Taiyuan Basin Research Articles

Numerical Analysis of the Dynamic Response of a High-Speed Railway Foundation across a Ground Fissure Zone—A Case Study of the Datong–Xi’an High-Speed Railway Crossing a Ground Fissure in the Taiyuan Basin, China

Numerical Analysis of the Dynamic Response of a High-Speed Railway Foundation across a Ground Fissure Zone—A Case Study of the Datong–Xi’an High-Speed Railway Crossing a Ground Fissure in the Taiyuan Basin, China

Mechanisms of evolution and pollution source identification in groundwater quality of the Fen River Basin driven by precipitation

Groundwater pollution has attracted widespread attention as a threat to human health and aquatic ecosystems. However, the mechanisms of pollutant enrichment and migration are unclear, and the spatiotemporal distributions of human health risks are poorly understood, indicating insufficient groundwater management and monitoring. This study assessed groundwater quality, human health risks, and pollutant sources in the Fen River Basin(FRB). Groundwater quality in the FRB is good, with approximately 87 % of groundwater samples rated as “excellent” or “good” in both the dry and rainy seasons. Significant precipitation elevates groundwater levels, making it more susceptible to human activities during the rainy season, slightly deteriorating water quality. Some sampling points in the southern of Taiyuan Basin are severely contaminated by mine drainage, with water quality index values up to 533.80, over twice the limit. Human health risks are mainly from As, F, NO3−, and Cr. Drinking water is the primary pathway of risk. From 2019 to 2020, the average non-carcinogenic risk of As, F, and NO3− increased by approximately 28 %, 170 % and 8.5 %, respectively. The average carcinogenic risk of As and Cr increased by 28 % and 786 %, the overall trend of human health risks is increasing. Source tracing indicates As and F mainly originate from geological factors, while NO3− and Cr are significantly influenced by human activities. Various natural factors, such as hydrogeochemical conditions and aquifer environments, and processes like evaporation, cation exchange, and nitrification/denitrification, affect pollutant concentrations. A multi-tracer approach, integrating hydrochemical and isotopic tracers, was employed to identify the groundwater pollution in the FRB, and the response of groundwater environment to pollutant enrichment. This study provides a scientific basis for the effective control of groundwater pollution at the watershed scale, which is very important in the Loess Plateau.

Annual variations in characteristics and sources analysis of VOCs during the ozone season in the Taiyuan Basin, China, from 2020 to 2022

Annual variations in characteristics and sources analysis of VOCs during the ozone season in the Taiyuan Basin, China, from 2020 to 2022

Ecological environment quality of the Shanxi section of the Yellow River Basin under different development scenarios

Shanxi Province holds an important strategic position in the overall ecological pattern of the Yellow River Basin. To investigate the changes of the ecological environment in the Shanxi section of the Yellow River Basin from 2000 to 2020, we selected MODIS remote sensing image data to determine the remote sensing ecological index (RSEI) based on the principal component analysis of greenness, humidity, dryness, and heat. Then, we analyzed the spatial and temporal variations of ecological quality in this region to explore the influencing factors. We further used the CA-Markov model to simulate and predict the ecological environment under different development scenarios in the Shanxi section of the Yellow River Basin in 2030. The results showed that RSEI had good applicability in the Shanxi section of the Yellow River Basin which could be used to monitor and evaluate the spatiotemporal variations in its ecological environment. From 2000 to 2020, the Shanxi section of the Yellow River Basin was dominated by low quality habitat areas, in which the ecological environment quality continued to improve from 2000 to 2010 and decreased from 2010 to 2020. The high quality habitat areas mainly located on the mountainous areas with superior natural conditions and rich biodiversity, while the low ecological quality areas were mainly in the Taiyuan Basin and the northern part of the study area, where the mining industry developed well. Climate factors were negatively correlated with ecological environment quality in the northern and central parts of the study area, and positively correlated with that in the mountainous area. Under all three development scenarios, the area of cultivated land, forest, water and construction land increased in 2030 compared to that in 2020. Compared to the natural development scenario and the cultivated land protection scenario, the ecological constraint scenario with RSEI as the limiting factor had the highest area of new forest and the lowest expansion rate of cultivated land and construction land. The results would provide a reference for land space planning and ecological environment protection in the Shanxi section of the Yellow River Basin.

Origin and current activity of the Jiaocheng ground fissure, a 48 km long fissure in northern China

Since the mid to late 20th century, several regions in Northern China have had to contend with ground fissure disasters, with the FenWei basin and North China Plain being clear examples. The Jiaocheng ground fissure, which spans a total of 48 km in the FenWei basin, has caused severe damage to infrastructure along its path. In recent years, the local activity of the Jiaocheng ground fissure has worsened. Field investigations, mapping, trenching, geophysical surveys and other methods have been used to determine the origin mechanism and current activity characteristics of the Jiaocheng fissure. The Jiaocheng ground fissure has a general orientation stretching 48 km in the NE-NEE direction, inclining towards the SE. The Jiaocheng ground fissure involves horizontal tension and vertical dislocation, with segmented activity on the surface, with the most intense activity observed in the northern Qingxu section. In profile, it displays characteristics of synsedimentary faults and primary-secondary fissure combinations. The activity of the Jiaocheng fault underlies the occurrence of the Jiaocheng ground fissure. Over-extraction of groundwater has exacerbated the fissure activity. Recently, the Jiaocheng ground fissure has displayed localized intensification and the emergence of new fissures stretching towards the north. This has been linked to heightened tectonic activity in the northern region of the Taiyuan basin.

The dynamic amplification effect of a site with earth fissures: a case study in the Taiyuan Basin, China

The dynamic amplification effect of a site with earth fissures: a case study in the Taiyuan Basin, China

Land use function change and its driving force of the "production-living-ecological" space in Fenhe River Basin from 1980 to 2020.

It is of importance to analyze land use function change and driving factors of the production-living-ecological space of national territory to realize the coordinated development. Based on land use remote sensing data in 1980, 1990, 2000, 2010, and 2020, we used the methods of land use dynamics, transfer matrix, center of gravity shift and geographic detector to analyze the pattern of production-living-ecological space in Fenhe River Basin and explore the influencing factors. The results showed that the area proportion of production-living-ecological space in the basin was ecological space > production space > living space from 1980 to 2020. The ecological space and agricultural production land showed a decreasing trend, with a decrease of 72441.19 and 105882.96 hm2, respectively. The living space and industrial production land showed an upward trend, with an increase of 119503.02 and 58821.13 hm2, respectively. There was significant difference in the land use function change of production-living-ecological space. Agricultural production land had the largest transferred area, accounting for 47.9% of the total. The largest transfer area of industrial land was agricultural land, which accounted for 61.3% of the total from 2000 to 2010. The occupation of agricultural land by urban living land was mainly distributed in the marginal area of various urban areas of Taiyuan Basin. Among them, the increasing area of urban living land in Taiyuan City showed a trend of gradual southward expansion. The center of gravity migration of urban land for living and industrial production land was the most obvious, and that for living showed the trend of first moving south and then moving north, while industrial production land moved northward significantly. The influence of social-economic factors on the land use change was obviously stronger than that of natural factors, while the interaction between social-economic factors had a stronger explanatory power. The results would provide reference for clarifying the relationship between land function transformation and optimizing land use function of production-living-ecological space.

Quantifying seasonal ground deformation in Taiyuan basin, China, by Sentinel-1 InSAR time series analysis

The presence of seasonal surface deformation in areas that are subsiding/uplifting due to groundwater pumping/recovery can indicate aquifer recharge. Understanding seasonal surface deformation and the existing relationship between triggering factors and the deformation supports effective groundwater management and subsidence mitigation. Here, we investigated seasonal ground displacement in response to the aquifer compaction and expansion due to groundwater extraction and recharge within the Taiyuan basin, Northern China. We obtained Sentinel-1 InSAR displacement time series data from 2017 to 2020 and quantitatively analyzed relationships between triggering factors (e.g., groundwater level and precipitation) and the associated displacements using wavelet-based methods. The results show that the seasonal deformation is concentrated in the irrigated areas within the central basin, with a 1-year periodicity. The annual peak-to-peak amplitude was up to 2 cm, peaking in March with troughs in August. This seasonality present in the displacement time series is found to be ‘in-phase’ with respect to groundwater levels and ‘anti-phase’ in relation to natural precipitation. These results are consistent with the irrigation pumping practices in the basin, with increased groundwater withdrawals to meet irrigation water demand prior to the monsoon precipitation season, resulting in water level decline and land subsidence from mid-March to mid-August. During the non-irrigated season, groundwater extraction decreases, and the aquifer systems are replenished by precipitation, causing water levels to recover and the ground to uplift from late August to mid-March. A geological analysis indicates that the seasonal deformation is controlled by the thickness of Quaternary strata and fault systems, which play a critical role in forming the earth fissures in the basin. The elastic and inelastic deformation was separated; the estimated elastic/inelastic ratio was about 0.6–0.7, indicating that the inelastic component dominates the main aquifer compaction in the central basin. Our analysis is of hydrologic importance which could be used to guide groundwater management and subsidence mitigation, given the inter-basin water diversion projects in this arid basin.

Characteristics and mechanism of Jiaocheng ground fissure in Taiyuan Basin, China

The Fenwei Basin, which has 612 developed ground fissures, is the most concentrated and severe area in China and even the world for ground fissure hazards. The Jiaocheng ground fissure in the Taiyuan Basin is known for being the longest length and causing the most damage and impact in China. To discover the origin of the Jiaocheng ground fissure, surveying, trenching, and geophysical exploration were used to study its geological basis, developmental characteristics, and genetic processes. With a total length of 46 km and an impact bandwidth of 80–120 m, it is one of the longest ground fissures found in the world. The fissure is located on the hanging wall of the Jiaocheng fault, and the NE strike is quite consistent with this fault. Houses, roads, fields, and other structures have all been damaged to varying degrees by the fissure. The most common movements are vertical slip, horizontal tension, and right-lateral slide. According to trenching and shallow seismic profiling, this fissure has synsedimentary features. The Jiaocheng ground fissure was formed due to the interaction of several forces. First, the Jiaocheng fault shifted because of regional extension, creating a rupture system in the surface strata. Pumping activity contributed to the formation of the current ground fissure. This research has significant implications for understanding fissure mechanics and preventing and mitigating ground fissures.

Retro‐Foredeep Basin Evolution in Taiwan: Zircon U‐Pb and Hf Isotope Constraints From the Coastal Range

AbstractThe evolution of the Taiwan orogen, an active arc–continent collision zone between the North Luzon Arc (NLA) and the Eurasian continent, is recorded in arc volcanic and sedimentary rocks from the Taiyuan retro‐foredeep basin of the Coastal Range, eastern Taiwan. Arc volcanism in the northern NLA experienced a magmatic culmination at 9–7 Ma and a cessation at 5.3–4.3 Ma, based on new U‐Pb zircon ages. Young zircon U‐Pb ages (1.74, 1.58, and <1.2 Ma) and low Hf isotope signatures also indicate that three biotite‐bearing lapilli tuff layers deposited in the retro‐foredeep basin are associated with eruptions of the Lutao volcano in the southern NLA. NLA volcanism terminated progressively from ∼5.3 to <1.2 Ma in response to a southward‐propagating collision zone that followed the uplift of the orogenic wedge by <3 myr. The detrital zircon U‐Pb age spectra suggest no significant shift in provenance during the Plio‐Pleistocene period and indicate that the synorogenic sediments in the Taiyuan Basin were mainly derived from the Yuli Belt, a Miocene accretionary complex in the eastern Taiwan orogen. The accumulation rate of synorogenic deposits varies from 0.53 to 5.14 km/myr, with an abrupt increase at ∼2 Ma. This new rate is higher than the previously proposed rates and that of the pro‐foreland basin to the west, suggesting asymmetric and rapid uplift of the Taiwan orogen.

Study on Spatial Optimal Allocation of Land Use in Fenhe River Basin

Land is the basis for the survival and development of human society, and it is also an important factor that restricts the development of social economy. It has become an urgent problem to make scientific and efficient use of limited land resources, formulate a scientific land use optimization scheme and promote the efficient and sustainable development of land resources. In this paper, taking Fenhe River Basin as the research area, combining the current land use data, remote sensing data and social and economic data, ArcGIS is used to evaluate the land suitability, and based on the planning constraint index and land use allocation theory, the land use space is optimally allocated. The main conclusions of the optimization are as follows: (1) Through the analysis of the present situation of land use and the comparative analysis of the quantitative constraint indicators of various regions, the total area of cultivated land has decreased compared with the original situation of land use, but it has actually increased and decreased, mainly in rural residential areas with convenient irrigation conditions and low and flat terrain, which are distributed on both sides of Fenhe River and its tributaries in Fenhe Valley, mainly in areas with serious soil and water loss in Luliang Mountains. It is required to return farmland to forest and grassland, and to restore it to forestry land and grassland, so as to keep good land in Taiyuan Basin and Linfen Basin. (2) The total area of urban and rural construction land has increased compared with the original land use status, mainly in the general suitable areas of agricultural land affected by human beings around the existing towns. Qingxu County has a good development foundation, which can give play to the urban-rural linkage mechanism and establish a satellite city of Taiyuan. (3) The overall layout of ecological land has not changed much, mainly increasing the water source protection in ningwu county, the dam along the river in the scenic spot, and the construction of green belt along the river of Fenhe River and its tributaries.

Health‑risk assessment and distribution characteristics of fluoride in groundwater in six basins of Shanxi Province, Middle China.

High-fluoride (F) pollution is an environmental problem that severely affects the growth of human beings, animals and plants. High-fluoride groundwater severely harms the health of Shanxi residents and leads to frequent endemic diseases. This study aims to analyze the differences and possible health risks of fluoride among six basins in Shanxi Province, North China, using 338 groundwater samples collected from wells, infer the main sources of fluoride in the groundwater, and provide valuable suggestions for fluoride contamination in regional groundwater. The results revealed that F in the Yuncheng basin had the highest health risk. In addition to the Changzhi basin, the groundwater at the sampling points in other basins had adverse effects on human health. The main source of fluoride in groundwater is the dissolution of fluoride-containing minerals, which has little to do with human activities. The groundwater in Shanxi Province tends to be alkaline, and the fluorite saturation index is less than 0 in most circumstances, indicating that fluorite is in an unsaturated state, and fluoride will continue to dissolve into groundwater under suitable conditions. Clustering analysis shows that the high-fluoride areas are mainly distributed in the Yuncheng basin and the southern part of the Xinzhou basin. Fluoride-rich groundwater in a basin often exists only in a certain area, and the distribution of confined water and unconfined water in high-fluoride areas is different. Fluoride contamination in the Changzhi basin is not severe. For the high-fluoride areas in the Datong basin, Xinzhou basin, Taiyuan basin and Linfen basin, utilizing water from other areas or exploiting groundwater from other aquifers to diminish the harm of high-fluoride groundwater intake for a long period is suggested. For the Yuncheng basin, adopting membrane-based processes or variable temperature drop fluoride technology to make groundwater contaminated by excessive fluoride potable is recommended.

Depositional Record and Geochemistry Constraints on the Late Miocene–Quaternary Evolution of the Taiyuan Basin in Shanxi Rift System, China

The formation and evolution of the Shanxi Rift System are topics of interest because of its unique geometry and critical position in the North China Plate. However, it is not easy to delineate the late Cenozoic tectonic processes. This is partly because it is difficult to preserve the normal faults’ movement evidence and related deposits for dating in the outcrop. Besides, the current studies are heavily weighted in the low-temperature thermochronology, whereas it is insensitive to the exhumation after the late Miocene. To decipher the late Cenozoic evolution, we offer a case of detailed depositional recording from the boreholes of the Taiyuan Basin in the Shanxi Rift System. Fluvial, deltaic, and lacustrine environments were identified by the systematic stratigraphic analysis of boreholes. Two mega transgressions generated basin-wide lakes at ca. 5.8–4.4 and ca. 2.2–1.6 Ma. We attribute the lake expansion events to tectonism because climatic variations from reconstruction of the paleoclimate with geochemistry index and previous work exhibited asynchronous processes with the transgression. Thus, the basin experienced two stages of extensive subsidence after the rift occurred. This result is conducive to obtaining a more complete late Cenozoic tectonic evolution of the Shanxi Rift System.

Mapping the 2021 October Flood Event in the Subsiding Taiyuan Basin by Multitemporal SAR Data

A flood event induced by heavy rainfall hit the Taiyuan basin in north China in early October of 2021. In this study, we map the flood event process using the multi-temporal synthetic aperture radar (SAR) images acquired by Sentinel-1. First, we develop a spatiotemporal filter based on low-rank tensor approximation (STF-LRTA) for removing the speckle noise in SAR images. Next, we employ the classic log-ratio change indicator and the minimum error threshold algorithm to characterize the flood using the filtered images. Finally, we relate the flood inundation to the land subsidence in the Taiyuan basin by jointly analyzing the multi-temporal SAR change detection results and interferometric SAR (InSAR) time-series measurements (pre-flood). The validation experiments compare the proposed filter with the Refined-Lee filter, Gamma filter, and an SHPS-based multi-temporal SAR filter. The results demonstrate the effectiveness and advantage of the proposed STF-LRTA method in SAR despeckling and detail preservation, and the applicability to change scenes. The joint analyses reveal that land subsidence might be an important contributor to the flood event, and the flood recession process linearly correlates with time and subsidence magnitude.

Analysis of Aerosol Vertical Distribution Characteristics and Meteorological Elements of Haze Day in the Taiyuan Basin

Analysis of Aerosol Vertical Distribution Characteristics and Meteorological Elements of Haze Day in the Taiyuan Basin

Land subsidence and rebound in the Taiyuan basin, northern China, in the context of inter-basin water transfer and groundwater management

The freshwater scarcity and sustainability of overexploited aquifers have been recognized as a big threat to global water security for human development. Consequently, much research has focused in the past on negative consequences of groundwater abstraction, but somewhat less has been documented about the impacts of adequate management practices to address water shortages. Here, using an integrated analysis of InSAR displacement data, groundwater, and geophysical modeling we show how combined management provisions and inter-basin water transfer project has affected the aquifer system in Taiyuan basin in North China. Following groundwater recovery, the alleviation of land subsidence was found with rates being reduced by up to ~70% in the period 2017–2020 with respect to the period 2007–2010. The increase in pore pressure caused by rising groundwater in Taiyuan city, north of the basin, turned four subsidence centers with rates exceeding 110 mm/yr in the 1980 to uplift centers with rates up to +25 mm/yr between 2017 and 2020. A simple linear elastic model for homogenous subsurface properties can explain InSAR-measured surface displacements well. In the central basin, we found a significant seasonal displacement with annual amplitude up to 43 mm (negative peak in autumn and positive peak in spring) related to the groundwater recharge and discharge due to agricultural pumping irrigation. Using cross-wavelet method, we showed a relatively short time lags (less than one month) between surface deformation and water level changes in the central basin, indicating the low-permeability clayey units have a limited influence in delaying the compaction of aquifer system. Quantifying the effects of adequate groundwater management measures and large-scale engineering approaches like inter-basin water transfer to recharge pumped aquifers provide insight for local governments and decision-makers to properly evaluate the impacts of their policy in recovering the sustainability and efficiency of aquifers in water-deficient basins.

Large-area ground deformation investigation over Taiyuan Basin, China 2007–2011 revealed by ALOS PALSAR imagery

Large-area ground deformation investigation over Taiyuan Basin, China 2007–2011 revealed by ALOS PALSAR imagery

A probabilistic method for mapping earth fissure hazards

Earth fissures caused by tectonic forces, human activities, or both seriously threaten the safety of people’s lives and properties. The Taiyuan Basin, a Cenozoic downfaulted basin located in the centre of the Fen-Wei Basin tectonic belt, in northwestern China, presents the ideal study area for a hazard assessment of earth fissures. A total of 104 earth fissures have been observed in the Taiyuan Basin, with a total length of approximately 128 km. In this paper, we proposed a probabilistic method for mapping earth fissure hazards by integrating the analytic hierarchy process (AHP), the area under the curve (AUC), and the certainty factor model (CFM). Geomorphic units, geologic formations, active faults and land subsidence zones of the Taiyuan Basin were mapped in detail. Correlations between these factors and earth fissures were evaluated through spatial modelling in ArcGIS. The AUC was introduced into the AHP to weight each factor and thus, to derive an earth fissure susceptibility map. Finally, the modelled earth fissure susceptibility was compared with a digital inventory of earth fissures to develop a probability function and map the spatial variability in failure probability through the CFM. The study indicates that active faults have the greatest contribution to the generation of earth fissures. Earth fissures are prone to develop in the piedmont alluvial-diluvial clinoplain and the transitional zone near the geomorphic boundary. This mapping procedure can assist in making rational decisions regarding urban planning and infrastructure development in areas susceptible to earth fissures.

Seasonal Variations of Carbonyls and Their Contributions to the Ozone Formation in Urban Atmosphere of Taiyuan, China

Ambient carbonyls are critical precursors of ozone (O3) and secondary organic aerosols (SOA). To better understand the pollution characteristics of carbonyls in Taiyuan, field samplings were conducted, and 13 carbonyls were detected in an urban site of Taiyuan for the four seasons. The total concentration of carbonyls in the atmosphere was 19.67 ± 8.56 μg/m3. Formaldehyde (7.70 ± 4.78 μg/m3), acetaldehyde (2.95 ± 1.20 μg/m3) and acetone (5.57 ± 2.41 μg/m3) were the dominant carbonyl compounds, accounting for more than 85% of the total carbonyls. The highest values for formaldehyde and acetone occurred in summer and autumn, respectively, and the lowest occurred in winter. The variations for acetaldehyde were not distinct in the four seasons. Formaldehyde and acetone levels increased obviously in the daytime and decreased at night, while acetaldehyde did not show significant diurnal variations. Higher temperature and stronger sunlight intensity could facilitate the photochemical reaction of volatile organic compounds (VOCs) and enhance the O3 levels in summer. Formaldehyde and acetaldehyde contributed 70–95% of carbonyls’ ozone formation potential (OFP) caused by carbonyls with the highest totals of 268.62 μg/m3 and 38.14 μg/m3, respectively. The highest concentrations of carbonyls from south and southwest winds in summer suggest that the coke industries in the southern Taiyuan Basin should be, firstly, controlled for the alleviation of ozone pollution.

High-resolution crustal velocity structure in the Shanxi rift zone and its tectonic implications

The Shanxi rift zone, located in the Trans-North China Orogen (TNCO) of the North China Craton (NCC), is wellknown for hosting large intraplate earthquakes in continental China. The TNCO is a suture zone formed by the amalgamation of the eastern and the western blocks of the NCC. After its formation, it was reactived and deformed by later tectonic activities, which result in complex lithospheric heterogeneities. Thus, the detailed crustal structure of the Shanxi rift zone is critical for understanding the tectonics and seismogenic mechanism in this area, which will shed new lights on the formation and dynamic evolution of the NCC. In this study, we applied ambient noise tomography based on 18 months continuous records from 108 seismic stations located in Shanxi and its surroundings, in order to constrain its detailed crustal structure. We measured 4437 Rayleigh wave phase velocity dispersion curves in the period of 5–45 s from the cross-correlation functions. Next, a surface wave direct inversion algorithm based on surface-wave ray tracing was used to resolve a 3-D S-wave velocity model in the upper 60 km with lateral resolution of ∼50–80 km. The tomographic images show that the sedimentary thickness of the Taiyuan Basin is less than 5 km. At depth of 0–10 km, we observe a good correlation between the imaged structural variations with geological and topographic features at the surface. For example, the center of rift shows low-velocity anomalies and the uplifting areas on both sides are characterized by high velocity anomalies. The western and eastern boundaries of the slow materials coincide with the faults that control the basin. The slow material extends from the shallow surface to depth of about 15 km but it getting smaller in shape at deeper depth. For the Taiyuan Basin, Linfen Basin, and Yuncheng Basin in the central and southern parts, the structure is dominant by slow materials in the upper crust but changes to strong high-velocity anomalies in the lower crust and the uppermost mantle at depth deeper than 25 km. We interprete these high-velocity anomalies to be associated with the cold remnant of the underplated basalt in the lower crust that were formed in early Tertiary before the basin was stretched. We also observe the low-velocity anomaly beneath the Datong volcanic area, which extends from the uppermost mantle to a depth of 20 km vertically and migrates from west to east laterally. It may reflect the upwelling channel of the magmatic material in Datong. Moreover, the strong low-velocity anomalies presented north of 38°N could be related to the heated crustal materials with paritial melting as a result of the intensive magmatic activities of the Datong Volcano since the Cenozoic. In our study region, seismicity mainly concentrates in the depth range of 5–20 km and we find that most earthquakes appear to occur in places where velocity changes from high to low rapidly, with slight higher concentration in the faster material areas. In summary, our high-resolution 3-D crustal velocity model provides important seismological constraints to understand the tectonic evolution and seismicity across the Shanxi rift zone.