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In this paper, convolutional neural networks (CNNs) were used to distinguish between tectonic and non-tectonic seismicity. The proposed CNNs consisted of seven convolutional layers with small kernels and one fully connected layer, which only relied on the acoustic waveform without extracting features manually. For a single station, the accuracy of the model was 0.90, and the event accuracy could reach 0.93. The proposed model was tested using data from January 2019 to August 2019 in China. The event accuracy could reach 0.92, showing that the proposed model could distinguish between tectonic and non-tectonic seismicity. [ABSTRACT FROM AUTHOR]

Downward continuation is a very useful technique in the interpretation of potential field data. It would enhance the short wavelength of the gravity anomalies or accentuate the details of the source distribution. Taylor series expansion method has been proposed to be one of the best downward continued methods. However, the method using high-order vertical derivatives leads to low accuracy and instability results in many cases. In this paper, we propose a new method using a combination of Taylor series expansion and upward continuation for computing vertical derivatives. This method has been tested on the gravitational anomaly of infinite horizontal cylinder in both cases with and without random noise for higher accurate and stable than Hilbert transform method and Laplace equation method, especially in the case of noise input data. This vertical derivative method is applied successfully to calculate the downward continuation according to Taylor series expansion method. The downward continuation is also tested on both complex synthetic models and real data in the East Vietnam Sea (South China Sea). The results reveal that by calculating this new vertical derivative, the downward continuation method gave higher accurate and stable than the previous downward continuation methods. [ABSTRACT FROM AUTHOR]

As a data-driven approach, the performance of deep learning models depends largely on the quantity and quality of the training data sets, which greatly limits the application of deep learning to tasks with small data sets. Unfortunately, sometimes we need to use limited small data sets to complete our tasks, such as distributed acoustic sensing (DAS) data denoising. However, using a small data set to train the network may cause overfitting, resulting in poor network generalization. To solve this problem, we propose an approach based on the combination of a generative adversarial network and a deep convolutional neural network. First, we used a small noise data set to train a generative adversarial network to generate synthetic noise samples, and then used these synthetic noise samples to augment the noise data set. Next, we used the augmented noise data set and the signal data set obtained through forward modelling to construct a synthetic training set. Finally, a denoising network based on a convolutional neural network was trained on the constructed synthetic training set. Experimental results show that the augmented data set can effectively improve the denoising performance and generalization ability of the network, and the denoising network trained on the augmented data set can more effectively reduce various kinds of noise in the DAS data. [ABSTRACT FROM AUTHOR]

HY-2A is China's first satellite altimeter mission, launched in Aug. 2011. Its geodetic mission (GM) started from 2016 March 30 till present, collecting sea surface heights for about five 168-d cycles. To test how the HY-2A altimeter performs in marine gravity derivation, we use the least-squares collocation method to determine marine gravity anomalies on 1′ × 1′ grids around the South China Sea (covering 0°–30°N, 105°E–125°E) from the HY-2A/GM-measured geoid gradients. We assess the qualities of the HY-2A/GM-derived gravity over different depths and areas using the bias and tilt-adjusted ship-borne gravity anomalies from the U.S. National Centers for Environmental Information (NCEI) and the Second Institute of Oceanography, Ministry of Natural Resources (MNR) of P. R. China. The RMS difference between the HY-2A/GM-derived and the NCEI ship-borne gravity is 5.91 mGal, and is 5.33 mGal when replacing the HY-2A value from the Scripps Institution of Oceanography (SIO) V23.1 value. The RMS difference between the HY-2A/GM-derived and the MNR ship-borne gravity is 2.90 mGal, and is 2.76 mGal when replacing the HY-2A value from the SIO V23.1 value. The RMS difference between the HY-2A and SIO V23.1 value is 3.57 mGal in open sea areas at least 20 km far away from the coast. In general, the difference between the HY-2A/GM-derived gravity and ship-borne gravity decreases with decreasing gravity field roughness and increasing depth. HY-2A results in the lowest gravity accuracy in areas with islands or reefs. Our assessment result suggests that HY-2A can compete with other Ku-band altimeter missions in marine gravity derivation. [ABSTRACT FROM AUTHOR]

SUMMARY: On the 9th of January 2020, an Mw 6.4 strike-slip earthquake took place north of the Asian margin of the Bering Sea. The earthquake occurred within the known reverse-right-lateral active fault zone, called Khatyrka–Vyvenka, which transverses the Koryak Highland from SE to NW and is thought to be a surface manifestation of the Asian portion of either the Bering plate boundary or the northern edge of the Alaskan stream. No other strong earthquake has ever been recorded in this remote uninhabited area and the few existing seismic stations provide poor quality earthquake locations. We adopt SAR interferometry (InSAR) technique to define an improved location of the Koryak 2020 earthquake and constrain the seismic source. The analysis of the 2020 event revealed a previously unknown active fault of left-lateral kinematics that is possibly hidden and strikes NW transversely to the Khatyrka–Vyvenka fault zone. Although several mechanisms could account for left-lateral kinematics of this fault, we propose that the structure is part of a more extended NW fault structure, that formed in pre-neotectonic times and has played a role of a pre-existing rheological discontinuity. This revived NW structure together with a similar structure located easterly, so far aseismic, make the plate/stream boundary segmented, step-like in plan view. The step-like boundary geometry may be the result of internal transform deformation of a rigid plate, but it is better explained by deflections of the Alaskan stream edge at local crustal asperities, which are pre-Cenozoic terrains. [ABSTRACT FROM AUTHOR]

A sequence of three strong (M W7.2, 6.4, 6.6) earthquakes struck the Pamir of Central Asia in 2015–2017. With a local seismic network, we recorded the succession of the foreshock, main shock and aftershock sequences at local distances with good azimuthal coverage. We located 11 784 seismic events and determined 33 earthquake moment tensors. The seismicity delineates the tectonic structures of the Pamir in unprecedented detail, that is the thrusts that absorb shortening along the Pamir's thrust front, and the strike-slip and normal faults that dissect the Pamir Plateau into a westward extruding block and a northward advancing block. Ruptures on the kinematically dissimilar faults were activated subsequently from the initial M W 7.2 Sarez event at times and distances that follow a diffusion equation. All main shock areas but the initial one exhibited foreshock activity, which was not modulated by the occurrence of the earlier earthquakes. Modelling of the static Coulomb stress changes indicates that aftershock triggering occurred over distances of ≤90 km on favourably oriented faults. The third event in the sequence, the M W 6.6 Muji earthquake, ruptured despite its repeated stabilization through stress transfer in the order of –10 kPa. To explain the accumulation of M W > 6 earthquakes, we reason that the initial main shock may have increased nearby fault permeability, and facilitated fluid migration into the mature fault zones, eventually triggering the later large earthquakes. [ABSTRACT FROM AUTHOR]

The performance of supervised deep learning-based denoising methods relies on massive amounts of high-quality training data set with labels. However, data labelling is a time-consuming and tedious process, and the lack of labelled data set has become a major bottleneck affecting the development of supervised deep learning-based denoising methods. In recent years, denoising methods that only use unlabelled noisy data set for training have received more and more attention. Although these methods get rid of the dependence on labels, they usually have some specific requirements on the training data set. For example, the paired training data are required to be multiple noisy observations for each scene or obey a specific noise distribution, etc., which are often very challenging to meet in practical applications. In this study, we propose an improved self-supervised denoising framework based on Noise2Noise that only uses noisy seismic data set for training, and we name it sample2sample. The proposed denoising framework does not require multiple repeated acquisitions of seismic data to obtain multiple independent noisy observations for each scene used for training, and has no specific requirement for the noise distribution prior. Specifically, we introduce a random sampler to generate paired subsamples from some individual noisy seismic data for training. The corresponding elements in the two paired subsamples are adjacent in the original seismic data and approximately meet the training premise of Noise2Noise, that is the paired training data have the same signal. In addition, considering that there are some subtle differences in the signals of the paired subsamples generated by sampling, we also introduce a regularization loss to compensate for this. We conducted a qualitative and quantitative analysis of the denoising performance of the proposed method through further experiments, including synthetic data experiments and field data experiments. [ABSTRACT FROM AUTHOR]

The M w 7.9 2015 April 25 Gorkha earthquake is the latest of a millenary-long series of large devastating Himalayan earthquakes. It is also the first time a large Himalayan earthquake and its aftershocks were recorded by a local network of seismic stations. In the 5 yr following the main shock, more than 31 000 aftershocks were located by this permanent network within the ruptured area, including 14 362 events with M L greater than 2.5, 7 events with M L > 6, including one large aftershock with M w 7.2 on 2015 May 12. In 2020, 5 yr after the main shock, the seismicity rate along the ruptured fault segments was still about 5 times higher than the background seismicity before the Gorkha earthquake. Several bursts of earthquakes, sometimes organized in clusters, have been observed from a few days to several years after the main shock. Some of these clusters were located at the same place as the clusters that happened during the decades of interseismic stress build-up that preceded the large earthquake. They also happened in the vicinity of the high frequency seismic bursts that occurred during the main shock. These heterogeneities contribute to a persistent segmentation of the seismicity along strike, possibly controlled by geological structural complexities of the Main Himalayan Thrust fault. We suggest that these pre-2015 clusters revealed the seismo-geological segmentation that influences both the coseismic rupture and the post-seismic relaxation. [ABSTRACT FROM AUTHOR]

We investigate the presence of seismic structures in the Earth's mantle by searching for seismic signals that travel off the great circle path direction and are reflected or scattered off structures in the lower mantle. We focus on areas of current and past subduction beneath Eurasia by using events from Indonesia and Japan recorded at the broad-band stations in Germany, Morocco and Namibia. Applying seismic array techniques, we measure the direction and traveltime of the out-of-plane arrivals and backtrace them to their location of reflection/scattering. We backtrace the signals as P -to- P and S -to- P waves and extend the methodology to P -to- S waves. There seems to be a low number of reflection points in the regions beneath Eurasia in our study. Investigating possible causes, we find that the focal mechanism influences the presence of out-of-plane reflected waves. However, the potential coverage with our data set is large and should allow detection, but there may potentially be few seismically visible structures in the region. Most of our backtraced reflectors are located beneath southern Asia and are found shallower than 1500 km depth. They correlate well with the edges of prominent high velocity anomalies in tomographic inversions beneath southern Asia, which have been interpreted as remnants of fossil slabs of the subduction of the Tethys Oceans. We also observe few reflectors deeper than 1600 km that are located away from subducting regions and their positions coincide with the eastern edge of the African low velocity anomaly. These observations suggest that the presence of reflectors in the mid-lower mantle is not exclusively related to current or past subducting regions, but widespread throughout the mantle. [ABSTRACT FROM AUTHOR]

We investigate the nature of seismic anisotropy in the Kumaun–Garhwal Himalaya by analysing the core-refracted shear wave splitting phases (SK(K)S, PKS) recorded at 42 broad-band seismic stations. The mean fast anisotropy directions are mostly in ∼E–W and a few in the ∼NE–SW directions. The mean delay time decreases progressively from ∼1.0 s beneath the Sub Himalaya to ∼0.5 s at the Higher Himalaya. The observed seismic anisotropy suggests that the ongoing deformation beneath the Kumaun–Garhwal Himalaya has a large contribution by the relative rotation of the asthenospheric flow by the compression along the Indian Plate motion direction in this collision zone. Also, the southward transportation of the mid-crustal flow is probably responsible for the reduction in the delay times in the Main Central Thrust zone and further north than in the Sub-to-Lower Himalaya. The splitting parameters indicate a complex pattern of deformation beneath the Kumaun–Garhwal Himalaya with contribution from crust, lithospheric mantle and asthenospheric mantle. [ABSTRACT FROM AUTHOR]

Knowing the pre-collisional extent of the northern Indian Plate margin ('Greater India') is vital to understanding the tectonic evolution of the India–Asia collision and the formation of the Himalayan–Tibetan orogen. However, suitable geological units for palaeomagnetic investigations along the Himalayan belt are limited, which makes it difficult to reconstruct Greater India during the pre-collisional period in Late Cretaceous to Palaeogene. Often the palaeomagnetic results from the Zongpu Formation at Gamba in southern Tibet (∼88.5°E) were used for estimates of Greater India, but their validity was recently questioned. As a contribution to closing the data gap, we performed a palaeomagnetic study of the Palaeocene/Lower Eocene Dibling limestone (DL) in the western Tethyan Himalaya of Zanskar (34.0°N/76.6°E). The results from 27 sites revealed a well grouping (k  = 71.7) syntectonic magnetization with best grouping at 52 per cent unfolding. The remagnetization of the DL was acquired shortly after ∼54 Ma, at the latest at ∼49 Ma, and is probably carried by fine-grained magnetite formed during the early orogenic phase. Assuming proportional tilting of the fold limbs, the corresponding palaeolatitude of 11.8 ± 2.4°N suggests a maximum Greater India extent of 810 ± 420 km and a first continental contact with the southern Eurasian margin at ∼12°N in the western part of the suture zone. The tectonostratigraphic equivalence of the DL with the Zongpu Formation at Gamba and a great similarity in their magnetic properties supports a secondary origin of the Gamba results. Through understanding the mechanism of remagnetization in the DL, an early orogenic remanence acquisition is also indicated for the Zongpu Formation, and thus the Gamba results deserve further credit for Greater India reconstructions. However, we note a large inconsistency of the available Late Cretaceous and Palaeogene palaeolatitude data from the Tethyan Himalaya by up to ∼20°, corresponding to differences of up to ∼2000 km in the size of Greater India. These discrepancies require further palaeomagnetic work in the Tethyan Himalaya, and in particular we recommend comparative studies at same locations and of same units. [ABSTRACT FROM AUTHOR]

Heat flow data are essential for understanding lithospheric dynamics. As a petroliferous basin, a large number of boreholes have been drilled during hydrocarbon exploration and production in the northern part of Songliao Basin, Northeast China. Meanwhile, the data on crustal structures, core samples and formation temperatures have been accumulated, which provide an opportunity for understanding the thermal state of the basin. Based on the temperature data from both Drilling Stem Test and continuous steady-state logging profiles, together with the systematic analysis of the thermal properties of rock samples, we present a new heat flow map of the northern Songliao Basin with significantly increased number of heat flow sites. The northern Songliao Basin is characterized by relatively high geothermal gradients and high heat flow for sedimentary basins. The heat flow values range from 44.4 to 95.0 mW m–2 with an average of 67.2 ± 12.8 mW m–2, and the geothermal gradients range from 21 to 59 °C km–1 with an overall average of 41.7 °C km–1. Heat production from sedimentary covers accounts for about 4.5 mW m–2 at the site of Well SK-2. Furthermore, based on the crustal structures revealed by previous seismic studies, lithospheric thermal structures are analysed and compared among different structural units of the basin. A thinned thermal lithosphere with a thickness of ∼65 km is found beneath the Central downwarp and the Southeast uplift in which a large part of the heat flow is mantle derived. The Western Slope exhibits a moderate heat flow value and a thicker thermal lithosphere with thickness greater than 110 km. From the perspective of the geothermal state of the lithosphere, the regional geodynamics related to the Mesozoic lithosphere stretching and the subduction of the Pacific Plate are discussed. [ABSTRACT FROM AUTHOR]

We combine Global Positioning System (GPS) velocity field with Interferometric Synthetic Aperture Radar (InSAR) results to study the interseismic deformation across the western Altyn Tagh fault (ATF) at longitude 86° E. GPS and InSAR data are consistent after correcting for the contribution from vertical deformation in the InSAR line-of-sight map. InSAR and GPS data identify an area of ∼2 mm a−1 sinistral shear and ∼6 mm a−1 of NS shortening located ∼150 km south of the ATF near the Manyi fault system. Excluding the data located in that area, Bayesian inversion of a 2-D profile across the ATF indicate a locking depth of 14.8 ± 3.5 km and a slip rate of 8.0 ± 0.4 mm a−1, lying at the lower range of previously published estimates. In addition, we find no significant offset between the fault at depth and the surface fault trace and no asymmetry of the interseismic profile that implicitly reveal lateral variations of the elastic strength across the ATF. Detailed analysis of InSAR profile across the fault show no distinguishable surface creep along the western ATF. Our study highlights how different data sets, data selection and model assumption might impact results on the ATF slip rate, locking depth and rheological contrast across the fault. [ABSTRACT FROM AUTHOR]

The Qinling–Dabie orogenic belt, which contain the arc-shaped Dabbashan orocline and is the world's largest belt of HP/UHP metamorphic rocks, formed by a long-term complex amalgamation process between the North China Block and the Yangtze Block. To understand the collision processes and tectonic evolution, we constructed a 3-D S -wave velocity model from the surface to a depth of ∼120 km in the eastern Qinling-Dabie orogenic belt and its adjacent region by inverting 5–70 s phase velocity dispersion data of Rayleigh waves extracted from ambient noise data. Our 3-D model reveals low velocities in the middle–lower crust and high velocities in the upper mantle beneath the orogenic belt, suggesting the delamination of the lower crust. Our results support a two-stage exhumation model for the HP/UHP rocks in the study area. First-stage exhumation was caused by the slab breaking away from the subducted Yangtze Block during the Early–Middle Triassic. Partial melting of the lithospheric mantle caused by slab breakoff-related asthenospheric upwelling weakened the lithospheric mantle beneath the orogenic belt, and continued convergence of the two continental blocks led to further thickening of the lower crust. Such processes promoted lower-crust delamination, which triggered the second-stage exhumation of the HP/UHP rocks. In the Dabbashan orocline, two deep-rooted high-velocity domes, that is, Hannan–Micang and Shennong–Huangling domes, acted as a pair of indenters during the formation stage. High-velocity lower crust was observed beneath the Dabbashan orocline. In addition, our 3-D model reveals that high-velocity lithospheric mantle extends from the Sichuan Basin to the Dabbashan orocline, with a subhorizontal distribution, providing strong support for the high-velocity lower crust. We also observed the destruction of lithospheric mantle beneath the Yangtze Block; the destruction area is bounded by the North–South Gravity Lineament, suggesting that the destruction mechanism of the Yangtze Block may be similar to the North China Block. [ABSTRACT FROM AUTHOR]

As early as the Palaeozoic era, the Tianshan Mountains constituted a vast tectonic convergent zone within the Eurasian Continent; by this time, they had already experienced multiple intense crustal movements. The Indian Plate is currently advancing northwards by more than 50 mm yr–1; it affects the Tianshan Mountains even though they are located more than 1000 km away. However, many of the geological issues of the Tianshan Mountains, especially in eastern Tianshan, which remains an active tectonic zone today, are not yet fully understood. There have been few studies into the seismogenic characteristics of this intense seismic zone. Here, the full gravity gradient tensor (GGT) within the eastern Tianshan Mountains was calculated using a stable spatial frequency domain algorithm, based on high-spatial resolution (7.2 arcsec or approximately 200 m) Global Gravity Model plus (GGMplus) surface gravity data. In addition, the features of the geological structures in the eastern Tianshan Mountains were interpreted using different combinations of GGT components. Moreover, the possibilities of using different combinations of GGT components to identify the distributions and strikes of faults was discussed regarding the potential earthquake rupture risks in this study area. The results show that (1) the distributions and strikes of the main fault zones in the study area are highly consistent with the linear geological features within the mountains, as extracted from a combination of different GGT components, including terrain effects. (2) Removing the terrain effects revealed that the different components of GGT (derived from the complete Bouguer gravity anomaly) vary gently, showing few linear geological features that could be extracted. This implies that satellite gravity data are not sufficient to analyse the characteristics of small underground geological structures (spatial scales less than ∼10 km). (3) Comparing the spatial distributions of fault traces depicted at different ages with the linear geological features of the study area revealed that discovering more unknown faults increased the coincidence ratio of linear geological features to fault traces in the area (from 27 to 40 per cent). In conclusion, the findings of this study represent a valuable reference for further revealing the seismogenic characteristics of this area, where there is a lack of detailed surface fault structure measurements due to its inaccessibility. The proposed method could also recognize faults in other similar areas with harsh conditions that are not feasible for ground-based surveys, such as forests and glaciers. [ABSTRACT FROM AUTHOR]

The increasing density of geodetic measurements makes it possible to map surface strain rate in many zones of active tectonics with unprecedented spatial resolution. Here we show that the strain tensor rate calculated from GPS in the India–Asia collision zone represents well the strain released in earthquakes. This means that geodetic data in the India–Asia collision zone region can be extrapolated back in time to estimate strain buildup on active faults, or the kinematics of continental deformation. We infer that the geodetic strain rates can be assumed stationary through time on the timescale needed to build up the elastic strain released by larger earthquakes, and that they can be used to estimate the probability of triggering earthquakes. We show that the background seismicity rate correlates with the geodetic strain rate. A good fit is obtained assuming a linear relationship (⁠|$\dot{N} = \lambda \ \cdot \dot{\epsilon }$|⁠ , where |$\dot{N}$| is the density of the rate of M w ≥ 4 earthquakes, |$\dot{\epsilon }$| is strain rate and λ = 2.5 ± 0.1 × 10−3 m−2), as would be expected from a standard Coulomb failure model. However, the fit is significantly better for a non-linear relationship (⁠|$\dot{N} = \gamma _1 \cdot \dot{\epsilon }^{\gamma _2}$| with γ1 = 2.5 ± 0.6 m−2 and γ 2 = 1.42 ± 0.15). The b -value of the Gutenberg–Richter law, which characterize the magnitude–frequency distribution, is found to be insensitive to the strain rate. In the case of a linear correlation between seismicity and strain rate, the maximum magnitude earthquake, derived from the moment conservation principle, is expected to be independent of the strain rate. By contrast, the non-linear case implies that the maximum magnitude earthquake would be larger in zones of low strain rate. We show that within areas of constant strain rate, earthquakes above M w 4 follow a Poisson distribution in time and and are uniformly distributed in space. These findings provide a framework to estimate the probability of occurrence and magnitude of earthquakes as a function of the geodetic strain rate. We describe how the seismicity models derived from this approach can be used as an input for probabilistic seismic hazard analysis. This method is easy to automatically update, and can be applied in a consistent manner to any continental zone of active tectonics with sufficient geodetic coverage. [ABSTRACT FROM AUTHOR]

The Kohistan Island Arc (KIA) occupies the northwestern region of the Himalayan Mountains, sandwiched between Asia and India plates. Its formation, collision with plate boundaries, and evolution has been controversially discussed for a couple of decades. To better understand this, a palaeomagnetic study has been conducted on the Jutal dykes (ca. 75 Ma), intruded in the northeastern part of the KIA. Comprehensive rock magnetic investigations reveal that the magnetic carrier minerals are pyrrhotite and magnetite. An intermediate temperature component (ITC) predominates the natural remanent magnetization and shows good coincidence within-site; it is carried by pyrrhotite and is considered reliable, yielding a mean direction at D g/ I g = 11.5°/39.9° (k g = 28.4, α 95 = 3.5°) before and D s/ I s = 8.6°/12.1° (k s = 5.1, α 95 = 9.1°) after tilt correction. A high-temperature component that is carried by magnetite exhibits random distribution within-site. The fold test for the ITC is negative, indicating a post-folding origin. Scanning electron microscopy combined with energy-dispersive X-ray spectroscopy indicates that the magnetic carrier minerals were influenced by metamorphism or thermochemical fluids. The comparison of mean palaeolatitude (22.6 ± 3.5°N) of the ITC with the collisional settings and thermal history of the study area implies that the remagnetization occurred at ∼50–35 Ma, consistent with the previous reported palaeomagnetic data of the KIA. We propose a tectonic model that shows the evolution of the Jutal dykes, supporting the concept that India collided with the KIA first, followed by a later collision with Asia. [ABSTRACT FROM AUTHOR]

Calculations of Coulomb stress changes have shown that moderate to large earthquakes may increase stress at the location of future earthquakes. Coulomb stress transfers have thus been widely accepted to explain earthquake sequences, especially for sequences occurring within parallel or collinear fault systems. Relating, under this framework, successive earthquakes occurring within more complex fault systems (i.e. conjugate fault system) is more challenging. In this study, we assess which ingredients of the Coulomb stress change theory are decisive for explaining the succession of three large (M w 7+) earthquakes that occurred on a conjugate fault system in the NE Lut, East Iran, during a 30-yr period. These earthquakes belong to a larger seismic sequence made up of 11 earthquakes (M w 5.9+) from 1936 to 1997. To reach our goal, we calculate, at each earthquake date, the stress changes generated by the static deformation of the preceding earthquakes, the following post-seismic deformation due to the viscoelastic relaxation of the lithosphere, and the interseismic deformation since 1936. We first show that accurately modelling the source and receiver fault geometry is crucial to precisely estimating Coulomb stress changes. Then we show that 7 out of 10 earthquakes of the NE Lut sequence, considering the uncertainties, are favoured by the previous earthquakes. Furthermore, the last two M 7+ earthquakes of the sequence (1979 and 1997) have mainly been favoured by the moderate M w ∼ 6 earthquakes. Finally, we investigate the link between the Coulomb stress changes due to previous earthquakes and the rupture extension of the next earthquake and show that a correlation does exist for some earthquakes but is not systematic. [ABSTRACT FROM AUTHOR]

We investigate the crustal seismic structure of the Garhwal Himalayan region using regional and teleseismic earthquake waveforms, recorded over 19 closely spaced broad-band seismic stations along a linear profile that traverses from the Sub Himalayas to Higher Himalayas. The regional earthquake traveltime analysis provides uppermost mantle P- and S- wave velocities as 8.2 and 4.5 km s−1, respectively. The calculated receiver functions from the teleseismic P waveforms show apparent P-to-S conversions from the Moho as well as from intracrustal depths, at most of the seismic stations. These conversions also show significant azimuthal variations across the Himalayas, indicating complex crustal structure across the Garhwal Himalaya. We constrain the receiver function modelling using the calculated uppermost mantle (Pn and Sn) velocities. Common conversion point stacking image of P-to-S conversions as well as receiver function modelling results show a prominent intracrustal low shear velocity layer with a flat–ramp–flat geometry beneath the Main Central Thrust zone. This low velocity indicates the possible presence of partial melts/fluids in the intracrustal depths beneath the Garhwal Himalaya. We correlate the inferred intracrustal partial melts/fluids with the local seismicity and suggest that the intracrustal fluids are one of the possible reasons for the occurrence of upper-to-mid-crustal earthquakes in this area. The results further show that the Moho depth varies from ∼45 km beneath the Sub Himalayas to ∼58 km to the south of the Tethys Himalayas. The calculated lower crustal shear wave velocities of ∼3.9 and 4.3 km s−1 beneath the Lesser and Higher Himalayas suggest the presence of granulite and partially eclogite rocks in the lower crust below the Lesser and Higher Himalayas, respectively. We also suggest that the inferred lower crustal rocks are the possible reasons for the presence and absence of the lower crustal seismicity beneath the Lesser and Higher Himalayas, respectively. [ABSTRACT FROM AUTHOR]

The cumulative displacement by multiple slip events along faults may generate composite damage zones (CDZ) of increasing width, and could modify the hydraulic and mechanical properties of the faults. The internal architecture and fracture distribution within CDZs at the subsurface are analysed here by using seismic attributes of variance, curvature and dip-azimuth of the 3-D seismic reflection data from tight sandstone reservoirs in northeast Sichuan, China. The analysed faults intersect the reservoir within a depth range of 2.4–3.0 km. The damage intensity mapping revealed multiple CDZs with thicknesses approaching 1 km along faults ranging 3–15 km in length, and up to 1000 m of cumulative slip. The identification of numerous fault cores and associate damage zones led us to define three classes of CDZs: banded shape, box shape and dome shape. The mechanical strength contrasts and distortion of fault cores suggest potential weakening and strengthening (healing) mechanisms for formation of CDZs that can be extended to faulting processes and earthquake simulations. [ABSTRACT FROM AUTHOR]

Using a numerical integral method, we deduced a set of formulae for the co-seismic internal deformation in a spherically symmetric earth model, simultaneously taking self-gravitation, compressibility and realistically stratified structure of the Earth into account. Using these formulae, we can calculate the internal deformation at an arbitrary depth caused by an arbitrary seismic source. To demonstrate the correctness of our formulae, we compared our numerical solutions of radial functions with analytical solutions reported by Dong & Sun based on a homogeneous earth model; we found that two sets of results agree well with each other. Our co-seismic internal Green's functions in the near field agree well with the results calculated by the formulae of Okada, which also verifies our Green's functions. Finally, we calculated the Coulomb stress changes on the Japanese Islands and Northeast China induced by the Tohoku-Oki M w 9.0 earthquake using the methods described above. We found that the effect of layered structure plays a leading role on the near field, while curvature occupies a dominant position on the deep region of the far field. Through a comparison of the Coulomb stress changes at a depth of 10 km on a layered earth model calculated by our method along with the corresponding results of Okada, we found that the discrepancy between them in near field was ∼31.5 per cent, and that of far field was >100 per cent of the signals. [ABSTRACT FROM AUTHOR]

A uniformly spaced linear transect through the northeastern Tibetan Plateau was constructed using 54 stations from ChinaArray Phase II. We used a set of colocated earthquakes to form receiver function beams that were then used to construct a 2-D image of main converting boundaries in our region and to investigate lateral changes in main impedance contrasts along the transect. The image revealed obvious mid-crustal low-velocity zones beneath the Qilian Orogen and the Alxa Block. We developed a new procedure that uses harmonically decomposed receiver functions to characterize seismic anisotropy, and that can determine both the orientations of symmetry axes and their type (fast or slow). We tested our technique on a number of synthetic models, and subsequently applied it to the data from the transect. We found that: (1) within the upper crust the orientations of slow symmetry axes are nearly orthogonal to the strike directions of faults, and thus anisotropy is likely caused by the shape preferred orientation of fluid-saturated cracks or fractures and (2) together with the low-velocity zones revealed from receiver functions stacks, anisotropic layers in the middle-to-lower crust could be explained by the crustal channel flow that was proposed for this region by previous studies. The shear within the boundary layers of crustal flow forms anisotropy with symmetry axes parallel to the flow direction. [ABSTRACT FROM AUTHOR]

The Democratic People's Republic of Korea (North Korea) conducted its sixth and largest affirmed underground nuclear test on 2017 September 3. Analysis of Interferometric Synthetic Aperture Radar (InSAR) data revealed detailed surface displacements associated with the nuclear explosion. The nuclear explosion produced large-scale surface deformation causing decorrelation of the InSAR data directly above the test site, Mt. Mantap, while the flanks of the Mountain experienced displacements up to 0.5 m along the Line-of-Sight of the Satellite. We determined source parameters of the explosion using the Bayesian inversion of the InSAR data. The explosive yield was estimated as 245–271 kiloton (kt) of TNT, while the previous yield estimations range from 70–400 kt. We determined the nuclear source at a depth of 542 ± 30 m below Mt. Mantap (129.0769°E, 41.0324°N). We demonstrated that the Bayesian modelling of the InSAR data reduces the uncertainties in the source parameters of the nuclear test, particularly the yield and source depth that are otherwise poorly resolved in seismic methods. [ABSTRACT FROM AUTHOR]

We applied a nonlinear teleseismic tomography algorithm to explore the 3-D structure of the upper mantle beneath the Arabia–Eurasia continental collision zone in western Iran. An unprecedented data set consisting of 32 738 teleseismic P- wave relative arrival time residuals from 129 permanent and temporary seismic stations was inverted for seismic imaging. The seismic images suggest a thick high-velocity lithospheric mantle beneath the Zagros. Other high-velocity domain observed below ∼300 km depth, which is not connected to the Zagros lithosphere, is interpreted as a slab segment in the upper mantle beneath the collision zone. The low-velocity anomalies beneath Central Iran and Alborz, consistent with a weak upper mantle, may be a result of upwelling asthenosphere and partial melting of the subducted lithosphere. [ABSTRACT FROM AUTHOR]

The converted wave data (P -to- s / S -to- p), commonly termed as receiver functions, contain noise of various origins. Such noises may influence the modeling and may sometimes lead to over interpretations of the data. In order to suppress noise, we use a robust sparsity enhancing tool, that is, the Seislet Transform, to process receiver function data by applying regularization in the seislet domain. The transform utilizes the multiscale orthogonal basis and the basis functions are oriented along the dominant seismic phases following local linearity. The inversion results of both the synthetic and field examples from the Hi-CLIMB network and station HYB from the Indian shield show an excellent performance over the original data sets. [ABSTRACT FROM AUTHOR]

Knowledge of deformation at plate boundaries has been improved greatly by the development of observational techniques in space geodesy. However, most theoretical and numerical models of coseismic deformation have remained very simple and do not include realistic Earth structure. 3-D material heterogeneity and topography are often neglected because simple models are assumed to be sufficient and available tools cannot easily accommodate complex heterogeneity. In this study, we demonstrate the importance of 3-D heterogeneity using a spectral element method that incorporates topography and 3-D material properties. Using a parabolic hill model and a topographic model of the 2010 Maule earthquake, we show that topographic features can alter the shape of observed surface deformation patterns. We also estimate the coseismic surface deformation due to a model of the slip distribution of the 2015 Gorkha earthquake using realistic topography and 3-D elastic structure, and find that the presence of topography causes changes in the shape of observed surface displacement patterns, while material heterogeneities primarily affect the magnitude of observed displacements. Our results show that the inclusion of topography in particular can affect predictions of coseismic deformation modelling. [ABSTRACT FROM AUTHOR]

An earthquake (2016M5.8_GY) occurred on 2016 September 12 due to the SW−NE movement of the Yangsan fault in the Gyeongsang Basin in the southeastern part of the Korean Peninsula and caused more than 600 aftershocks within a 1-yr period. To find out the causes and process of the 2016M5.8_GY and its aftershocks, we conducted 3-D underground structural modelling of the Gyeongju area using a combined interpretation of geological, petrophysical and potential field data and hence calculated the density- and magnetic-field distribution along the Yangsan fault. The results of the 3-D modelling indicate that the 2016M5.8_GY occurred in the ductile region of the fault, which is tectonically very unstable, and is therefore highly likely to be significantly affected by any future seismological stress. The crustal structure most affected by the 2016M5.8_GY is the Cretaceous Granite, which has a vertical relief of approximately 12 km with a relatively lower density material than the surrounding country rock and rests on highly ductile material. Therefore, the lower part of the granite is enough unstable that it was probably crushed by the main shock. That is also the reason, why so many aftershocks occurred beneath the granites. [ABSTRACT FROM AUTHOR]