Tectonophysics最新文献

On March 10, 2011, an Ms. 5.8 earthquake struck Yingjiang City, western Yunnan, China, causing destructive damage. Due to the very sparse distribution of seismic stations on the southwestern border of China, its seismogenic structure and mechanism remain controversial. In this study, with the aid of machine-learning-based detection and location workflow and template matching technique, we detect 10,356 events ranging from December 1, 2010, to April 30, 2011. The high-precision earthquake catalog shows that the foreshocks initiated in the extensional stepover connecting the northeast and middle segments of the Dayingjiang fault and then bilaterally extended northeast and southwest, with migration fronts that can be simulated by fluid diffusion model with diffusivities of 0.8 m²/s and 0.19 m²/s, respectively. The mainshock occurred at the southwest end of the foreshock sequence and then probably activated the northwest-trending blind fault. In addition, we determine the full moment tensor solutions for the mainshock, six large foreshocks, and one aftershock, with magnitudes ranging from 3.03 to 5.80, in which the mainshock was characterized by an obvious negative isotropic (ISO) component. The static Coulomb failure stress change caused by five Mw ≥ 4.0 foreshocks on the mainshock fault plane is ∼24 kPa, reaching the typical static triggering threshold. Therefore, we suggest that both the fluid diffusion and stress perturbation contribute to triggering the mainshock. This study advances our understanding of the spatiotemporal evolution, seismogenic mechanism, and hazard implication for the Yingjiang Ms. 5.8 earthquake and provides additional evidence of natural fluid-triggered seismicity in western Yunnan.

Thirty-nine 1D shear wave velocity profiles, obtained by jointly inverting receiver functions and Rayleigh wave group velocities, are used to investigate the crustal structure of the Bushveld Complex in northern South Africa. Data from teleseismic earthquakes recorded on broadband seismic stations between 1997 and 1999 and 2015–2020 were used to compute P-wave receiver functions. Rayleigh wave group velocities between 5 and 30 s period were obtained from an ambient noise tomography and combined with group velocities between 30 and 60 s period from a published continental-scale surface wave tomography model. Moho depths of 45–47.5 km are found under the center of the complex compared to 40 km thick crust, on average, surrounding the complex, indicating ∼5–7 of crustal thickening. The bottom ∼10 km or more of the lower crust across much of the Bushveld Complex has a Vs ≥ 4.0 km/s, consistent with a mafic composition, whereas in most areas around the margins of the complex the thickness of the mafic lower crust is much less than 10 km. In the upper crust higher velocity structure (Vs > 3.6 km/s) above 15 km depth underlain by lower velocity structure is seen in many locations, suggesting the presence of mafic/ultramafic layering. These results favor the continuous-sheet model for the structure of the Bushveld Complex because the ensemble of 1D models is characterized by three diagnostic features consistent with that model: (1) thicker crust under the center of the complex than away from the complex; (2) a greater thickness of high-velocity (i.e., mafic) layering in the lower crust under the complex compared to away from the complex; (3) high-velocity (i.e., mafic/ultramafic) layering in the upper crust beneath much of the complex. The lack of upper crustal mafic/ultramafic layering beneath some parts of the complex is consistent with the post-emplacement tectonic and magmatic history of the complex.

Identifying accurate seismogenic faults is critical for studying the mechanisms of induced earthquakes. On February 24th and 25th, 2019, three moderate earthquakes with magnitudes of M_S 4.7, M_S 4.3, and M_S 4.9 occurred successively in the shale gas development area of Weiyuan, China. We utilized high-resolution three-dimensional (3D) seismic data to identify two pre-existing faults (F1 and F2) that were responsible for the three moderate earthquakes. InSAR data were used to validate the rationality of the two seismogenic faults. Furthermore, we analyzed the impact of fluid diffusion on fault F1 near the fracturing well and calculated the Coulomb failure stress (CFS) generated on fault F2 by the M_S 4.7 and M_S 4.3 earthquakes to analyze the interactions between these events. The results indicated that fluid diffusion caused by hydrofracturing induced the M_S 4.3 and M_S 4.7 earthquakes on F1. The static Coulomb stress changes from these two earthquakes subsequently triggered the larger M_S 4.9 earthquake on F2. This study provides a case of a cascading process in which induced earthquake events triggered a more distant and higher-magnitude earthquake. This triggering scenario reminds us that earthquake-to-earthquake interactions may be more hazardous than a “typical” inducing mechanism and challenges current risk management practices.

Epidote is a common hydrous mineral present in subduction zones subject to greenschist metamorphic conditions – and potentially an important control on the fault stability-instability transition observed under greenschist facies. We explore controls on this transition through shear experiments on simulated epidote gouge at temperatures of 100–500 °C, effective normal stresses of 100–300 MPa and pore fluid pressures of 30–75 MPa. We use rate-and-state friction to define these controls of temperature, effective stress and pore fluid pressure on gouge stability. Experimental results indicate that the epidote gouge is frictionally strong (μ ∼ 0.73) and the frictional strength is insensitive to variations in temperature or pressure. With increasing temperature, the epidote gouge exhibits a first transition from velocity-strengthening to velocity-weakening at sub-greenschist conditions (T < 100 °C) before transitioning to velocity-strengthening under greenschist metamorphic conditions (T > 300 °C). Elevating the pore fluid pressure or decreasing the effective stress promotes unstable sliding. The transition in gouge rheology at varied temperatures and pressures is explained by the competition between granular flow-induced gouge dilation and pressure solution-induced gouge compaction. Our results demonstrate that the rate-and-state frictional stability of epidote gouges support the potential for a fault stability-instability-stability transition for subduction under greenschist metamorphic conditions.

Resulting from the convergence of the Yangtze and North China Cratons, the Qinling-Dabie orogenic zone (QD) represents an important element in the central China orogenic system. To fully comprehend the craton evolution and lower crustal flow from the Tibetan Plateau, it is important to understand the crust and mantle structure of the QD. We reconstructed the three-dimensional lithospheric structure beneath the QD with high resolution using the joint inversion of receiver functions and ambient noise. Observations reveal that a high-velocity anomaly in the middle to lower crust beneath the western Qinling (WQL) orogen obstructs the eastward extension of a crustal low-velocity anomaly originating from the Tibetan Plateau. This finding provides unambiguous evidence that the WQL orogen is not crossed by eastward lower crustal flow from the Tibetan Plateau. The lithospheric mantle beneath the Weihe Rift and East Qinling orogen exhibits low-velocity characteristics, indicating that eastward asthenospheric flow from the Tibetan Plateau has caused substantial thermal-chemical erosion in the uppermost mantle beneath these regions. The results additionally indicate that the uppermost mantle high-velocity anomalies beneath the Dabie orogen is confined in a limited area and extends only to a depth of 70 km. We propose that during the Triassic, deeply subducted continental lithosphere returned into the uppermost mantle, forming the high-velocity anomalies beneath the Dabie orogen.

GNSS (Global Navigation Satellite System) data in northern Sulawesi and western Halmahera reveals a pattern of coseismic displacement that was caused by the 7 July 2019 (Mw 6.9) and 14 November 2019 (Mw 7.1) Molucca Sea earthquakes. The coseismic slip of these earthquakes are obtained via inversion on rectangular fault planes of surface GNSS coseismic deformation offsets. The 7 July 2019 earthquake ruptured on an east-dipping fault with a maximum slip of ∼35 cm located at ∼4 km depth and ∼ 100 km north-west of the epicenter. The 14 November 2019 earthquake also ruptured on an east-dipping fault, which has a maximum slip of ∼64 cm located at ∼22 km depth and ∼ 20 km south-west of the epicenter. The coseismic slip distribution of the 14 November earthquake is spatially aligned to that of an earthquake of similar magnitude that took place on 15 November 2014 in the same region. This observation points to the possibility of synchronization, thus raising the prospect of a future earthquake exceeding magnitude 7. If the stress was completely released during the 2014 event, it would have been necessary to reload that portion of the fault at a rate significantly larger than the observed convergence rate of ∼5.8 cm/yr. This suggests that partial ruptures are likely controlling the recurrence time of large earthquakes in the region.

We performed an analysis of present-day stress profiles for four wells penetrating the Lower Palaeozoic shale sequence of the Baltic Basin (northern Poland). Breakouts, hydraulic fracturing and leak-off tests were used to calibrate stress models based on anisotropic elastic shale properties. Initial stress models, balancing the lengths of the modelled and observed breakouts, indicated a degradation of the mechanical properties reconstructed from the density and dipole acoustic tool logs. Taking this adverse effect into account, final stress models were calculated which showed a stratification of the stress regime consistent with the lithostratigraphic shale units. A clear dominance of the horizontal stress-generating gravity factor over the horizontal tectonic strain was demonstrated. The obtained values of tectonic strain in the shale sequence compared to the previously determined strain in the crystalline basement of the same study area suggest a significant role of viscoelastic relaxation of the shale sequence.

The Haiyuan Arcuate Tectonic Belt (HATB) in the northeastern Tibetan Plateau features the interactions of three intersecting blocks: the eastern Qilian Shan, the Alxa Block, and the Ordos Block. While the HATB has displayed active responses to the ongoing collision between the Indian and Eurasian plates, the exact process behind the formation of this arcuate belt remains unclear. In pursuit of further insights into this topic and a deeper comprehension of the tectonic responses in NE Tibet, we conducted receiver function calculations using teleseismic waveforms recorded by two seismic short-period dense arrays spanning the western and eastern HATB, respectively, extending into the Alxa and Ordos Blocks. The CCP results in the HATB show major structural features that are different from those of adjacent blocks, mainly characterized by structural discontinuities in the crust due to severe deformation, including bending and uplifting in the lower crust. Together with previous geological studies, the bending interfaces in the lower crust of the HATB illuminate the existence of a crustal-scale tectonic accretionary wedge within the HATB, which originated in the Early Paleozoic. Furthermore, a decoupled deformation is seen within the HATB, with the lower crust undergoing shortening and the upper crust experiencing sequential stepwise thrusting towards the north. These scenarios, coupled with the resistance from the rigid Alxa and Ordos Blocks, lead to the conclusion that the arcuate shape of this belt is influenced by the weak crust of the HATB, which primarily orients the northeast, where the weak lithosphere of the Helan tectonic belt is situated between the Alxa and Ordos Blocks. Meanwhile, the progression of a series of thrusting faults in the upper crust within the HATB extends outward, involving adjacent blocks in plateau's growth.

The size-frequency distribution of many geological and geophysical variables, in relation to fractures, faulting and seismicity, is well described by a statistical distribution of the power law type which is characterized by its exponent. For earthquake magnitudes, the exponent is the well-known b-parameter of the Gutenberg-Richter scaling law. In this paper we:

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  provide a strict statistical derivation of the distribution law of earthquake magnitudes,

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  show that the maximum likelihood estimator of the b-parameter is unbiased,

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  demonstrate that the maximum likelihood estimator is invariant to the value chosen as the minimum magnitude threshold in so far as it is larger than the magnitude of completeness of the earthquake catalogue, and

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  provide a new estimator based on the minimization of the Kolmogorov-Smirnov statistic and provide a strategy for detecting and mapping the spatio-temporal variation of the b-parameter in seismic swarms.

The findings are illustrated with simulated data and a case study with real data.

The Three Gorges Reservoir, one of the largest water conservation system in the world, has been of keen interest to scientists globally since its impoundment. After construction of the dam, there has been a significant increase in seismic activity in the head area of the reservoir. It is generally accepted that earthquakes in this region are predominantly caused by the Jiuwanxi and Xiannvshan faults. This study focused on the stress changes occurring in the research area. A three-dimensional finite element model of the reservoir area was constructed using the geological structure and digital ground elevation data of the reservoir area. The fluid-solid coupling theory was applied to calculate the dynamic spatial changes in pore pressure and Coulomb stress in the faults and surrounding rocks during reservoir impoundment. The findings indicated that the added head pore water pressure at the bottom of the reservoir had a maximum impact range of approximately −2800 m on the surrounding rock, whereas the Xiannvshan and Jiuwanxi faults had a maximum diffusion range of approximately −4300 m. Rock permeability also played a significant role in the water storage process. During the 1 56 m water impoundment stage, owing to rapid water storage activity, stress could not be transmitted to both sides in a timely manner, resulting in the formation of an extreme stress change zone at −4000 m inside the fault. This may have been the reason for the frequent earthquakes during this stage. The 17 5 m cycle water storage stage also exhibited a significant degree of seismicity, potentially attributable to the long-term infiltration of reservoir water and accumulation of stress in the previous stage. The stress in the study area at the four stages are in a process of accumulation-release-accumulation-release.