controlled by the evolution and retreat of the Cadomian Arc. Density of such extensional structures increases with depth as displayed by the potential data. The prevailing structural trends in the Central Eastern Desert (CED) are NW–SE and WNW–ESE. Both trends are highly prompted by the timing of deformation upon the Najd Fault System, and are themselves dissected by a relatively younger NE–SW shearing trend. Lineament density in the CED is subordinate for both subsurface and near surface structures. The South Eastern Desert exhibits compressional and extrusion-related structures of two main prominent trends; WNW-to-NW (to the western part) and the N-, NNE- to NE (to the eastern part). The previously mentioned Neoproterozoic trends are remarkably influenced by the Oligocene–Miocene Red Sea-Gulf of Suez rift related fractures in the vicinity of the rift shoulder. The remarkable change in trends and densities of structural trends, especially in the NED, is interpreted in terms of concealing of the older structures by the younger extensional structures which in turns reflect an N-ward progressive deformation in the entire ENS. Gravity data are more appropriate in delineating the structural trends compared to the magnetic data which are largely affected by lithological variations and/or alteration zones and magnetic mineralogy.

Integration of Topex Satellite Gravity and DEM SRTM Imagery for Subsurface Structure Identification at Tiris Geothermal Area, Lamongan Volcano Complex, Probolinggo, East Java

Tiris area is one area that has geothermal prospects. Tiris area is located in the Tiris sub-district, Probolinggo regency, East Java. This study aims to identify the subsurface structure of the Tiris geothermal area using Topex gravity and DEM SRTM imagery data. The Topex gravity data obtained is still in Free Air Anomali (FAA) form, so it is necessary to make Bouguer and terrain corrections to get a complete Bouguer anomaly. A residual Bouguer anomaly is needed to study shallow subsurface structures. This anomaly is obtained by separating the residual and regional anomalies from the complete Bouguer anomaly using the moving average method. The study area has residual Bouguer anomaly values ranging from -0.5 to 2.9 mGal. The low residual Bouguer anomalies are seen in the body of the Lamongan volcano (LV) and the Tiris geothermal area (TGA). The study of subsurface structures is focused on the LV and the TGA through Forward 2D modeling. The A-A' cross-section illustrates the subsurface of LV, while the B-B' cross-section illustrates the subsurface of TGA. Cross-section A-A' modeling results show that the source of the low anomaly is in the second layer D2 = 1.24 g/ at a depth of 4.0 to 0.6 km below sea level and is suspected to be magma because it has a low density. Cross-section B-B' modeling obtained a low anomaly source at the second layer D2 = 2.03 g/ at a depth of 4.0 to 2.8 km below sea level and is suspected to be intrusive rock. The lineament display on the 2D of DEM SRTM map for the 45o and 135o azimuths shows that the TGA region has a high lineament density, and the orientation of the surface geological structure in the study area is dominant in a northwest-southeast direction.

Deep-Learning-Based Method for Estimating Permittivity of Ground-Penetrating Radar Targets

Correctly estimating the relative permittivity of buried targets is crucial for accurately determining the target type, geometric size, and reconstruction of shallow surface geological structures. In order to effectively identify the dielectric properties of buried targets, on the basis of extracting the feature information of B-SCAN images, we propose an inversion method based on a deep neural network (DNN) to estimate the relative permittivity of targets. We first take the physical mechanism of ground-penetrating radar (GPR), working in the reflection measurement mode as the constrain condition, and then design a convolutional neural network (CNN) to extract the feature hyperbola of the underground target, which is used to calculate the buried depth of the target and the relative permittivity of the background medium. We further build a regression network and train the network model with the labeled sample set to estimate the relative permittivity of the target. Tests were carried out on the GPR simulation dataset and the field dataset of underground rainwater pipelines, respectively. The results show that the inversion method has high accuracy in estimating the relative permittivity of the target.

Optimizing Image Scale and Resolution to Obtain Lineament Related Structures using ALOS PALSAR Dual Orbit Data

Geological structures identification using conventional optical remote sensing encountered some problems due to atmospheric and surface conditions. Therefore, we exploited the Synthetic Aperture Radar (SAR) in Ascending and Descending orbits to characterize lineaments related to geological structures. We have analyzed two images from the Phased Array type L-band Synthetic Aperture Radar (PALSAR) onboard the Advanced Land Observing Satellite (ALOS). The adjustment of their resolution in the multilooking process was applied to obtain the best understanding between detected lineament and reference fault. The cross-polarized data of ALOS PALSAR level SLC were multilooked with variation of resolution from 15 to 120 m. Obtaining the optimum detected lineament-related faults, which applied the Yamaguchi criterion to extract the lineament visually in the multilooked images. The Kamojang-Guntur Geothermal Fields in West Java were selected as a study area because there is a challenge to confirm geological structure in the area with high alteration and high weathering conditions. According to the proposed criterion, we have obtained those images with resolution of 75 to 90 from the analysis images can represent surface geological structure conditions, and the correlation with existing published geological map. The field verifications have been confirmed that the geological structure indication from images has been proved with the existence of slickenside and morphological conditions such as drainage of rivers, ridges, and valleys. Based on lineaments, stress regimes, and field verifications, it was concluded that Yamaguchi criterion analysis with image resolution 75 to 90 m can describe the presence of regional structures and interpreted structures.

Structural Characteristics and Deformation Evolution of an Intra-Continental Fold-Thrust Belt in Eastern Sichuan: Insights Into Analogue Sandbox Models

The Eastern Sichuan area is a typical fold-and-impulse zone. It is bounded by the Qiyue Mountain fault, with septal folds on the east and on the west. Based on previous geological mapping and surface geological structure research as well as the fault-related folding theory, in this study, tectonophysical simulation experiments were conducted to investigate the tectonic geometry and kinematics of the Eastern Sichuan fold-impulse zone. The experiments were conducted by selecting different experimental materials and changing the physical properties of the cover, the friction between the cover and the basement, the number of slip layers, the burial depth, and other factors. Finally, the similarity of the results to the morphological characteristics of the real geological structure was assessed. The results show that the deformation style is influenced to some extent by the brittle shear strength and the ductile shear strength of the basement, and the different interlayer cohesions between the competent and incompetent layers, the high rheology, and the burial depth of the slickensides played crucial roles in the formation of the final evolution of the Eastern Sichuan fold fault zone. The step-like system, composed of a lower crustal crystalline basement detachment surface, regional boundary fractures, and the overlying Cambrian and Triassic synclines, is a necessary condition for the formation of the Eastern Sichuan fold and fault zone and controls the overall evolution of the zone. Under the action of multiple phases of tectonic activity with different main stress orientations, the geological phenomenon of multiple superimposed phases of folding in different directions observed at present in the Eastern Sichuan area was formed. The well-developed karst features are not conducive to the construction of a project tunnel in the area where tightly closed back-slope, fissures, and normal faults have developed in the Eastern Sichuan fold and fault zone.

Interpretation of Subsurface Fault Through Multi-Level Second Vertical Derivative Gravitational Data in Bittuang Geothermal Working Area, South Sulawesi, Indonesia

The research was conducted in Bittuang, Tana Toraja Regency, South Sulawesi Province, as one of the geothermal prospect areas and targets for the initial stage of the Government exploration drilling program for the 2020-2024 period. One aspect of geothermal is the manifestation control structure as a fluid migration path from below the surface. Therefore, identification of existing structures in the Bittuang geothermal area was carried out and confirmed the surface geological structure contained in the Bittuang geothermal geological map. In determining the presence of a fault and knowing its characteristics such as the type of fault, the direction of the dip, and the magnitude of the dip of the fault, the gravity data is processed using the multi-level second vertical derivative (ML-SVD) method. To strengthen the interpretation, the results from the ML-SVD were matched with the data from the horizontal gradient (HG) method and the geological data of the structure of the study area. From this process, there are 27 faults in the Bittuang geothermal area, two of which are indicated as controlling faults for the manifestation of the Balla group and the Cepeng group. This research is expected to describe faults in the Bittuang geothermal area, which can support detailed exploration activities.

Pn anisotropic tomography and mantle dynamics underneath the South China Sea and surrounding areas

Abstract In the present study, we present a new Pn anisotropic tomographic model of the uppermost mantle underneath the South China Sea and surrounding areas through the inversion of 23,042 high-quality hand-picked Pn arrival-time data. Our model shows obvious lateral heterogeneities in velocity and anisotropy that are closely correlated to surface geological structure in the study region. High Pn velocity anomalies are generally observed in the structurally stable areas, such as the South China block and Pearl-River Mouth basin. Obvious low Pn velocity anomalies are imaged underneath the Tengchong volcano, the Hainan volcano, and other volcanoes along the Ryukyu, Luzon, Philippine, and Java arcs, whereas the low-to-high-velocity transition zone is visible underneath the volcanoes in the Sumatra arc. Most large crustal earthquakes occurred in the low Pn velocity areas in the Ryukyu, Luzon, and Philippine arcs and the low-to-high Pn velocity transition zones in the Sumatra and Java arcs. These results suggest that these volcanism and earthquake occurrences may be closely related to the deep dynamics of the subduction of the Philippine Sea and Australian plates. High Pn velocity areas with trench-perpendicular fast directions of Pn propagation are revealed in the Ryukyu, Philippine, Sumatra, and Java trenches, presenting the subducting Philippine Sea and Australian plates, whereas low Pn velocity areas with trench-parallel fast directions are visible in the arc areas, suggesting the compressed mantle flows there. Our results provide new seismological evidence for understanding the mantle dynamics beneath the South China Sea and surrounding areas.

Interpretation of gravity data to delineate the subsurface structures and reservoir geometry of the Aluto–Langano geothermal field, Ethiopia

The Aluto–Langano geothermal field is located within the central-southern part of the Main Ethiopian Rift, about 200 km southeast of Addis Ababa. This study analyzes gravity data to define the geometry of the geothermal reservoir and subsurface geological structures controlling the geothermal system. A Bouguer anomaly map of the area was generated using a Bouguer density of 2.49 g/cm3, which was obtained from the gravity dataset using different optimization methods. In order to highlight the structures of interest in the calderic system, a regional-residual separation was conducted. The map produced shows two dominant high Bouguer anomalies underlying Aluto caldera associated with the presence of basaltic rocks and the occurrence of fumarolic activity. Analysis of horizontal gradients of gravity data showed that the area is characterized by the presence of high gradient anomalies associated with geothermal manifestations. A 3-D gravity model for assessing the geothermal heat source architecture within the Aluto caldera area is also presented. The model was constrained using the estimated densities of rock cuttings obtained from drilled wells. The results from 3-D modeling imaged a high-density body at depth surrounded by a less-dense body (∼2.55 g/cm3), which is interpreted to be the geothermal reservoir. The model was then interpreted together with surface manifestations, lithology, and geological structures. The less-dense body found west of the Adonsha fault lineament could be the expression of the second reservoir, which is likely deeper than the productive reservoir found along the NNE–SSW trending structure (i.e., the Artu Jawe Fault Zone) in the central area.

ОТРАЖЕНИЕ МАГМАТИЗМА И ТЕКТОНИКИ В ГЕОФИЗИЧЕСКИХ ПОЛЯХ ЮГО-ЗАПАДНОГО ПРИМОРЬЯ

The results of a comprehensive interpretation of geophysical fields (magnetic, gravity and radioactivity) of the South-Western Primorye (Khasansky district) are presented. The authors give schemes reflecting the relationship of geophysical fields with various surface and deep geological structures, and discontinuous disturbances of various orders. The deep boundaries of the local granitoids massifs of the Jurassic age emerging on the surface are determined. The distribution areas of Cenozoic magmatism are outlined. The anomalous zone of increased values of the radioactive field, associated with the introduction of magmatic bodies with a high content of potassium, was identified. An assumption is made about the existence of a long-lived fluid-permeable zone that controls magmatic processes in the region.

Upper crustal velocity and seismogenic environment of the &lt;em&gt;M&lt;/em&gt;7.0 Jiuzhaigou earthquake region in Sichuan, China

On August 8, 2017, a magnitude 7.0 earthquake occurred in Jiuzhaigou County, Sichuan Province, China. The deep seismogenic environment and potential seismic risk in the eastern margin of Tibetan Plateau have once again attracted the close attention of seismologists and scholars at home and abroad. The post-earthquake scientific investigation could not identify noticeable surface rupture zones in the affected area; the complex tectonic background and the reason(s) for the frequent seismicity in the Jiuzhaigou earthquake region are unclear. In order to reveal the characteristics of the deep medium and the seismogenic environment of the <i>M</i>7.0 Jiuzhaigou earthquake region, and to interpret the tectonic background and genesis of the seismicity comprehensively, in this paper, we have reviewed all available observation data recorded by the regional digital seismic networks and large-scale, dense mobile seismic array (China Array) for the northern section of the North–South Seismic Belt around Jiuzhaigou earthquake region. Using double-difference seismic tomography method to invert the three-dimensional P-wave velocity structure characteristics of the upper crust around the Jiuzhaigou earthquake region, we have analyzed and discussed such scientific questions as the relationship between the velocity structure characteristics and seismicity in the Jiuzhaigou earthquake region, its deep tectonic environment, and the ongoing seismic risk in this region. We report that: the P-wave velocity structure of the upper crust around the Jiuzhaigoug earthquake region exhibits obvious lateral inhomogeneity; the distribution characteristics of the shallow P-wave velocity structure are closely related to surface geological structure and formation lithology; the <i>M</i>7.0 Jiuzhaigou earthquake sequence is closely related to the velocity structure of the upper crust; the mainshock of the <i>M</i>7.0 earthquake occurred in the upper crust; the inhomogeneous variation of the velocity structure of the Jiuzhaigou earthquake area and its surrounding medium appears to be the deep structural factor controlling the spatial distribution of the mainshock and its sequence. The 3D P-wave velocity structure also suggests that the crustal low-velocity layer of northeastern SGB (Songpan–Garzê Block) stretches into MSM (Minshan Mountain), and migrates to the northeast, but the tendency to emerge as a shallow layer is impeded by the high-velocity zone of Nanping Nappe tectonics and the Bikou Block. Our results reveal an uneven distribution of high- and low-velocity structures around the Tazang segment of the East Kunlun fault zone. Given that the rupture caused by the Jiuzhaigou earthquake has enhanced the stress fields at both ends of the seismogenic fault, it is very important to stay vigilant to possible seismic hazards in the large seismic gap at the Maqu–Maqên segment of the East Kunlun fault zone.

The missing craton edge: Crustal structure of the East European Craton beneath the Carpathian Orogen revealed by double-difference tomography

The Trans-European Suture Zone (TESZ) is the most important and extensive continental suture in Europe, marking the edge of the East European Craton (EEC), from the North Sea to the Black Sea. It corresponds to significant changes in surface geology and deep crustal structure, evident in seismic, gravitational and magnetic studies. However, the TESZ disappears beneath the Eastern Carpathians accretionary nappes and Neotethys ophiolites, thrusted over the subducted EEC passive margin in Romania that may have experienced progressive southward break-off generating post-collisional volcanism and anomalous seismicity in the Vrancea region of the South-East Carpathians. To illuminate the missing TESZ section and investigate the change in crustal properties from the Precambrian EEC across the collisional orogen and the impact of related volcanism, we determined a 3D seismic model of P and S wave velocities of the Eastern Carpathians in Romania. With the advent of new permanent broadband and short-period seismic stations of the Romanian Seismic Network, we were able to lower the earthquake magnitude detection threshold to 0.5 ML, largely expanding the earthquake database and create seismic images of the crust across the Carpathians. Using double-difference tomography and waveform cross-correlation differential times, we relocated local crustal earthquakes between 2010 and 2017 and jointly inverted for the 3D P and S -wave velocity structure down to the Moho discontinuity. Our study provides the highest resolution 3D crustal seismic model of this area to date and emphasizes the manifestation of surface tectonic boundaries at lower crustal depths. The TESZ is highlighted at mid-to lower-crustal depths beneath the Carpathian nappes, east of the post-collisional volcanic belt, as a gently-dipping transition from high Vp in the eastern footwall, the dipping Precambrian basement, to low Vp beneath the Carpathian Orogen to the west and a downward decrease in Vp/Vs. In parts of the volcanic region, Vp/Vs ratios increase suggesting the presence of fluids, mafic material and partial melts that may have altered the seismic structure of the TESZ. Relocated hypocenters are vertically distributed in the velocity transition zone from low to high especially beneath the youngest volcanoes in the south, consistent with a hypothetically active magmatic plumbing system. In the northern segment, where volcanoes are older, relatively high velocities are estimated and seismicity tends to cluster in the top 10 km, likely connected to a recently reactivated fault system. Seismicity in the foreland of the East Carpathians aligns along the NW-SE trending major crustal faults and continues beneath the thrust belt, suggesting the East European Craton basement extends beneath the orogen as far as the volcanic belt.

Quasi-waveform seismic tomography of crustal structures in the capital circle region of China

Seismic tomography is one of the main tools to explore the interior of the earth. In this study, the quasi-waveform seismic tomographic method is used for the first time to reveal the crustal structures in the capital region of China. 3-D high-resolution VP, VS and the Poisson’s ratio models are generated by inverting 29839 direct P- and 29972 direct S-wave traveltimes selected from 3231 local earthquakes. The results reveal strong crustal heterogeneities. The velocity anomalies at shallow depths are well consistent with surface geologic structures. The relatively low-velocity anomaly layer in the middle crust may be the result of multiple phases of tectonic activity. Earthquakes generally occurred on the boundaries of high- and low-velocity and Poisson’s ratio anomalies. There are obvious low-velocity anomalies below the hypocenters of the Tangshan earthquake and the historical Sanhe-Pinggu earthquake, implying the existence of fluids. The similar velocity structures around the hypocenters of the two earthquakes indicate that the occurrences of the two earthquakes may be related to the same mechanism. The highresolution velocity models provide important observational constraints on the small-scale heterogeneities and dynamic mechanism of the crust in the capital region of China.