Sichuan-Yunnan Block Research Articles

Geological and geomorphological evidence of holocene activity along the maisu fault in the northern Sichuan–Yunnan block, Tibetan Plateau

The Sichuan–Yunnan block is located in the eastern of Tibetan Plateau and exhibits strong tectonic and earthquake activity. The Maisu fault is an E–W-trending fault within this block. Via interpretations of remote-sensing imagery and field surveys, we identified a earthquake surface rupture zone that has developed along the Maisu fault; we then estimated its Holocene activity. The surface rupture extends westward from the town of Puma, Sichuan Province, to the village of Worilong, Xizang Province, and has a length of approximately 45 km. According to a fault outcrop and carbon-14 dating of a profile near the village of Yongqu, Xizang Province, the most recent earthquake along this rupture may have occurred after 1850 ± 30 BP. The Maisu fault extends eastward and may intersect the Garzê–Yushu fault. Accordingly, as a secondary fault, the Maisu fault likely accommodates the partitioned horizontal slip deformation of the Garzê–Yushu fault.

Potential seismicity by impoundment of the Baihetan Reservoir, Xiaojiang Fault Zone, Southwest China

Baihetan reservoir, the second-largest hydropower station in China, is located at the east boundary of the Sichuan-Yunnan block, one of the most seismic active zones in continental China. Many studies have shown that the potential risk of reservoir-induced seismicity will ramp up when the crustal stress field is remarkably perturbed following its impoundment. In this work, based on the fully-coupled poroelastic theory, we built models with different parameters to quantitatively analyze the displacement and stress field changes caused by the Baihetan reservoir impoundment. The preliminary results show that the maximum subsidence reaches about 0.3 m, and the stress change is about 0.4 MPa at 5 km after five years of impoundment. Most strikingly, we find a significant increase of Coulomb Failure Stress Change (ΔCFS) on the fault planes at the vicinities of the surrounding Xiaojiang Fault, Daliangshan Fault, and Lianfeng Fault, suggesting high reservoir induced seismicity risks. However, the occurrence of induced/triggered earthquakes is not only related to the regional tectonic loading, but also the rock strength. Moreover, the reservoir water level rise rate will lead to different spatial–temporal patterns of the induced micro-seismicity, however, this difference will gradually disappear as the reservoir continues to operate, and the large earthquake is mainly affected by geological tectonics.

Is clustered seismicity an indicator of regional stress? Insights from earthquake sequences in Yongning-Luguhu faulted basin, Southwest China

Using hypocenter relocation, moment tensor inversion, stress field inversion, and fault slip tendency analysis, this study systematically investigated three M5.5–5.8 earthquake sequences that occurred after 2000 in the Yongning-Luguhu faulted basin in the middle of the Lijiang-Xiaojinhe fault zone within the Sichuan-Yunnan block, Southwest China. Our results show that since the 2008 Wenchuan Earthquake, the tectonic stress pattern in this area may have changed and that b-values estimated for the earthquake sequences show evidence of an increasing trend in stress in the study area. Seismicity in the small-scale faulted basin adjacent to the large-scale fault zone is a possible indicator of regional stress. We also note that the aftershocks of the M5.7 earthquake sequence in 2012 and the M5.5 earthquake sequence in 2022 show relatively clear fluid diffusion-triggering characteristics. Overpressure of deep fluids is still the main factor driving seismic activity in the region, and we propose that the background tectonic stresses have not yet reached critical levels.

Coseismic Displacement and Slip Distribution of the 21 May 2021 Mw 6.1 Earthquake in Yangbi, China Derived From InSAR Observations

On 21 May 2021, a Mw 6.1 earthquake struck Yangbi County, Yunnan Province, China. In this study, InSAR data from Sentinel-1 SAR images were processed to image the coseismic deformation fields of the Yangbi earthquake. Then, the optimal slip model was obtained by applying the particle swarm optimization method. The interferometry results revealed that the earthquake triggered obvious surface deformation near the epicenter, while the earthquake did not produce an obvious surface rupture zone from field investigation. The optimal slip model suggests that the strike of the seismogenic fault responsible for this event is 139°, the dip angle is 81°, and the average rake angle is -170°. Additionally, the slip was concentrated mainly at depths of 2–8 km, the maximum dip-slip amount was 0.5 m, and the cumulative seismic moment reached 1.43 × 1018 N⋅m, equivalent to a Mw 6.1 earthquake. The geodetic and geophysical inversion results demonstrate that the Yangbi earthquake was dominated by a steeply dipping dextral strike-slip rupture. The rupture fault generally strikes NNW-SSE, which is consistent with that of the Weixi-Qiaohou fault, and may be a relatively new fault formed by an E-W-oriented extension of the western boundary of the Sichuan-Yunnan block. Finally, based on the InSAR results in combination with the spatial distribution characteristics of ground fissures and the strong historical earthquakes, we analyzed the tectonic background preceding the Yangbi earthquake and analyzed the relationship between the Yangbi earthquake and strong historical earthquakes in the region, thereby providing empirical evidence for analyzing seismic risk and fault rupture parameters, interpreting seismic deformation characteristics, and better understanding the seismogenic background of the western boundary of the Sichuan-Yunnan block.

Variation of Thermal Infrared Brightness Temperature Anomalies in the Madoi Earthquake and Associated Earthquakes in the Qinghai-Tibetan Plateau (China)

We processed MODIS data received from ground receiving stations into the spatial range of the Qinghai-Tibetan Plateau (China) and the eastern margin of the plateau, and then 283 K was set as the threshold value to remove the area covered by clouds. The monthly background field was calculated based on 17 years’ data, then we obtained the spatial Brightness Temperature anomaly of the current month by deducting the background field. Furthermore, the Brightness Temperature anomaly curves for secondary tectonic blocks in the plateau were calculated. The data indicated that since June 2020, the Brightness Temperature radiation within the Qinghai-Tibetan Plateau began to increase abnormally, starting from the western part of the study area and expanding eastward to cover the entire plateau. In January 2021, such an anomaly was seen again, extending to the Sichuan-Yunnan Block in the easternmost part of the study area in april. With the ongoing anomaly, a series of moderate and strong earthquakes occurred in the Qinghai-Tibetan Plateau, and finally, on 22 May 2021, the M7.4 earthquake struck the Madoi County. Moreover, according to the internal Brightness Temperature time series curves of the different secondary tectonic blocks, the Brightness Temperature has increased simultaneously since the beginning of 2020. A twofold standard deviation was found in the middle-east segment of the Bayanhar Block and the Qiangtang Block in October 2020, and an almost twofold standard deviation was found in March, while a twofold standard deviation was found in the Sichuan-Yunnan Block in april 2021. The occurrence of earthquakes in the plateau before the Madoi earthquake coincided with an upward trend of the time series curve. The spatial anomaly of Brightness Temperature over the Qinghai-Tibetan Plateau disappeared and the Brightness Temperature time series curve dropped drastically after the Madoi earthquake. The development of spatial anomaly of Brightness Temperature and the time series curve both coincide with the occurrence of earthquakes and are consistent with the generation of tectonic stress in the Qinghai-Tibetan Plateau. Our study showed that thermal infrared Brightness Temperature radiation reasonably reflects regional stress development and enables the detection of anomalies prior to moderate and strong earthquakes.

Hydrochemical Characteristics of Earthquake-Related Thermal Springs along the Weixi–Qiaohou Fault, Southeast Tibet Plateau

The Weixi–Qiaohou Fault (WQF) is considered an important zone of the western boundary of the Sichuan–Yunnan block, and its seismicity has attracted much attention after a series of moderate–strong earthquakes, especially the Yangbi Ms6.4 earthquake that occurred on 21 May 2021. In the present research, we investigate major and trace elements, as well as hydrogen and oxygen isotopes, of 10 hot springs sites located along the WQF, which are recharged by infiltrated precipitation from 1.9 to 3.1 km. The hydrochemical types of most analyzed geothermal waters are HCO3SO4-Na, SO4Cl-NaCa, and SO4-Ca, proving that they are composed of immature water and thus are characterized by weak water–rock reactions. The heat storage temperature range was from 44.1 °C to 101.1 °C; the circulation depth was estimated to range between 1.4 and 4.3 km. The results of annual data analysis showed that Na+, Cl−, and SO42− in hot springs decreased by 11.20% to 23.80% north of the Yangbi Ms5.1 earthquake, which occurred on 27 March 2017, but increased by 5.0% to 28.45% to the south; this might be correlated with the difference in seismicity within the fault zone. The results of continuous measurements of NJ (H1) and EYXX (H2) showed irregular variation anomalies 20 days before the Yangbi Ms6.4 earthquake. In addition, Cl− concentration is more sensitive to near-field seismicity with respect to Na+ and SO42−. We finally obtained a conceptual model on the origin of groundwater and the hydrogeochemical cycling process in the WQF. The results suggest that anomalies in the water chemistry of hot spring water can be used as a valid indicator of earthquake precursors.

Advances in the deep tectonics and seismic anisotropy of the Lijiang-Xiaojinhe fault zone in the Sichuan-Yunnan Block, Southwestern China

The Sichuan-Yunnan Block (SYB) is located at the SE margin of the Qinghai-Tibetan Plateau (TP). Under the influence of the southeastward movement of material originated from the TP, intense crustal deformation, frequent seismic activity, and complex geological structures are observed in the SYB. The Lijiang-Xiaojinhe fault (LXF) goes through the central part of the SYB, dividing it into two blocks from north to south, and forming an intersecting fault system with the surrounding faults. This paper firstly introduces the morphology and the nature of the LXF, the distribution of the regional surface displacements and the focal mechanisms, and then analyzes the medium deformation and the effects of faults. Moreover, according to the regional tectonics and geophysical patterns, the paper discusses the characteristics of the north-south blocks of the SYB and the abrupt change of deep structure along the LXF zone. Since seismic anisotropy is an essential property for detecting crustal stress, deep structures and dynamical mechanisms, this paper is dedicated to the advances in seismic anisotropy at different depths and different scales in the study area. There are noteworthy differences in the anisotropic features between the north part and the south part of the SYB, possibly associated with a clear boundary adjacent to the LXF. Such phenomenon suggests some close correlation between anisotropic zoning boundary and the LXF, although this boundary is not consistent with the LXF in strike. The results from the deformation of the crust and the upper mantle elucidate the distribution patterns of the crust-mantle coupling in the north part and the crust-mantle decoupling in the south part, even though this conclusion needs to be further verified by more studies. Presently, the scientific understanding of the deep tectonics and the media deformation around the “generalized” LXF i.e. the LXF with the Jinpingshan fault on its eastern side, is still insufficient, and related equivocal topics deserve more in-depth studies.

Lithospheric flexural isostasy background of the 2017 Ms7.0 Jiuzhaigou earthquake and its implications for material extrusion in the northeastern Bayan Har block

We applied a 2-dimensional, non-spectral technique to investigate the spatial variations of the lithospheric effective elastic thickness (Te) in the eastern Bayan Har block and its adjacent areas. A program was designed to calculate the Moho flexure induced by topography loading, as well as the Bouguer gravity anomalies caused by that Moho flexure. The Te values were determined by minimizing the RMS differences between the observed and calculated Bouguer gravity anomalies. The results suggested that Te varies significantly from eastern Tibet to the Sichuan Basin. The eastern Bayan Har block and northern Sichuan-Yunnan block have low Te values (0 < Te < 20 km), indicating an easily deformable lithosphere. The Sichuan Basin has high Te values (40 km < Te < 100 km) acts as a rigid block that resists the eastward extrusion of plateau materials. The moderate Te values (30 km < Te < 40 km) found under the Longmen Shan fault belt indicate that the lithosphere of the Sichuan Basin plays an important role in supporting the Longmen Shan topography. The extremely low Te (Te < 10 km) observed in the seismogenic zone of the Ms7.0 Jiuzhaigou earthquake indicated that the topography is compensated locally, which is significantly different from the Longmen Shan topography. A banded region of relatively low Te values (< 40 km) stretching from the northeastern Bayan Har block to the southwestern Ordos block sketches out an escape channel for plateau materials, and challenges the existing of crustal flow on the north side of the Sichuan Basin.

Mantle-Derived Helium Distribution and Tectonic Implications in the Sichuan-Yunnan Block, China.

The geochemical characteristics of mantle degassing observed on the surface of the earth can indicate the origin and migration path of mantle fluids. Compared with the plate boundary tectonic environment, the intraplate tectonic environment does not have a large number of active volcanoes and active faults, and the observation of mantle volatiles in hot spring gas is relatively limited. We selected the Sichuan–Yunnan block to discuss mantle degassing based on the carbon and noble gas isotopes of the spring gases and previous studies on the fault slip rate and geophysical research. A total of five hot spring gas samples (including two free gases and three dissolved gases) were collected from the Sichuan–Yunnan block. Chemical and isotopic compositions were analyzed in N2-dominant hot spring gases. The 3He/4He ratio (0.068–0.541 Ra) indicates the occurrence of mantle-derived helium throughout the Sichuan–Yunnan block, which has been diluted by a crustal radiogenic 4He component. The occurrence of mantle-derived helium in the study areas ranges from 0.74 to 5.67%. The lower proportion of mantle-derived helium in YNWQ and HGWQ than that in other spring gases near the Jinghe-Qinghe fault may be caused by the smaller scale of fault around YNWQ and HGWQ than the Jinghe-Qinghe fault. The correlation between 4He, 20Ne, and N2 concentrations implies a common trapping mechanism for 4He, 20Ne, and N2 in hot spring gases. The 40Ar/36Ar ratios and N2/Ar ratios indicate that N2 and Ar are mostly meteoric, and YNWQ and HGWQ have more crustal-derived Ar contribution (40.56 and 51.49%, respectively). The δ13C(CO2)o values calculated by Rayleigh fractionation and CO2 concentration suggest that CO2 has inorganic and organic origins. The plot of Rc/Ra versus δ13C(CO2) indicates that the spring gas CO2 origin in the Sichuan–Yunnan block is mainly derived from mixing of limestone and organic sediments with minor mantle CO2. The δ13C(CH4) versus CH4/3He values indicate that the origin of methane is thermogenic and microbial oxidation. The low mantle-derived helium distribution pattern is most likely controlled by the weak fault activity rate, the small fault scale, and not obvious magmatic activity in the Sichuan–Yunnan block.

Construction of 3D geological model and multi-scale numerical analysis of in-situ stress field of long tunnel in fault development area

Affected by faults of different scales, there is a big difference between the local stress and regional stress in the crust. The Xianglushan tunnel of Central Yunnan Water Diversion Project is located in the Sichuan-Yunnan block with violent tectonic activity, and the tunnel passes through complex strata and fault development areas. Due to the limitations of testing methods, the measured stress data in Xianglushan tunnel project are mainly distributed in the relatively intact rock mass, so it is difficult to reflect the stress characteristics in the fault-affected area. Therefore, based on the tectonic geological data of Xianglushan tunnel, this article explores the construction method of multi-scale 3D geological model of fault development area. The corresponding information processing mechanism is established to realize the interactive interpretation, supplement and correction of fault data, and then gradually approach the actual situation of the geological body in the modeling process. According to the modeling method, a generalized large-scale model and a refined sub-model were established separately for the whole Xianglushan tunnel project area and the Longpan-Qiaohou fault affected area. At the same time, combined with the stress test data of Xianglushan tunnel, the first-stage stress field inversion calculation of large scale model is carried out based on multiple regression analysis method. The macroscopic stress field distribution characteristics of the strata and fault zones along the tunnel are preliminarily explored. From the first-stage inversion result, the displacement boundary conditions of the sub-model can be obtained by interpolation in the displacement field, which is applied to sub-model for the secondary elastic-plastic iterative calculation. Finally, the construction of the refined sub-model and the secondary elastic-plastic iterative calculation not only optimize the detail features of the initial stress field obtained by the first-stage inversion analysis, but also reveal the disturbance law of the complex strata and the Longpan-Jiaohou fault on the initial stress state, which has a high guiding significance for the construction and support of tunnel engineering.

Stress evolution before the 2014 Ludian M6.5 earthquake: Insights from groundwater and geodetic measurement

Because the meta-instability stage before an earthquake can be found in rock-deformation experiments in the laboratory, great expectations are placed on the detection of possible precursors in earthquake forecasting and prediction. However, varietal observations are difficult to compare, and it is very difficult to distinguish to which stage the observed value belongs. The top priority is to study the relationship between the observations and the meta-instability stage in a uniform system of measurement. The 2014 Ludian M6.5 earthquake, which occurred in the southeastern region of the Qinghai-Tibet Plateau, was surrounded by 122 observatories for groundwater, electromagnetism, and surface deformation within a 300-km radius. Many data were observed before and after the Ludian earthquake, which provided a great case to study the relationship between the observations, stress evolutions, and meta-instability stage before the earthquake. We determined the detailed spatiotemporal variations in groundwater temperature, water level, chemical ion measurement, and crust deformation by strain meter and tilt meter. Following the previous study on the relationship between the observations and stress field, we simply classified all these data from 78 observatories into three groups: stress increased, stress disturbed, and uninfluenced. The results show that the stress field displayed a four-quadrant pattern around the seismogenic structure of the 2014 Ludian earthquake, which is called the Baogunao-Xiaohe Fault and is known as a buried branch fault of the Daliangshan fault. Along with the orientation of fault-block movement, the stress decreased at the back while increasing at the front. In addition, some stress perturbations were found along nearby large active faults, such as the Xianshuihe-Anninghe-Zemuhe-Xiaojiang fault zone, which is a deep-cutting active fault belt confining the Sichuan-Yunnan block. Based on the spatiotemporal evolution of the stress field and distribution of aftershocks, we simplified the seismogenic structure to a set of conjugated strike–slip faults. The nonlinear friction finite-element method was applied to simulate the stress variations before and after the extensive disengagement and slip of nodes on the fault plane. The computed stress field is consistent with the observed phenomenon and the meta-instability stage could be distinguished out from our simulation. Our research can be used to help detach earthquake precursors and determine whether the earthquake system is in the meta-instability stage.

A case study of 10 years groundwater radon monitoring along the eastern margin of the Tibetan Plateau and in its adjacent regions: Implications for earthquake surveillance

The eastern margin of the Tibetan Plateau has been characterized by frequent disastrous earthquakes, which has induced changes in groundwater such as variations in isotopes, ions, and dissolved gases. In this study, the fluctuation of groundwater radon recorded at six different monitoring stations from 2009 to 2018 was analyzed. With the statistical approach (the residual signal processing technique and the Hurst exponent estimates), groundwater radon variations caused by seismic events were unraveled, reflecting regional stress status changes. The results of this study indicated that (1) groundwater radon exhibits distinct differences between the eastern margin of the Sichuan-Yunnan Block and the South China Block; (2) the precursory anomalies of groundwater radon are principally categorized into three types: positive, negative, and dual behaviors; (3) the relatively low level of the groundwater radon results from the underlying rock masses resisting the compressional stress and the locking on the Xianshuihe fault, the Anninghe fault, and the Zemuhe fault. Additionally, the seismic response zone of each monitoring station was delineated using historical seismic precursors.

Recent Crustal Deformation Based on Interpolation of GNSS Velocity in Continental China

We used the interpolation method of two-dimensional vector velocity field data based on Green’s function to conduct coupled interpolation with a Poisson’s ratio of 0.5 for 1966 horizontal velocity field data from 1999 to 2017 and obtained the uniform velocity field and strain rate field with a grid of 1°. The main results are as follows: the eastern Himalayan structure as the center, the eastern Lhasa block, the eastern Qiangtang block, the Sichuan-Yunnan block, and the Burma block form a strong deformation rate zone of continuous deformation in the fan-shaped region, which has been a strong deformation rate zone for earthquakes of magnitude 7 or higher in continental China since 1963. Besides, the eastward movement of crustal material in the Tibetan Plateau is blocked by the stable South China block. Therefore, the direction of crustal material movement is deflected, which gradually forms a clockwise rotation motion system centered on the eastern Himalayan structure. Finally, our research shows that the influencing factors of strong earthquakes include velocity change, non-uniform strain distribution, accumulation of larger strain, and the difference of the second strain rate invariant. Strong earthquakes are closely related to the difference in energy accumulation in space.

Evaluation of crustal inhomogeneity parameters in the southern Longmenshan fault zone and adjacent regions

Broadband teleseismic waveform data from 13 earthquakes recorded by 70 digital seismic stations were selected to evaluate the inhomogeneity parameters of the crustal medium in the southern Longmenshan fault zone and its adjacent regions using the teleseismic fluctuation wavefield method. Results show that a strong inhomogeneity exists beneath the study region, which can be divided into three blocks according to its structure and tectonic deformation features. These are known as the Sichuan-Qinghai Block, the Sichuan-Yunnan Block, and the Mid-Sichuan Block. The velocity fluctuation ratios of the three blocks are approximately 5.1%, 3.6%, and 5.1% in the upper crust and 5.1%, 3.8%, and 4.9% in the lower crust. The inhomogeneity correlation lengths of the three blocks are about 10.1 km, 14.0 km, and 10.7 km in the upper crust and 11.8 km, 17.0 km, and 11.8 km in the lower crust. The differences in the crustal medium inhomogeneity beneath the Sichuan-Yunnan Block, the Sichuan-Qinghai Block, and the Mid-Sichuan Block may be related to intensive tectonic movement and material flow in the crust and upper mantle.

The 2014 Mw 6.1 Ludian Earthquake: The Application of RADARSAT-2 SAR Interferometry and GPS for this Conjugated Ruptured Event

Although the Zhaotong–Ludian fault is a seismically active zone located in the boundary between the Sichuan–Yunnan block and the South China block, it has not experienced a large earthquake greater than Mw 7 since at least 1700. On 3 August, 2014, an Mw 6.1 earthquake (the Ludian earthquake) ruptured the Zhaotong active belt in Ludian County, Yunnan province, China. This earthquake was the largest earthquake recorded in the region since 2000, and it provides us with a unique opportunity to study the active tectonics in the region. The analysis of the aftershocks showed that two conjugate faults could have been involved in the event. We first used Global Positioning System (GPS) data and C-band RADARSAT-2 imagery to map the coseismic surface deformation. We then inverted the derived coseismic deformation for the slip distribution based on the constructed conjugate fault model. Finally, the coulomb failure stress due to the Ludian earthquake was estimated to investigate the potential seismic hazards in this region. Our investigations showed that the Ludian earthquake was mainly a bilateral rupture event. The major slip of the main shock was located at depths of 0–5 km, which is close but does not superpose with the aftershocks that are mostly located at depths of 5–20 km. Interestingly, the seismic moment released by the aftershocks (6.9 × 1018 N∙m) was greater than that of the main shock (2.6 × 1018 N∙m). This evidence suggests that the accumulated elastic strain at depths of 0–20 km could have been fully released by the Ludian earthquake and its subsequent aftershocks. Furthermore, our analysis of the coulomb failure stress changes due to the main shock showed that the aftershocks could be the result of dynamic triggering rather than static triggering.

Inverting Rayleigh surface wave velocities for crustal thickness in eastern Tibet and the western Yangtze craton based on deep learning neural networks

Abstract. Crustal thickness is an important factor affecting lithospheric structure and deep geodynamics. In this paper, a deep learning neural network based on a stacked sparse auto-encoder is proposed for the inversion of crustal thickness in eastern Tibet and the western Yangtze craton. First, with the phase velocity of the Rayleigh surface wave as input and the theoretical crustal thickness as output, 12 deep-sSAE neural networks are constructed, which are trained by 380 000 and tested by 120 000 theoretical models. We then invert the observed phase velocities through these 12 neural networks. According to the test error and misfit of other crustal thickness models, the optimal crustal thickness model is selected as the crustal thickness of the study area. Compared with other ways to detect crustal thickness such as seismic wave reflection and receiver function, we adopt a new way for inversion of earth model parameters, and realize that a deep learning neural network based on data driven with the highly non-linear mapping ability can be widely used by geophysicists, and our result has good agreement with high-resolution crustal thickness models. Compared with other methods, our experimental results based on a deep learning neural network and a new Rayleigh wave phase velocity model reveal some details: there is a northward-dipping Moho gradient zone in the Qiangtang block and a relatively shallow north-west–south-east oriented crust at the Songpan–Ganzi block. Crustal thickness around Xi'an and the Ordos basin is shallow, about 35 km. The change in crustal thickness in the Sichuan–Yunnan block is sharp, where crustal thickness is 60 km north-west and 35 km south-east. We conclude that the deep learning neural network is a promising, efficient, and believable geophysical inversion tool.

3D Viscoelastic Finite Element Study of Post-Seismic Influence of the Wenchuan Earthquake in the Sichuan–Yunnan Region

We created a three-dimensional (3D) viscoelastic finite element model of the crust to study post-seismic influences of the Wenchuan earthquake, considering both the horizontal fragmentation and vertical strata of the crustal material in the Sichuan–Yunnan region, and the asymmetrical static displacement of hanging wall and footwall on the fault plane in relation to the earthquake. Results indicate that (1) a far-field accumulated horizontal deformation of approximately 0–20 mm occurred inside the Sichuan–Yunnan block, 10 years after Wenchuan earthquake, due to the medium’s viscoelastic relaxation effect, and the deformation was greater in the north than in the south. (2) The far-field accumulated vertical deformation of the medium’s viscoelastic relaxation in the Sichuan–Yunnan block was small (0–4 mm) during the same period. (3) A belt of high horizontal and vertical deformation gradients are evident along the Xianshuihe and East Kunlun faults. Coincidentally, several significant earthquakes (MS 5.4 Daofu 2010, MS 5.7 Luhuo 2011, MS 6.3 Kangding 2014 and MS 7.0 Jiuzhaigou 2017) were located on the belt described above. (4) The seismic activity of the faults of increasing Coulomb failure stress is stronger than that of other faults, generally.

Contemporary crustal tectonic movement in the southern Sichuan-Yunnan block based on dense GPS observation data

We analyzed 360 permanent and campaign GPS data from 1999 to 2017 in the southern Sichuan-Yunan block, and obtained crustal horizontal deformation in this region. Then, we derived the strain rate using a multi-scale spherical wavelet method. Results reveal a complex pattern of tectonic movement in the southern Sichuan-Yunnan block. Compared to the stable Eurasian plate, the maximum rate of the horizontal deformation in the southern Sichuan-Yunnan block is approximately 22 mm/a. The Xiaojiang fault shows a significantly lower deformation—a left-lateral strike-slip movement of 9.5 mm/a. The Honghe fault clearly shows a complex segmental deformation from the north to south. The northern Honghe fault shows 4.3 mm/a right strike-slip with 6.7 mm/a extension; the southern Honghe fault shows 1.9 mm/a right strike-slip with 1.9 mm/a extension; the junction zone in the Honghe and Lijiang–Xiaojinhe faults shows an obvious clockwise-rotation deformation. The strain calculation results reveal that the maximum shear-strain rate in this region reaches 70 nstrain/a, concentrated around the Xiaojiang fault and at the junction of the Honghe and Lijiang–Xiaojinhe faults. We note that most of the earthquakes with magnitudes of 4 and above that occurred in this region were within the high shear strain-rate zones and the strain rate gradient boundary zone, which indicates that the magnitude of strain accumulation is closely related to the seismic activities. Comparison of the fast shear-wave polarization direction of the upper-crust with the upper-mantle anisotropy and the direction of the surface principal compressive strain rate obtained from the inversion of the GPS data reveals that the direction of the surface principal compressive strain is basically consistent with the fast shear-wave polarization direction of the upper crust anisotropy, but different from the polarization direction of the upper mantle. Our results support the hypothesis that the principal elements of the deformation mechanism in the southern Sichuan-Yunnan block are decoupling between the upper and lower crust and ductile flow in the lower crust.

Geological and geomorphic evidence for activity of the Mengzi fault along the southeastern margin of the Tibetan Plateau

Few studies have been conducted on the activity of the Mengzi fault system, which is located to the southeast of the Sichuan-Yunnan block. The Mengzi fault is distributed along the eastern margin of the Mengzi basin. However, little is known about the location, length, and activity of the fault or the occurrence of paleo-earthquakes during the late Quaternary. This paper studies the geometric distribution and activity of the fault via remote sensing imagery interpretation, fault outcrops in the area, faulted landforms and chronological analyses. The results show that the Mengzi fault extends from Nandong (Kaiyuan basin) in the north through the eastern margin of the Mengzi basin to Pingbian in the southeast, eventually intersecting with the Honghe fault. The Mengzi fault is approximately 250 km long and is characterized as a reverse strike-slip fault. We survey and calculate the offset gullies and terraces dislocated by the fault; combined with sample dating, we calculate the horizontal slip rate of the fault since 30 ka BP to be 0.91–1.56 mm/a. Three paleo-events since 30–40 ka BP are observed with a 10-ka major earthquake recurrence period. We analyze the relationships among the Mengzi fault, southern Xiaojiang fault, and Qujiang-Jianshui fault and note a slip rate loss of ≥8 mm/a from the southern Xiaojiang fault that was absorbed by reverse dextral strike-slip faulting and lateral shortening of the Qujiang-Jianshui fault and the Mengzi fault. The Mengzi fault plays an important role in the adjustment of the large slip rate reduction along the southern Xiaojiang fault. Moreover, the Mengzi fault has a similar tectonic position to the seismogenic faults of the 2008 Wenchuan earthquake, 2013 Lushan earthquake and 2014 Ludian earthquake, and the Mengzi fault and southern Xiaojiang fault both accumulate the energy generated by the southeastward movement of the Sichuan-Yunnan block. Studies on the Mengzi fault are helpful for renewing our understanding of the tectonic patterns of the southeastern Sichuan-Yunnan block and delimiting the boundary of the southeastern block margin. Furthermore, because the cities of Mengzi and Kaiyuan are located along the Mengzi fault, this study has practical significance for earthquake disaster mitigation.

Correlations between the radon concentrations in soil gas and the activity of the Anninghe and the Zemuhe faults in Sichuan, southwestern of China

The Anninghe fault (ANHF) and the Zemuhe fault (ZMHF) with left-lateral strike-slip, located along the eastern boundary of the Sichuan-Yunnan block (southwestern of China), are some of the most active faults. These faults mainly control the seismicity of southwestern area of China. Measurement of soil gas radon (Rn) emitted from fault along the ANHF and the ZMHF has been carried out for the research of tectonic activity. We obtained the Rn concentrations at 394 sampling points along 15 profiles across the ANHF and the ZMHF in 2016. The measurement results show that the values of Rn in the ANHF are significantly higher than that in the ZMHF. The relative coefficient KQ of Rn activity attained in profiles of the ANHF ranges from 3.3 to 9.1, which are obviously higher than that of 2.1–2.5 in profiles of the ZMHF. The radon flow brings up the deeper and radon-richer gas upward through the high-level cracked strata caused by the tectonic activity accounts for the anomalously high values attained. The spatial variation of Rn in the concentration profile and the relative coefficient KQ calculated indicate that the tectonic activity of the south segment of the ANHF is significantly higher than that of the north segment of the ZMHF.