Search

Earth’s upper atmosphere over low latitudes is influenced by the equatorial electrodynamics. Its influence can also be modulated by the thermospheric winds. The effect of these two forces on the thermospheric neutral behaviour in the daytime can be investigated by measuring the variations in the OI 630.0 nm dayglow emission rates as these serve as effective tracers of the altitude region of their origin. The capability to measure dayglow emissions over a large field-of-view presents a unique opportunity to investigate the upper atmospheric behaviour over large spatial extents. Diurnal measurements of OI 630.0 nm dayglow emission rates have been carried out from two different locations in India using multi-wavelength imaging echelle spectrographs, MISE, which, together cover a spatial range from 5 ° -18 ° magnetic latitude. These investigations have been carried out during January-February 2020 (winter season) when solar flux variation was nearly negligible (varied only by 4 solar flux unit from 68-72 solar flux unit). Therefore, latitudinal variations in the daily averaged oxygen dayglow emission intensities have been analyzed to assess their response to the variations in the strength of the equatorial electrodynamics and the meridional winds. The results reveal that these forces show varying effects with the poleward meridional wind contributing to an enhancement in the OI 630.0 nm dayglow emission rates closer to the magnetic equator and a decrement as one moves away from the equator. The equatorial electric field effect, however, continues to be equally effective in the low-latitudes region under consideration.

Jiaodong is the largest gold deposit concentration area in China. In recent years, great breakthroughs have been made in deep prospecting, and it has become the third largest concentration area of gold deposits in the world. A series of prospecting discoveries in the Jiaodong area provided the basic conditions for summarizing the regional metallogenic law, constructing the deposit model, carrying out research on the genesis of the deposit and innovating metallogenic theory. The Jiaodong gold deposits are mainly distributed in Northwest Jiaodong, Qi-Peng-Fu and Mu-Ru metallogenic regions, and mainly occur in Precambrian metamorphic rock series, Jurassic Linglong-type granite, Early Cretaceous Guojialing-type granite and Laiyang group. The orebodies are controlled by Sanshandao, Jiaojia, Zhaoping, Xilin-Douya, Jinniushan and other major ore-controlling fault zones. The main mineralization types include altered rock-type, quartz vein-type, stockwork type, sulfide quartz vein-type, interlayer detachment zone-type, altered conglomerate-type, basin margin fault breccia-type and pyrite carbonate vein-type and so on. The important progress of deep prospecting is summarized: two super giant deposits have been evaluated in the Sanshandao and Jiaojia areas; the amount of altered rock-type gold resources identified by deep prospecting has exceeded that of quartz vein-type gold deposits in the Linglong gold deposit field; the Hushan large scale gold deposit has been discovered in Qixia gold deposit field; and the Liaoshang large-size gold deposit is a new discovery of deep prospecting in Pengjiakuang gold deposit field. The ore-controlling faults of gold deposits have several changing steps with dip angle varying from steep to gentle along the dip angle. The gold ore bodies are mainly enriched in sections along the steep, gentle turning points and relatively gentle parts of the fault dip angle, forming a stepped distribution pattern. In the Early Cretaceous, the subduction and rollback of the Paleo-Pacific plate induced crust mantle interaction, resulting in large-scale magmatic and fluid activities. The crust tension and magma uplift form the dome extension structure of granite, which provides a channel for the migration of ore-forming fluid and also provides favorable space for the deposition of ore-forming fluid.

In this study, the crustal shear velocity structure beneath the Indo-Burma Ranges (IBR) is estimated using inversion modeling of the receiver functions computed at seismic stations operated during 2011–2018. The inversion results describe significant variations in the shear-wave velocity (Vs), velocity ratio (Vp/Vs) and crustal thickness in and around the IBR. The Moho deepens from 41 km in the west to 70 km in the east of the IBR. It is also observed that most of the earthquakes are confined to the crust in the west of the Indo-Burma arc, whilst the east of the IBR is associated with sub-crustal earthquakes, supporting the fact that the Indian plate is dipping beneath the Burma Plate. The dip angle of the Moho in the north of the IBR is smaller (7.8o) than that in its south (9.5o). We found that an average crustal Vs of 3.5 km/s for the highest value of the Vp/Vs ratio (> 1.8) was prevalent at all the stations in the IBR, associated with mafic prehnite-pumpellyite facies basalt and hydroxyl zeolite facies basalt, which suggests that the subducting Indian slab beneath the Myanmar microplate in and around the IBR is under a state of dehydration, where intra-slab earthquakes at varying depths (> 50 km) occur. This observation is highly consistent with the earlier 3-D tomographic results obtained independently.

© 2021 The Author(s). Published by Oxford University Press on behalf of The Royal Astronomical Society.Flat slab subduction has been ascribed to a variety of causes, including subduction of buoyant ridges/plateaus and forced trench retreat. The former, however, has irregular spatial correlations with flat slabs, while the latter has required external forcing in geodynamic subduction models, which might be insufficient or absent in nature. In this paper, we present buoyancy-driven numerical geodynamic models and aim to investigate flat slab subduction in the absence of external forcing as well as test the influence of overriding plate strength, subducting plate thickness, inclusion/exclusion of an oceanic plateau and lower mantle viscosity on flat slab formation and its evolution. Flat slab subduction is reproduced during normal oceanic subduction in the absence of ridge/plateau subduction and without externally forced plate motion. Subduction of a plateau-like feature, in this buoyancy-driven setting, enhances slab steepening. In models that produce flat slab subduction, it only commences after a prolonged period of slab dip angle reduction during lower mantle slab penetration. The flat slab is supported by mantle wedge suction, vertical compressive stresses at the base of the slab and upper mantle slab buckling stresses. Our models demonstrate three modes of flat slab subduction, namely short-lived (transient) flat slab subduction, long-lived flat slab subduction and periodic flat slab subduction, which occur for different model parameter combinations. Most models demonstrate slab folding at the 660 km discontinuity, which produces periodic changes in the upper mantle slab dip angle. With relatively high overriding plate strength or large subducting plate thickness, such folding results in periodic changes in the dip angle of the flat slab segment, which can lead to periodic flat slab subduction, providing a potential explanation for periodic arc migration. Flat slab subduction ends due to the local overriding plate shortening and thickening it produces, which forces mantle wedge opening and a reduction in mantle wedge suction. As overriding plate strength controls the shortening rate, it has a strong control on the duration of flat slab subduction, which increases with increasing strength. For the weakest overriding plate, flat slab subduction is short-lived and lasts only 6 Myr, while for the strongest overriding plate flat slab subduction is long-lived and exceeds 75 Myr. Progressive overriding plate shortening during flat slab subduction might explain why flat slab subduction terminated in the Eocene in western North America and in the Jurassic in South China.

Using the ground-based multi instruments observations in Hainan and Southeast Asia, as well as space-based observations, the long lasting events of ionospheric irregularities occurred in the equatorial region of East Asia on the night of March 31, 2014 are studied. Concurrent observations at Hainan station indicate that ionospheric irregularities measured with different instruments showed distinct spatial structures and temporal variation, mainly from sunset to midnight (19-23LT) and even to post midnight. The 3-m-scale irregularities detected by VHF radar first attenuated and disappeared at 01LT, then the 400-meter-scale irregularities (GPS scintillations) vanished at 03LT, and finally the larger-scale irregularities (spread F) ended at 05LT. It provides a directly observational evidence that the larger the scale is, the latter the irregularities decay after midnight. Ionospheric irregularities near the magnetic equator (ME) and the equatorial abnormal peaks (EAPs) mainly occurred from after sunset to midnight (19-01LT), but there were distinct morphological differences after midnight. The irregularities near ME were attenuated and disappeared rapidly, while irregularities near EAPs weakened obviously, but lasted until dawn. The plasma depletions observed by the SWARM satellite in the latter half of the night were clearly related to the TEC fluctuations at JOG2 site, the ionospheric scintillations and spread F at Hainan station. The quasiperiodic structures of plasma bubbles observed by C/NOFS satellite were clearly related to the occurrence of ionospheric irregularities in Hainan and Southeast Asia, which indicated that the seeding of atmospheric gravity waves may play an important role in the generation of ESF/EPB irregularities, even in the latter half of the night.

Difficult understanding of gravity effects on the 2D vertical and inclined faults for the delineation of subsurface structure for gravity exploration is slow and cumbersome. Hence, a fast and efficient algorithm is established for the interpretation of gravity anomaly over 2D inclined and vertical fault. The method can simultaneously determine all parameters such as the depth to the top (z) and base (h), dip angle (α), amplitude coefficient (k), and location of the fault plane on the surface (x0) of a hidden thin faulted slab from the observed gravity data. The developed algorithm can effectively interpret all parameters for dipping and vertical fault even though there is no subsurface drilling information. Interpretation of all the parameters suggests that there is no uncertainty for 2D inclined and vertical fault. However, if the detachment tip of the fault is at a larger depth, then the dip of the fault shows some uncertainty. The present code has been applied to non-noisy synthetic anomaly data and Gaussian noisy anomaly. Furthermore, the algorithm was also verified on three field examples from Egypt, and the USA for exploration. The appraised value of all the parameters is found to be in decent agreement with earlier published works and borehole information wherever available.

SUMMARY The 26 May 2019 MW 8.0 Peru intraslab earthquake ruptured the subducting Nazca Plate where the dip angle of the slab increases sharply and the strike angle rotates clockwise from the epicentre to north. To obtain a detailed seismic source model of the 2019 Peru earthquake, including not only the rupture evolution but also the spatiotemporal distribution of focal mechanisms, we performed comprehensive seismic waveform analyses using both a newly developed flexible finite-fault teleseismic waveform inversion method and a back-projection method. The source model revealed a complex rupture process involving a back-propagating rupture. The initial rupture propagated downdip from the hypocentre, then unilaterally northward along the strike of the slab. Following a large slip occurring ∼50–100 km north of the hypocentre, the rupture propagated bilaterally both further northward and back southward. The spatial distribution of focal mechanisms shows that the direction of T-axis azimuth gradually rotated clockwise from the epicentre northward, corresponding to the clockwise rotation of the strike of the subducting Nazca Plate, and the large-slip area corresponds to the high-curvature area of the slab iso-depth lines. Our results show that the complex rupture process, including the focal-mechanism transition, of the Peru earthquake was related to the slab geometry of the subducting Nazca Plate.

Trans-ionospheric signals such as the Global Positioning System (GPS) signals propagating through irregular ionospheric plasma structures may experience phase variation and amplitude fades causing degradation in system performance. This paper investigates the latitudinal structure of equatorial F-region irregularities (EFI) in the African region at different seasons using a GPS-based proxy index (rate of change of TEC index (ROTI)). The results obtained indicate the dependence of EFI on local time, season and latitude. Its occurrence time ranges between 19:00 and 0:00LT depending on the season. Its appearance is earlier in equinoxes, particularly in March equinox, and late in June solstice. Further, its occurrence lies generally within ± 22° magnetic latitudes for all seasons. The probability of occurrence, which is observed to be generally higher in equinoxes than solstices, is almost comparable across the entire latitudes where it was observed in equinoxes while the observations in solstices indicate a dip around the magnetic equator. Furthermore, the average seasonal value of ROTI indicates a clear latitudinal dependence; with appearance within ± (0°-12°) magnetic latitude in solstices while it extends up to 18° in equinoxes, particularly in the south. While the hemispheric asymmetry in the average seasonal values is favored by the hemispheric asymmetry of latitudinal TEC profile, the TEC gradient could help delineate the latitude of its peak occurrence. In addition, severe irregularities are mostly observed in equinoxes than in solstices across all latitudes of occurrence and are most pronounced around the crest region mainly in the south. Conversely, moderate irregularities are typically observed in solstices and are most frequent around the magnetic equator and the poleward edge of equatorial ionization anomaly (EIA) region in the northern hemisphere.

The consequences to structures caused by permanent fault displacement has been investigated for dip-slip faulting, but not for the effect of the embedment depth on the interaction between a normal fault rupture and shallow embedded foundation. This study investigated the effect of the embedment depth on the interaction of normal fault rupture and shallow foundation using a numerical model validated with centrifuge experiments. It was found that a gapping interaction mechanism and foundation distress occurred at different foundation positions relative to the fault rupture outcrop for an embedded foundation in comparison with a surface foundation. The extent of this area depended on the combined influences of the foundation position, foundation surcharge, embedment depth, and fault dip angle. The sidewalls of the shallow embedded foundation were observed to act as kinematic constraints and had considerable influence on the rotation and displacement of the foundations. With regard to the level of rotation and displacement of the embedded foundation, the lateral earth pressure distribution on the footwall sidewall was similar to that of Rankine active earth pressure in a triangular distribution and on the hangingwall sidewall as a parabolic distribution of passive earth pressure. Foundations laid on loose soil exhibited less rotation than those on dense soil because the fault ruptures were absorbed or bifurcated around both sides of the foundation.

Multi-reflected echoes (MREs) and satellite traces (STs) are referred in literature as ionogram signatures of Travelling Ionospheric Disturbances (TIDs) which is a phenomenon that apparently drives spread F development mainly at nighttime mid-latitude ionosphere. A long-term statistical study has been undertaken to investigate the morphological aspect of these signatures over the lower midlatitude European station of Nicosia, Cyprus (35.19°N, 33.38°E geographic; magnetic dip. 29.38°N) by inspecting all ionograms recorded by the DPS-4D digisonde in the interval 2009–2016. The results underline the systematic manifestation of these TID signatures over Cyprus with a possible (although not quite clear) solar activity dependence and a distinctive seasonal and diurnal occurrence rate with a seasonal peak of STs during summer and of MREs during January to April. Based on the experimental results of the present study, the seasonal occurrence rate of MREs and STs is found to increase by 75% and 56% during high solar activity periods. Satellite traces are well known ionogram signatures of TIDs and mostly correlated to the nighttime spread F formation. The occurrence of mid-latitude spread Fs over European longitude sector normally increases during summer. The occurrences of TIDs are also prominent at this interval of the year over nighttime mid-latitude ionosphere. The presence of MREs as an ionogram signature of TIDs over mid-latitude ionosphere is unique in nature.

Tight and unconventional reservoirs have become the focus with the progress of petroleum exploration and development. Micro-fractures in these reservoirs can effectively improve reservoir permeability, and well-developed micro-fractures can serve to directly improve productivity. Compared with the centered electrical well logging method, the Micro Spherical Focused Logging (MSFL) is more suitable for micro-fracture identification due to its high resolution and near borehole wall measuring method. In this study, an anisotropic model is used to depict micro-fractured formation. First, a forward model with micro-fractured formation, borehole, logging instrument and surrounding rock is established. Subsequently, MSFL responses under different micro-fracture porosity, resistivity, dip angle and borehole radius, are calculated based on the finite element method (FEM). Finally, the MSFL responses under different micro-fracture parameters are analyzed with the response laws clarified.

A seafloor geomagnetic observatory in the northwest Pacific has provided very long vector geomagnetic time-series. It was found that the time-series include significant magnetic signals generated by a few giant tsunami events including the 2011 Tohoku Tsunami. Here we report that the tsunami-generated magnetic fields consist of the weak but first arriving field, and the strong but second arriving field—similar to the P- and S-waves in seismology. The latter field is a result of coupling between horizontal particle motions of the conductive seawater and the vertical component of the background geomagnetic main field, which have been studied well so far. On the other hand, the former field stems from coupling between vertical particle motions and the horizontal component of the geomagnetic main field parallel to tsunami propagation direction. The former field has been paid less attention because horizontal particle motions are dominant in the Earth’s oceans. It, however, was shown that not only the latter but also the former field is significant especially around the magnetic equator where the vertical component of the background magnetic field vanishes. This implies that global tsunami early warning using tsunami-generated magnetic fields is possible even in the absence of the background vertical geomagnetic component.

The Van Allen Probe B satellite simultaneously observed electromagnetic ion cyclotron (EMIC) and fast magnetosonic (MS) waves in a magnetic dip on April 29, 2017. During the magnetic dip, we found the coexistence of flux enhancements of ring current protons (∼11.2–∼51.7 keV) and flux reductions of relativistic electrons (∼1 MeV), which are identified as typical signatures of a magnetic dip in the inner magnetosphere. Based on linear theoretical calculations and observational analysis, the observed ring current protons show an anisotropic temperature distribution and partial shell distribution for different energies during the magnetic dip to provide free energies for the generation of EMIC and MS waves, respectively. Our findings indicate that the complex unstable distribution in the velocity phase space of ring current protons during the magnetic dip can trigger the simultaneous generation of EMIC and MS waves in the inner magnetosphere.

Subduction style is controlled by a variety of physical parameters. Here we investigate the effect of subducting plate length on subduction style using laboratory experiments of time-evolving buoyancy-driven subduction in 3-D space. The investigation includes two experimental sets, one with a lower (~740) and one with a higher (~1,680) subducting plate-to-mantle viscosity ratio (ηSP/ηM). Each set involves five models with a free-trailing-edge subducting plate and variable plate length (20–60 cm, scaling to 1,600–4,800 km), and one model with a fixed-trailing-edge subducting plate representing an infinitely long plate. Through determining the contact area between subducting plate and underlying mantle, plate length affects the resistance to trenchward motion of the subducting plate and thus controls the partitioning of the subduction velocity (vS) into the subducting plate velocity (vSP) and trench velocity (vT). This subduction partitioning thereby determines the subduction style by controlling the dip angle of the slab tip once it first touches the 660 km discontinuity. The low ηSP/ηM models display two subduction styles. Short plates (≤40 cm) induce a higher subduction partitioning ratio (vSP/vS), promoting trench advance and slab rollover geometries, whereas longer plates (≥50 cm) lead to a lower vSP/vS, producing continuous trench retreat and backward slab draping geometries. In contrast, the high ηSP/ηM models exclusively show trench retreat with draping geometries, as the high ηSP/ηM enables less slab bending before its tip touches the 660 km discontinuity. Our study indicates that future modeling work should consider the effects of plate length on the style and evolution of subduction.

Equatorial plasma bubbles (EPBs) can cause rapid fluctuations in amplitude and phase of radio signals traversing the ionosphere and in turn produce serious ionospheric scintillations and disrupt satellite-based communication links. Whereas numerous studies on the generation and evolution of EPBs have been performed, the prediction of EPB and ionospheric scintillation occurrences still remains unresolved. The generalized Rayleigh–Taylor (R–T) instability has been widely accepted as the physical mechanism responsible for the generation of EPBs. But how the factors, which seed the development of R–T instability and control the dynamics of EPBs and resultant ionospheric scintillations, change on a short-term basis are not clear. In the East and Southeast Asia, there exist significant differences in the generation rates of EPBs at closely located stations, for example, Kototabang (0.2°S, 100.3°E) and Sanya (18.3°N, 109.6°E), indicating that the decorrelation distance of EPB generation is small (hundreds of kilometers) in longitude. In contrast, after the initial generation of EPBs at one longitude, they can drift zonally more than 2000 km and extend from the magnetic equator to middle latitudes of 40° or higher under some conditions. These features make it difficult to identify the possible seeding sources for the EPBs and to accurately predict their occurrence, especially when the onset locations of EPBs are far outside the observation sector. This paper presents a review on the current knowledge of EPBs and ionospheric scintillations in the East and Southeast Asia, including their generation mechanism and occurrence morphology, and discusses some unresolved issues related to their short-term forecasting, including (1) what factors control the generation of EPBs, its day-to-day variability and storm-time behavior, (2) what factors control the evolution and lifetime of EPBs, and (3) how to accurately determine ionospheric scintillation from EPB measurements. Special focus is given to the whole process of the EPB generation, development and disruption. The current observing capabilities, future new facilities and campaign observations in the East and Southeast Asia in helping to better understand the short-term variability of EPBs and ionospheric scintillations are outlined.

Two earthquake swarms, the Qianguo swarm and Ningjiang swarm, have been observed since 2013 in the Songyuan area, Jilin Province, China. Using the seismic data of broadband permanent and temporal stations that recorded a larger number of earthquakes in the two swarms, we analyze the distribution of aftershocks and the focal mechanisms of these earthquakes and investigate the stress field and faulting properties of the Songyuan area. The relocated hypocenters of the Qianguo earthquakes are distributed along a NNW direction at focal depths of 4–13 km, while the dominant distribution of relocated Ningjiang earthquakes is along a NE direction at depths of 8–16 km. The focal mechanism solutions of 36 earthquakes with ML ≥ 3.6 demonstrate a combined strike-slip and thrust faulting mechanism with centroid depth concentrated at 3–7 km for the Qianguo swarm and a strike-slip mechanism with centroid depth at 5–7 km for the Ningjiang swarm. The full moment tensors of seven moderate–strong earthquakes show that these earthquakes contain a high percentage of non-double-couple components. The regional stress field is inverted using the focal mechanism parameters of the 36 earthquakes. In the Qianguo area, the horizontal maximum principal compressive stress ( $$\sigma_{1}$$ ) is in the E–W direction with a dip angle of 14.1°. In the Ningjiang area, the $$\sigma_{1}$$ is in the ENE–WSW direction with a dip angle of 9.2°. The $$\sigma_{1}$$ directions of the two areas are slightly different, but both approximately along an E–W direction. The comprehensive results of this study show that the Qianguo swarm is likely caused by the movement of a buried fault in the NNW direction oblique to the Changling–Dashanzi fault, whereas the Fuyu–Zhaodong fault in the NE direction is responsible for the Ningjiang swarm. Both the Qianguo and Ningjiang swarms are probably closely related to the thrusting effect of the subduction of the Pacific Plate into the Northeast Asian continental margin and the complex geological conditions in the Songyuan area.

Elastic reverse-time migration (ERTM) is becoming increasingly feasible with the development of high-performance computing. It can provide more physical information on subsurface structures. However, the crosstalk artifacts degrade the imaging resolution of ERTM. To obtain high-resolution ERTM imaging, we have developed additional constraints through a convolutional neural network (CNN) in the dip-angle domain. This procedure can significantly improve the image quality of ERTM by recognizing the dominant reflection events and rejecting the crosstalk artifacts in the dip-angle domain. This method can be divided into the following three steps. First, we generate the dip-angle gathers of ERTM using Poynting vectors shot by shot. Then, we stack all the dip-angle gathers over all the shots. Finally, we adopt the CNN to predict the dip-angle constraint, which can suppress the crosstalk artifacts and enhance the ERTM image quality. The picking method using CNN is an end-to-end procedure that can perform automatic picking without additional human intervention once the network is well-trained. The numerical examples have verified the potential of our method.

Fresnel zones are helpful for obtaining a high signal-to-noise ratio (S/N)-migrated result. A migrated dip-angle gather provides a simple domain for estimating 2D Fresnel zones for 3D migration. We...

Gridded aeromagnetic and aeroradiometric datasets of Isanlu sheet 225 were processed and interpreted in order to map the geological structures and characteristic lithology, as well as potential mineral ore zones in the area. Recorded aeromagnetic data were reduced to the magnetic equator before filtering operations which enhanced the magnetic anomalies. Analytic signal map was created, and subsequently a model was generated along a selected profile for better interpretation of the subsurface materials. On the aeroradiometric data were performed both enhancement and statistical operations. The lithological zones were identified. They are metasedimentary rocks, migmatites and igneous rocks occupying varying proportions within the entire area. Some potential radioactive or precious metal ore zones were also localized. A major structure interpreted in the study area trends in the SE–NW direction. Other surface structures were also delineated and general orientation was evaluated. This information may serve as a valuable database for further prospective research and mineral exploration in the region.

The present paper reports coordinated ionospheric irregularity measurements at optical as well as GPS wavelengths. Optical measurements were obtained from Tiny Ionospheric Photometer (TIP) sensors installed onboard the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) satellites. GPS radio signals were obtained from a dual frequency GPS receiver operational at Calcutta (22.58°N, 88.38°E geographic; geomagnetic dip: 32.96°; 13.00°N, 161.63°E geomagnetic) under the SCIntillation Network Decision Aid (SCINDA) program. Calcutta is located near the northern crest of Equatorial Ionization Anomaly (EIA) in the Indian longitude sector. The observations were conducted during the unusually low and prolonged solar minima period of 2008–2010. During this period, four cases of post-sunset GPS scintillation were observed from Calcutta. Among those cases, simultaneous fluctuations in GPS Carrier-to-Noise ratios (C/No) and measured radiances from TIP over a common ionospheric volume were observed only on February 2, 2008 and September 25, 2008. Fluctuations observed in measured radiances (maximum 0.95 Rayleigh) from TIP due to ionospheric irregularities were found to correspond well with C/N0 fluctuations on the GPS links observed from Calcutta, such effects being noted even during late evening hours of 21:00–22:00 LT from locations around 40° magnetic dip. These measurements indicate the existence of electron density irregularities of scale sizes varying over several decades from 135.6 nm to 300–400 m well beyond the northern crest of the EIA in the Indian longitude sector during late evening hours even in the unusually low solar activity conditions.

Seismic diffractions are ideal carriers of information on small-scale, discontinuous objects and can therefore be used to detect these geologic objects. However, recognizing diffractions is difficult because specular reflection with strong energy masks the weak diffraction. In this study, we propose a diffraction separation and imaging method based on a Mahalanobis-based and phase-based attenuation function used to modify the Kirchhoff migration formula in the full-azimuth dip-angle domain. In this domain, reflections are restricted to within the first Fresnel zone and are distributed in the vicinity of the stationary point, while diffractions are located across a wide range of azimuth and dip angles. Synthetic and field data applications suggest that this new method can effectively separate and image diffractions. The results also demonstrate the efficiency of the new method in clarifying subsurface small-scale objects, which can provide finer information about these structures for seismic interpretation.

Monitoring small magnitude induced seismicity requires a dense network of seismic stations and high-quality recordings in order to precisely determine events’ hypocentral parameters and mechanisms. However, microseismicity (e.g. swarm activity) can also occur in an area where a dense network is unavailable and recordings are limited to a few seismic stations at the surface. In this case, using advanced event detection techniques such as template matching can help to detect small magnitude shallow seismic events and give insights about the ongoing process at the subsurface giving rise to microseismicity. In this paper, we study shallow microseismic events caused by hydrofracking of the PNR-2 well near Blackpool, UK, in 2019 using recordings of a seismic network which was not designed to detect and locate such small events. By utilizing a sparse network of surface stations, small seismic events are detected using template matching technique. In addition, we apply a full-waveform moment tensor inversion to study the focal mechanisms of larger events (ML > 1) and used the double-difference location technique for events with high-quality and similar waveforms to obtain accurate relative locations. During the stimulation period, temporal changes in event detection rate were in agreement with injection times. Focal mechanisms of the events with high-quality recordings at multiple stations indicate a strike-slip mechanism, while a cross-section of 34 relocated events matches the dip angle of the active fault., Karlsruher Institut für Technologie (KIT) (4220), https://earthquakes.bgs.ac.uk/data/broadband_stationbook.html

The terrain conditions and geological structure of the mountainous areas in western China are complex. To meet the design requirements for the expressway, the route adopts the design of deep-buried super-long tunnels in hilly areas. If the highway tunnel passes through faulty structures, such as improper selection of structure and foundation form, it can cause collapse and structural instability during construction. Therefore, basic characteristics of faults and information that is more accurate should be provided during the survey process to avoid subsequent problems. The frequency and magnitude of earthquakes in this study area are high and available data show that the Xiaojiang fault was still active during the Quaternary Holocene period, which is a high-risk area for earthquakes. Therefore, a set of comprehensive technologies, such as geophysical prospecting, geological survey, and drilling, and was used to identify faults and their characteristics and to visually display underground information within a certain depth range. Here, the magnetotelluric method was used to analyze the electrical characteristics and structure of the highway tunnel. In addition, the electrical characteristics and structure of the Xiaojiang fault were discussed. Geological surveys and drilling agree well with the analysis and interpretation of the research results and further proposed a processing and interpretation method for correcting the dip angle of the fault using the statistical results from the long axis dip angle of the tensor polarization map for the wave impedance. Thus, a large amount of information in the tunnel site can be effectively obtained for predicting and evaluating the Xiaojiang fault. Furthermore, our results provide strong technical support for designing and operating highway tunnels, with meaningful experience and reference for future geophysical processing and interpretation.

In this paper, we analyzed low latitude geoelectric field variations and Geomagnetically Induced Current ( $$GIC$$ ), associated with disturbed geomagnetic field variations in West Africa. For this purpose, variations of geomagnetic field components $$H$$ , $$D$$ and $$Z$$ , and geoelectric field horizontal components $$Ey$$ and $$Ex$$ were examined during geomagnetically disturbed periods, with the daily means of the Ap index higher than 20 nT. Variations of geoelectric field components $$Ey$$ and $$Ex$$ were identified as associated with disturbed variations of the geomagnetic field. The $$GIC$$ was estimated from the observed $$Ey$$ and $$Ex$$ based on system parameters configuration with $$a = b = 50~\,{\text{A}}\,{\text{km/V}}$$ . The disturbance fluctuations in the geoelectric field components and the estimated GIC exhibit a diurnal trend, with higher amplitudes during the daytime. The impulses in the geoelectric field components and the estimated $$GIC$$ are stronger in the southern stations than in the northern stations. On the average, these impulses decrease from LAM to TOM, with a slight enhancement near the magnetic equator.

This study examines the ionosphere response to gravitational forces of the lunar phase and dynamical disturbances of the stratospheric sudden warmings (SSWs). The total electron content (TEC) of global ionosphere maps is employed to examine responses of the equatorial ionization anomaly (EIA) crests to lunar phases and twelve SSW events during 2000–2013. The most prominent feature in the ionosphere is the EIA, characterized by two enhanced TEC crests at low latitudes straddling the magnetic equator, which can be used to observe ionospheric plasma dynamics and structures. Results show that the EIA crest appearance time on new/full moons (first/third quarters) leads (lags) that of the overall 14-year average, which causes a pattern of TEC morning enhancements (suppressions) and afternoon suppressions (enhancements). A statistical analysis shows that SSWs can also significantly cause the early appearance of EIA crests, regardless of the lunar phase. Thus, both lunar phase and SSWs can significantly modulate the appearance time of EIA crest and ionospheric plasma dynamics and structures.

The need to accurately interpret geological models that approximate mineralized zones in a Basement Complex terrain necessitate the development of horizon loop electromagnetic method (HLEM) forward modeling solutions for such scenarios. The focus of the present work is on finding rapid forward modeling solutions for synthetic HLEM data as an aid in exploration for moderate to deep conductive mineral exploration targets.The main thrust is obtaining idealized HLEM models that are required for geological interpretation of the subsurface in such environment. The original HLEM equations developed by Wesley were extended to represent a horizontally stratified earth with a conductive approximated by shear zone. From these equations a computer program was written to calculate the HLEM responses for optimal conductor model with known values of coil separations (L), depth of burial (z) and angle of dip of the target.The thin conductive model was used because it is simple and suitable for different geological scenarios. The accuracy of the approximate forward solution has been confirmed for HLEM systems with various geometric ranges, frequencies and conductivities. Three models having varying overburden thickness, dip angle of target and source-receiver separation were used in the forward modeling. The effect of varying the dip angle,overburden thickness and coil separation was studied in all the three models used. The result obtained from the forward modeling showed that variation of the dip angle gave rise to changes in the amplitudes of the anomalies generated, while that of overburden and coil separation gave rise to changes in anomaly shape. Also, the geometry and position of the causative body were precisely delineated.

SUMMARY Accurate identification of the locations and orientations of small-scale faults plays an important role in seismic interpretation. We have developed a 3-D migration scheme that can image small-scale faults using diffractions in time. This provides a resolution beyond the classical Rayleigh limit of half a wavelength in detecting faults. The scheme images weak diffractions by building a modified dip-angle gather, which is obtained by replacing the two dip angles dimensions of the conventional 2-D dip-angle gather with tangents of the dip angles. We build the modified 2-D dip-angle gathers by calculating the tangents of dip angles following 3-D prestack time migration (PSTM). In the resulting modified 2-D dip-angle gathers, the Fresnel zone related to the specular reflection exhibits an ellipse. Comparing with the conventional 2-D dip-angle gather, diffraction event related a fault exhibits a straight cylinder shape with phase-reversal across a line related the orientation of the fault. As a result, we can not only mute the Fresnel zones related to reflections, correct phase for edge diffractions and obtain the image of faults, but also detect the orientations of 3-D faults using the modified dip-angle gathers. Like the conventional dip-angle gathers, the modified dip-angle gathers can also be used to image diffractions resulting from other sources. 3-D Field data tests demonstrate the validity of the proposed diffraction imaging scheme.

The brittleness prediction of shale formations is of interest to researchers nowadays. Conventional methods of brittleness prediction are usually based on isotropic models while shale is anisotropic. In order to obtain a better prediction of shale brittleness, our study firstly proposed a novel brittleness index equation based on the Voigt–Reuss–Hill average, which combines two classical isotropic methods. The proposed method introduces upper and lower brittleness bounds, which take the uncertainty of brittleness prediction into consideration. In addition, this method can give us acceptable predictions by using limited input values. Secondly, an anisotropic rock physics model was constructed. Two parameters were introduced into our model, which can be used to simulate the lamination of clay minerals and the dip angle of formation. In addition, rock physics templates have been built to analyze the sensitivity of brittleness parameters. Finally, the effects of kerogen, pore structure, clay lamination and shale formation dip have been investigated in terms of anisotropy. The prediction shows that the vertical/horizontal Young’s modulus is always below one while the vertical/horizontal Poisson’s ratio (PR) can be either greater or less than 1. Our study finds different degrees of shale lamination may be the explanation for the random distribution of Vani (the ratio of vertical PR to horizontal PR).

Walkaway VSP cannot obtain accurate velocity field, as it asymmetrically reflects ray path and provides uneven coverage to underground target, thereby presenting issues related to imaging quality. In this study, we propose combining traveltime tomography and prestack depth migration for VSP of an angle-domain walkaway, in a bid to establish accurate two-dimensional and three-dimensional (3D) velocity models. First, residual curvature was defined to update velocity, and an accurate velocity field was established. To establish a high-precision velocity model, we deduced the relationship between the residual depth and traveltime of common imaging gathers (CIGs) in walkaway VSP. Solving renewal velocity using the least squares method, a four-parameter tomographic inversion equation was derived comprising formation dip angle, incidence angle, residual depth, and sensitivity matrix. In the angle domain, the reflected wave was divided into up- and down-transmitted waves and their traveltimes were calculated. The systematic cumulative method was employed in prestack depth migration of a complex surface. Through prestack depth migration, the offset-domain CIGs were obtained, and dip angle was established by defining the stack section horizon. Runge—Kutta ray tracing was employed to calculate the ray path from the reflection point to the detection point, to determine the incident angle, and to subsequently calculate the ray path from the reflection point to the irregular surface. The offset-domain residual depths were mapped to the angle domain, and a new tomographic equation was established and solved. Application in the double complex area of the Tarim Basin showed the four-parameter tomographic inversion equation derived in this paper to be both correct and practical and that the migration algorithm was able to adapt to the complex surface.

In an effort to explore the morphology of nighttime spread F at the lower mid-latitude European station of Nicosia, Cyprus (35°N, 33°E geographic; magnetic dip. 29.38°N), all ionograms recorded by the DPS-4D digisonde during the interval 2009–2016 have been analyzed. Subsequent detailed investigation was performed to establish the possible effect of various triggering mechanisms on spread F within the framework of Perkins instability on a statistical basis by correlating spread F occurrence particular wave pattern signatures (Satellite Trace/Multiple-reflected Echoes), gravity wave signatures, F layer uplift (h'F) and unstable sporadic E layers. The results verify the systematic manifestation and therefore the significance of TIDs and unstable sporadic E layers as triggering factors responsible for seeding spread F development and underline the frequent appearance of multi-reflected echoes and satellite traces as dominant precursors of mid-latitude spread F over European latitudes for the first time. Furthermore clear seasonal characteristics and inverse solar activity dependence of spread F occurrence in lower European mid-latitudes is established similar to southern hemisphere studies undertaken previously.

Morphology of the subducting Indian lithosphere beneath Tibet is the key to understanding the tectonic evolution of the Tibetan plateau. In the present study we determine three-dimensional P-wave velocity images of the mantle under the entire Tibetan plateau and its adjacent areas by inverting ~200,000 teleseismic relative travel-time residuals recorded at 893 stations belonging to >32 portable seismic networks deployed in and around the plateau. An east-west varying high-velocity anomaly with a large dip angle down to the mantle transition zone is revealed clearly, which represents the subducting Indian slab. On the basis of the tomographic results, we propose a slab warping model to describe the complex subduction of the Indian plate beneath the plateau, which is characterized by lateral bending in addition to downward subduction. The complex geometry of the present Indian slab is possibly caused by non-uniform pulling of an earlier subducted slab in a process with the Indian slab detachment propagating laterally. As a result, a warping arc has formed beneath the Himalaya block to keep the Indian lithosphere intact. This slab warping model agrees well with many observations, such as magmatism and surface deformations in the western and southern Tibetan plateau.

Electromagnetic ion cyclotron (EMIC) waves at large L shells were observed away from the magnetic equator by the Magnetospheric MultiScale (MMS) mission nearly continuously for over four hours on 28 October 2015. During this event, the wave Poynting vector direction systematically changed from parallel to the magnetic field (toward the equator), to bidirectional, to antiparallel (away from the equator). These changes coincide with the shift in the location of the minimum in the magnetic field in the southern hemisphere from poleward to equatorward of MMS. The local plasma conditions measured with the EMIC waves also suggest that the outer magnetospheric region sampled during this event was generally unstable to EMIC wave growth. Together, these observations indicate that the bidirectionally propagating wave packets were not a result of reflection at high latitudes but that MMS passed through an off‐equator EMIC wave source region associated with the local minimum in the magnetic field., Key Points Several hours of EMIC wave activity were observed off‐equator in the outer magnetosphere with plasma conditions favorable for local growthChanges in direction of the wave Poynting vector indicate transition of source region from poleward, to local, to equatorward of spacecraftObservations confirm association of EMIC wave source region with local minimum‐B of the field line, possibly related to Shabansky orbits

The influence of aseismic ridges on subduction kinematics and dynamics, and on the deformation of the overriding plate, remains a topic of debate. This study presents laboratory-based geodynamic models that simulate the process of aseismic ridge subduction below an overriding plate. The analog experiments show that, depending on its size, subduction of an aseismic ridge can impact on the kinematics of subduction, overriding plate deformation and, to a lower extent, on the geometry of the slab and that of the trench. Specifically, it can cause a reduction of the subducting plate and trench retreat velocities, decreases overriding plate extension and even drives local forearc shortening for the widest ridge, increases the slab dip angle next to the aseismic ridge, but decreases the dip angle at the aseismic ridge in case the ridge is thick and wide, and produces a local perturbation (indentation) of the trench geometry. The magnitude of the described modifications relies to a comparable extent on aseismic ridge width and thickness. The effects of aseismic ridge subduction observed in our models compare to those of the Louisville Seamount Chain, the Cocos Ridge, and the d'Entrecasteaux Ridge system in nature and provide an explanation for the local deformation observed at these subduction zones.

We estimate the fault model of the 2 September 2009 Tasikmalaya intraslab earthquake based on the GPS data available in western Java, Indonesia. The focal mechanism of the earthquake was used to help construct two possible fault models: a west-dipping fault with a strike of 160.8° and an east-dipping fault with a strike of 34.0°. In this study, vertical information from GPS data is crucial for constructing the top depth of the fault. The subsidence information from GPS data located near the epicenter suggests that the earthquake involved a deeper fault model. While the amount of the moment release of the east-dipping fault (Model dipE) is equivalent to Mw 6.9, the moment release of the west-dipping fault (Model dipW) is equivalent to Mw 6.8. The GPS data inversion indicates that Model dipW produces a better fit than Model dipE. The tsunami simulation indicates that the tsunami height generated by the east-dipping fault is smaller than that generated by the west-dipping fault, implying that the maximum tsunami height of the latter is closer to agreement with the reported one. Unlike Model dipE, the stress transfer analysis of Model dipW indicates that most of the aftershocks were located in the region where ΔCFF is positive, suggesting positive stress from the ruptured triggered aftershocks. The combined analysis of GPS data, tsunami simulation, and stress transfer suggests that the fault ruptured during the 2009 earthquake was dipping westward with a steep dip angle.

We have developed a simple method to determine completely the model parameters of a buried dipping fault from gravity data (depths to the centers of the upper and lower portions of the faulted thin slab, dip angle, and amplitude coefficient). The method is based on defining the anomaly values at the origin and at four symmetrical points around the origin on the gravity anomaly profile. By defining these five pieces of information, the dip angle is determined for each value of the depth of the lower portion of the faulted thin slab by solving iteratively one nonlinear equation of the form f(α)= 0. The computed dip angles are plotted against the values of the depth representing a continuous depth-dip curve. The solution for the depth to the lower portion of the faulted thin slab (down-thrown block) and the dip angle of the buried fault is read at the common intersection of the depth-dip curves. Knowing the depth to the center of the lower portion of the faulted layer and the dip angle, the problem of determining the depth to the center of the upper portion of the faulted slab (up-thrown block) is transformed into the problem of solving iteratively a nonlinear least-squares equation, f(z) = 0. Because the depths and the dip angle are known, the amplitude coefficient, which depends on the thickness and density contrast of the thin slab, is determined using a linear least-squares equation. The method is applied to theoretical data with and without random errors. The validity of the method is tested on real gravity data from Egypt. In all cases examined, the model parameters obtained are in good agreement with the actual ones and with those given in the published literature.

Angle-domain common-imaging gathers (ADCIGs) are important in analyzing the subsurface discontinuities where reflection waves take place. In elastic reverse time migration (ERTM), dip-angle ADCIGs can be computed postmigration via subsurface offset extension. We have obtained dip-angle ADCIG premigration in ERTM by using the Poynting vector, which was easy to compute during wavefield propagation. The reflection normal of PP imaging is the bisector of the scattering angle, whereas that of PS imaging is not. We derive formulas for PP and PS dip-angle estimations, respectively, with some straightforward vector operations. Similar to the subsurface-offset one, our method also outputs dip-angle ADCIGs with the appearance of blocky horizontal coherence. According to local semblance analysis, the signal with a better horizontal coherence promises a higher semblance score, and vice versa. We can thus design a specular filter to suppress incoherent noises according to their corresponding local semblance scores. We validate our methods with numerical examples. The Graben and Marmousi data sets show that our methods work effectively in dip-angle ADCIG computation and the following noise suppression in ERTM. We also examine our methods with one field data set.

The paleogeography of East Asia in the supercontinent (Pangea) during the late Paleozoic to Mesozoic remains incomplete because the temporal and spatial constraints on the rigid attachment of East Asia to Eurasia is still unresolved. One fundamental obstacle to constructing a high-precision apparent polar wander (APW) path for East Asia is the uneven temporal coverage of available paleomagnetic poles with a severe lithologic bias towards sedimentary rocks. In reality, paleomagnetic data from sedimentary rocks are often biased towards a shallower paleomagnetic inclination, resulting in underestimation of paleolatitudes. In the present study, we tested whether the sedimentary paleopoles in the South China Block (SCB) and the North China Block (NCB) can be corrected for their shallow paleomagnetic inclinations. The shallow bias of paleomagnetic inclination in sedimentary rocks from the SCB was corrected using the elongation-inclination (E/I) method. According to paleogeographic comparisons, sedimentary paleopoles for the NCB and volcanic paleopoles for the SCB provide the most realistic paleogeographic distributions from 150 to 120 Ma. Therefore, we propose that the optimum paleogeography between the NCB and the SCB is as follows: (1) apparent polar wander (APW) path was constructed from volcanic data only from 150 Ma to 120 Ma; (2) for time intervals younger than 110 Ma, a new composite APW path was constructed both from volcanic and sedimentary results, with systematic E/I corrections.

We report detailed rock-magnetic and paleomagnetic results from a 27-m-long sedimentary core collected at depocenter of Lake Chapala, western Trans-Mexican Volcanic Belt. Rock-magnetic parameters point to PSD grain-size titanomagnetites as the dominant magnetic mineralogy. Observed variations, in both concentration and grain-size dependent magnetic parameters, reflect changes in the sediment supply through the studied depth, but mainly during the last half century. Stable characteristic remanent magnetization, representative of secular variation of the geomagnetic field, was retrieved after alternating field demagnetization of the natural remanent magnetization (NRM). The comparison of the obtained magnetic inclination and declination logs against the corresponding curves predicted by the Global Geomagnetic Field Model SHA.DIF.14k enables to establish an alternative sedimentation-rate independent age-depth model for the whole core, in reasonably good agreement with the one calculated using conventional radiocarbon dating. This approach works not only for Lake Chapala, but also for Lake Chalco. The correlation established between rock-magnetic parameters and paleoenvironments, based on diatom identification of representative samples within specific zones, can be considered as a novel proxy methodology. This approach opens a new perspective for new, more detailed rock-magnetic, paleomagnetic and paleoenvironmental investigations for better paleoclimatic reconstructions.

High-resolution airborne magnetic data of Oyo Metropolis has been used to map subsurface geological features which in most cases, corresponds to the area of interest for mineral expedition. The airborne magnetic data was first reduced to the magnetic equator (RTE); this is normally done to make the data simpler to interpret. The RTE map was subjected to Fast Fourier Transform to separate the regional and residual magnetic anomalies and estimate their depths. An average depth of 2500 m and 500 m were obtained for the top of regional and residual sources respectively. 3D-Euler deconvolution and tilt derivative interpretation techniques were combined and used on the RTE map, to locate and estimate depths to geologic contacts and pegmatite intrusions attributed to lineaments. The resulting 3D-Euler depth solutions were plotted on the tilt derivative map of the study area. A very good correlation was highlighted, indicating that the methods are good interpretative techniques for mapping magnetic lineaments. The estimated depths obtained from 3D-Euler deconvolution method are in the range of 108.1–544.3 m for structural index 1.0 corresponding to intrusions (dykes), and the result of structural index 0.0 corresponding to geologic contacts are in the range of 5.9–295.2 m. The estimated depth parameters coupled with known geological information, were further used to produce the best image view of the subsurface geological features of the study area, using 2-D profile modeling technique. The result obtained from the 2-D profile model is in good agreement with the results obtained from the 3D-Euler deconvolution and tilt derivative methods, indicating locations and depths of various intrusions (dykes) and geologic contacts. The obtained results from this study will help shed more light into the structural framework of the study area and can help potential explorer target new areas for mineral exploration.

The deviation of the IRI estimates of the monthly mean foF2 in the low mid latitude of 95°E–130°E longitude sector is investigated using simultaneous ground measurements at four stations during 2010–2014. The stations form two conjugate pairs of the same geo-magnetic latitude at two fixed longitudes enabling direct longitudinal and hemispheric comparison. The temporal, spatial, seasonal and solar activity variations of the deviations are discussed with reference to the longitudinal density variation in the transition region between low and midlatitudes. Cases of underestimation/overestimation as well as good estimate are noted. Underestimation (overestimation) in the daytime and overestimation (underestimation) in the nighttime of 95°E (130°E) are common. The longitudinal difference in the measurements suggests negative (positive) foF2 gradient from west to east in daytime (nighttime). In contrast, the IRI predicts flatter or increasing longitudinal profiles from 95°E to 130°E. The local time and longitudinal variation of the IRI deviations can be attributed to the combined role of the longitudinal EIA structure as well as midlatitude zonal wind-magnetic declination effect. The station/season independent deviations relate the role of solar activity representation in the IRI. These deviations may be attributed to the weak IRI response to rapid solar flux fluctuations.

In order to assess Ground penetrating radar (GPR) for imaging the shallow subsurface geometry and characteristics of the fault in Yushu area, details GPR measurements with 25 MHz, 100 MHz and 250 MHz frequencies antenna were firstly conducted in four sites along the Yushu fault after geomorphologic and geological investigation. The 25 MHz profiles delineated an excellent general view of deformation zones at a much wider area and greater depth. While the 100 MHz and 250 MHz data provided more detailed analysis of the shallow subsurface deformation about the geologic structure and the fault, including the stratigraphic structures, the dip angle and direction of the fault plane. The remarkable variation in the pattern and relative amplitude of electromagnetic waveform on the two-dimensional GPR profiles are all obvious and it is considered as the main fault zone with a nearly vertical fault with the dip angle of 70°–85°. High frequency GPR profiles show a good consistency with the trench sections at three sites. The geometry of the main fault zone can be depicted and deduced up to ~ 12 m deep or even deeper on the 25 MHz GPR profile in Yushu area and it is considered to be the result of the movement of active faults. What’s more, the geophysical features on GPR profile associated to the strike-slip fault are further summarized in different geological and geomorphological environment in Yushu area, the study also provides further evidence that GPR is valuable for fault investigation and palaeoseismic study in the Qinghai-Tibet Plateau area.

A comprehensive study is presented to illustrate the effect of an expanded polystyrene sheet (EPS) wall for absorbing fault rupture and its effects on shallow embedded foundations. Numerical modeling has been thoroughly validated through centrifuge test results. Various embedment depths and fault dip angles were considered to specify the critical foundation position on the soil surface. Furthermore, the impact of vertical trenching of an EPS wall, next to a rigid foundation, was investigated to mitigate surface foundation deformation. Results indicated that, due to high compressibility and low shear resistance that is not time dependent, an EPS wall panel protects the foundation by absorbing fault induced shear strains in the soil stratum. The effectiveness of such walls depends mainly on the geometry, location and mechanical characteristics of the walls, foundation location, bedrock fault offset, and fault dip angle.

Soft and hard interbedded rocks show obvious time-dependent deformation after deep tunnel excavations, and it is therefore necessary to research the mechanical behavior of the layered rock. However, it is hard to obtain ideal transversely isotropic rocks in fields, so rock-like specimens were poured by using artificial materials. Cyclic loading–unloading creep experiments were performed on the artificial layered cemented specimens with various layer angles (0°, 30°, 60° and 90°) at a 20 MPa confinement. Time-independent deformations and time-dependent deformations of the rock-like specimens were distinguished to investigate the visco-elasto-plastic deformation characteristics. Instantaneous elastic strain and instantaneous plastic strain had linear correlations with stress ratio, whereas creep strain, including visco-elastic strain and visco-plastic strain, increased nonlinearly with an increasing stress ratio. The specimens with a small layer angle had more noticeable time-independent and time-dependent deformations and larger steady-state creep rates than those of the specimens with a large layer angle. Attenuation creep and secondary creep could be observed at relative low stress levels, whereas accelerating creep until failure occurred at the creep failure stress level. The time for creep failure can be predicated according to the axial steady-state creep rate or volumetric creep curve. Damage in the rock-like specimens showed linear correlation with the stress ratio. Dip angle has a significant effect on the creep failure mode under cyclic loading–unloading conditions.

Waveform tomography and reverse time migration were used to derive a high resolution seismic migrated section with walkaway VSP data from an oil field in Abu Dhabi, United Arab Emirates. A high resolution velocity model that correlates well with the sonic log was obtained from waveform tomography and used for reverse time migration. A specific workflow, based on dip estimate, structural conditioning, noise attenuation, similarity/coherency attributes and binary filtering was used for the extraction of faults and fractures from the reverse time migrated section. Complex networks of lineaments were obtained and interpreted using rose diagrams. The lineaments associated with large throw in seismic section correlate well with manually interpreted faults from seismic, and the dip angle of lineaments with little to no throw correlate with the dip angle of interpreted fractures from FMI data, thus suggesting they might be related to fracture zones or fracture corridors.

Understanding lattice-preferred orientations (LPOs) of olivine is important in the study of seismic anisotropy and mantle flow of the upper mantle in the earth. Although olivine is the major mineral of the upper mantle, both amphibole and chlorite in a deformed peridotite may develop LPO and thus affect the seismic anisotropy in water-rich environments. However, it is not known whether the coexistence of LPO of amphibole and chlorite contributes to seismic anisotropy constructively or destructively. Therefore, the LPOs of amphibole, chlorite, and olivine in hydrated mantle peridotites from Bjorkedalen, southwestern Norway, were examined. In this study, the hydrated mantle peridotites showed three types of the LPOs of olivine, classified as A type LPO, B type-like LPO, and mixed LPO between the two types. Most of the olivines showed weak LPOs but amphibole and chlorite showed strong LPOs. High water content in olivine (440–690 ppm H/Si) and numerous hydrous inclusions in olivine and orthopyroxene indicated that water induced the fabric change of olivine from A type to B type-like LPO. The seismic velocity and seismic anisotropy were calculated on the basis of the LPOs of the studied minerals. It is found that the LPOs of both amphibole and chlorite can contribute to a strong trench-parallel seismic anisotropy in the subduction zone constructively, depending on the dip angle of flow. The results suggest that the development of the LPO of amphibole and chlorite can contribute significantly to seismic anisotropy of the subduction zone.

In this study, we quantify the mechanisms that govern two related observations with regard to deep (15–50 km) subduction-zone aseismic slow-slip events (SSEs): (i) the linear scaling relationship between seismic moments (M0) and event durations (T), and (ii) the direction-dependent slow-slip rupture speeds. Geological observations suggest that deep-subduction slow-slip shear zones are anisotropic and viscoplastic; the anisotropy is due to the presence of dip-parallel mafic lineaments created by seamount subduction, whereas the viscoplasticity is due to deformation of mixed brittle mafic and ductile felsic materials. We postulate that a dip-parallel (=slip direction) mafic lineament in an overall felsic slow-slip shear zone acts as a stress guide, which localizes initial slow-slip rupture in the dip direction. Subsequent stress concentration along the dip-parallel edges of the early ruptured lineament leads to along-strike rupture, with the rupture front propagating through the felsic shear zone. The second-phase slip-area expansion maintains a constant dip-parallel rupture-zone length, inherited from the length of the early ruptured mafic lineament. By combining an energy balance equation with a two-phase rupture model outlined above, we obtain the first analytical expression of the observed linear scaling law in the form of M0 = c0T, where c 0 = 4 γ 1 Δ z G 2 L 2 μ ¯ s − μ ¯ d ρgH Δ z η e cos δ + V FW 2 L Δ z μ ¯ s 2 − μ ¯ d 2 ρgH 2 − 4 γ 1 G L + Δ z − ρ Δ zLG V FW + v a 2 ; the observed value of this empirical constant is between 1011.5 and 1013.5 J/s. In the above expression, L, H, Δz, δ, G, ηe, μ ¯ s , and μ ¯ d are length, depth, thickness, dip angle, shear rigidity, effective viscosity, and effective coefficients of static and dynamic friction of the slow-slip shear zone, γ1 is surface-energy density of the initially ruptured mafic lineament, ρ is overriding-plate density, VFW and va are subducting-plate and slow-slip velocities, and g is surface gravity. Our model, based on the assumed shear-zone anisotropy, successfully predicts fast (~100 km/h) dip-parallel rupture along high-viscosity (~1020 Pa s) mafic lineaments and slow (2–10 km/day) strike-parallel rupture through low-viscosity (~1017 Pa s) felsic materials during a deep-subduction slow-slip event.

Traditional prediction methods for formation integrity collapse pressure are difficult to describe for shale microfractures, joints, and weakness planes. In this study, for multiple weakness planes, a reasonable prediction model for shale wellbore collapse instability is constructed using the Mohr–Coulomb criterion and the multistructural plane strength theory, which can analyze the wellbore collapse pressure considering different weak planes. The results show that the collapse pressure increases with the increase in the number of weak planes. With an increase in the number of weak planes, the safe drilling window becomes narrower, which leads to an increase in the drilling difficulty. With the change in the drilling azimuth, the weak plane dip region where the maximum collapse pressure is located for different weak plane groups is transformed. For a weak plane group, the maximum collapse pressure changes from a dip angle of 30°–60° to 0° and 90° with the drilling azimuth. When the dip angle of two weak plane groups is 0°, the collapse pressure of the wellbore is the lowest, and the maximum collapse pressure changes from 60°–90° to 30°–60° with the change in drilling azimuth. When drilling along the minimum horizontal ground stress azimuth, there is only a difference when the included angle of inclination is 60°–180° and 240°–360°, and the other azimuth coincide, which indicates that the influence of the weak plane inclination on the collapse pressure is weakened when drilling along the azimuth. The effect of multiple weak planes on the collapse pressure is more significant in the later stages of drilling. The example analysis shows that under the action of multiple weak planes, the method can predict the formation collapse pressure more comprehensively and accurately than previous methods, which is meaningful in guiding drilling engineering in shale formations.