Zhangjiakou Region Research Articles

Layered Archean lower continental crust: Constraints from granulite terrains and xenoliths

Our understanding of crust–mantle differentiation is hampered by the large uncertainty in the composition of the inaccessible lower continental crust. The lower-crustal estimates derived from granulite terrains are compositionally much more evolved than those based on granulite xenoliths and there is no consensus on the origin of the compositional discrepancy. Here an integrated study of granulite terrains, granulite xenoliths and lower crustal–derived magmatic rocks from the Zhangjiakou region of the northern North China craton and a combination of geochronological, mineralogical, geochemical and geophysical data enable us to establish a complete Archean lower crustal cross-section for the first time. It shows that the late Archean lower crust beneath the North China craton is layered, with an intermediate upper lower crust and a mafic lowermost crust. The tonalite–trondhjemite–granodiorite (TTG)-dominated granulite terrains may merely represent the upper lower crust. Our results successfully solve the long-standing question regarding the compositional difference between granulite terrains and xenoliths. It is inferred that a layered Archean lower crust persists in many other cratons (e.g., the Siberian, Slave, Superior, and Tanzanian cratons) and it has an overall composition different from current lower crustal estimates. We further show that the Archean mafic lower crust is temporally coupled with the upper crust. This finding is significant as it raises an important question whether the mafic lower crust and exposed TTGs are genetically linked, which may be a key to understanding crust generation and differentiation processes.

Research on Cast-in-situ Pumice Concrete Filler Wall Construction Technology

Since the existing masonry filler wall has the shortages such as: the heavy self-weight, low masonry mortar bond. we invented cast-in-situ insulation filler wall by using pumice concrete which is peculiar in Zhangjiakou region. This paper focused on construction technology for filler wall and problems that should be paid attention to on the construction. Cast-in-situ pumice concrete frame filler wall has the advantages of good entirety, simple construction, fast construction speed, it can reduce labor cost and improve the quality of engineering, thus achieves good technical and economic efficiency.

Fluoride enrichment mechanisms and related health risks of groundwater in the transition zone of geomorphic units, northern China

Although groundwater is the primary drinking water source in northern of China, little is known about generation mechanisms and related health risks of high fluoride groundwater at the geomorphic transition zones. Thus, 419 groundwater samples were collected from Zhangjiakou region, where is a typically geomorphic transition zone of the North China Plain and the Inner Mongolia Plateau, to conduct the hydrochemical analysis, geochemical modeling, multivariate statistical analysis, and health risks assessment. From the results, F− concentration in groundwater had a range of 0.05–9.71 mg L−1. About 37.1% and 26.2% of groundwater samples from Bashang region (BSR) and Baxia region (BXR), respectively, were over the 1.50 mg L−1, which were mainly distributed in the groundwater flow retardation area and/or evaporation discharge area. Thermodynamic simulations demonstrated that F-bearing minerals dissolution and Ca2+/Mg2+ removal via calcite/dolomite precipitation primarily governed high-F- groundwater formation in the whole study area. Competitive adsorption, evaporation, evaporites dissolution and salt-effect also affected F− enrichment in BSR. Desorption in alkaline environment, ion exchange and human activities played a vital role in F− enrichment at BXR. The multivariate statistical analysis revealed that the origin of F− contamination was geogenic in BSR; whereas, it was geogenic and anthropogenic in BXR. Besides, more than 71.8%, 51.0%, 36.1% and 25.5% of the study area exceeded the acceptable level (health index>1) for infants, children, adult males, and females, respectively. The health risks for different groups of people varied significantly and ranked: infants > children > males > females, suggesting that younger people were more susceptible to fluoride contamination. Meanwhile, females were more resistant to fluoride contamination than males. These findings are vital to providing insights on high-F- groundwater formation, investigate the situation of health risks, and conduct the integrated management for high fluoride groundwater in geomorphic transition zones at northern China.

Groundwater quality assessment and hydrogeochemical processes in typical watersheds in Zhangjiakou region, northern China.

It is of significance to elucidate the groundwater quality and hydrogeochemical processes for sustainable utilization of groundwater resources in water shortage regions. A total of 256 groundwater samples were collected in typical watersheds in Zhangjiakou, northern China. The hydrochemical parameters, conventional ions, and trace elements were measured, and δD and δ18O data were collected to delineate the groundwater quality and hydrogeochemical processes. The results showed that 32.91% of the groundwater could be directly used for drinking water sources in the Bashang Plateau, north of the study area. The F- and NO3--N were the main parameters above the standard threshold for drinking water. In contrast, the groundwater quality in the Baxia River Basins, south of the study area, was of a better scenario. Nonetheless, high concentrations of F-, total hardness, and SO42- were still observed. Most samples in the Bashang Plateau had relatively higher salinity than the Baxia River Basins. Both surface water and groundwater in the study area originated from local meteoric water with considerable hydraulic connections. The high-fluoride groundwater was primarily formed by dissolution of fluoride-rich minerals under conditions of high pH and Na+, low Ca2+, and rich in HCO3-. The dissolution of carbonate and silicate minerals accompanied by strong cation exchange and weak evaporation was the dominant water-rock interaction affecting the hydrochemical composition of groundwater, and anthropogenic NO3- input had an extra influence on hydrochemical process. This study provides a scientific guideline for the protection and allocation of local groundwater resources.

Platelet HPA Typing of Platelet Donors in Zhangjiakou Area

To analyze the polymorphism of the HPA1-5,15 system of the donors in Zhangjiakou area. DNA was extracted from the blood samples of the donors, PCR- SSP method was used to divide HPA1-6, 15 genotype. The gene frequency and genotype frequency were calculated, compared with the difference and regiahal specificity of the populations in our country and foregiens was compared other populations. The gene expression in the HPA-1, HPA-2 and HPA-4 systems were all homozygous aa, and the donors who expressed homozygous bb was not exessed. Among them, one heterozygous ab expression was found in both HPA-1 and HPA-4 systems (1%), and 14 cases of heterozygous ab expression were found in HPA-2 system (14%). The gene expression in the HPA-5 system was mainly homozygous aa (98%), and a very few expressed homozygous bb (2%) was found. The degree of heterozygosity of gene expression in the HPA-3 and HPA-15 systems was relatively high. The proprotion of the expression of aa, ab and bb in the HPA-3 system was respectively 46%, 40% and 14%, the proprotion of the expression of aa, ab and bb in the HPA-15 system was respectively 21%, 64% and 15%. The gene frequency of platelet-specific antigen HPA1-5,15 system in zhangjiakou region shows local characteristics. The heterozygosity degree of gene expression in the HPA-3 and HPA-15 systems are both high, suggesting that they are more likely to result in alloimmunization and ineffective platelet transfusion, which should be pays attention to.

A Study on Spatial Variation of Water Security Risks for the Zhangjiakou Region

Zhangjiakou region is situated in an agro-pastoral ecotone with a fragile ecosystem. While it has limited surface water resources available and serious groundwater over-exploitation, the city is located in the water conservation zone for the Beijing-Tianjin-Hebei coordinated development area, so its water security is crucial for the entire Beijing-Tianjin-Hebei region. Therefore, it is of vital significance to determine the zoning management of water resources and decision-making according to the magnitude of water resource security risks. This study built an indicator system for water security risk assessment in line with the principles of scientific validity, comparability, operability, and data availability, and this system gives weights to these indicators using the AHP approach. County-level multi-source data for the study area, based on water resource zones, were collected by using mathematical statistics and 3S technology. With normalized data and a weighting method the water security risks were calculated. The results showed large spatial variations of water security risks in Zhangjiakou on the scales of geomorphic and administrative units as well as river basins. High-risk areas are extensive in the Bashang Plateau, and extremely high risk values are found in the Baxia areas. On the watershed scale, high-risk areas are mainly distributed in the inland river basins and the Yongding River basin. The risk values of the Luanhe River, Chaobai River and Daqing River basins in the Zhangjiakou region tend to decrease from north to south. For the northern and western areas of the Bashang Plateau, the factor of “vulnerability of the disaster-prone environment” contributes the most to the water security risk level. Agricultural water use constrains industrial and ecological water use, but in the context of inadequate water resource endowments, the urban population concentration and industrial development are the main causes of water shortages and water pollution so they contribute more to water security risks. This study of the spatial variation of water security risks in Zhangjiakou can provide an important scientific reference for zone-based management and decision-making for reducing the water security risks in the farming-pastoral ecotone.

Positive effects of ecological restoration policies on the vegetation dynamics in a typical ecologically vulnerable area of China

Ecological restoration programmes have been initiated worldwide because of serious environmental degradation. However, the effectiveness of ecological restoration remains contentious because vegetation restoration is impacted by both climate change and human activities. Climate change is an important factor for vegetation restoration, especially in ecologically vulnerable areas. The Zhangjiakou region of northern China is located in a semiarid and subhumid climate region with apparent ecosystem vulnerability. Since the late 1970s, a series of ecological restoration programmes have been carried out in this area to recover the vegetation and ecosystems. Based on remote sensing images and ground-based meteorological data, two vegetation indexes (NDVI and EVI2) and aridity index were calculated to analyse (i) the vegetation dynamics and climate aridity trends in the Zhangjiakou region and (ii) the response of the vegetation dynamics to ecological restoration programmes and climate change between 1987 and 2017. The results indicated that vegetation cover increased with twice abrupt changes in 2002 and 2009, respectively, which simultaneously underwent an increase in aridity. Vegetation cover has increased rapidly since 2003 because of ecological recovery projects and policies. The contributions of climate change and human activities to vegetation cover were12%~ 5.2% and 88% ~ 94.8% during the period 2003–2009, respectively. The contributions of climate change and human activities to vegetation cover were2.1% ~ 2.4% and 97.9% ~ 97.6% during the period 2010–2017, respectively. The ecological restoration projects and policies effectively promoted vegetation cover in the study area.

Drought/wetting variations in a semiarid and sub-humid region of China

It has been widely accepted that the global climate will change, with a ‘dry gets drier, wet gets wetter’ pattern. Recent studies found that such changes were overestimated. Drought/wetting variations were detected in a semiarid and sub-humid transitional region of China. The aridity index based on precipitation and reference evapotranspiration was applied to estimate drought/wetting variations. The results showed that the climate in the annual mean of the Zhangjiakou region showed a wetting trend from 1960 to 2017. The wetting trend also occurred in January, February, May and the wet season, especially in June, and in September, October, December and the dry season, and the wetting trend was significant. However, the drought trend occurred in March, April, July, August and November. Moreover, the most obvious wetting and drought changes occurred in September and August, respectively. Climate drought/wetting conditions are primarily driven by precipitation. Although increasing temperatures resulted in enhanced evaporation, precipitation increased, which could offset the increase in evaporation. In addition to precipitation and temperature, humidity was an important influencing factor on drought/wetting variation. The impact of humidity on drought/wetting conditions was greater than that of temperature in January, February, July, August and September. The influencing factors of drought/wetting variations are complicated in the Zhangjiakou region and are mainly influenced by the southeast monsoon, southwest monsoon and plateau monsoon in the Tibetan Plateau, westerly circulation and West Pacific subtropical high.

Comparison of metamorphic zircons from granulite xenoliths and granulite terrain in northern North China Craton

We conduct a study of petrography, geochronology, trace element and Hf isotope compositions on zircons from granulite terrain and granulite xenoliths in the Zhangjiakou region, northern North China Craton. Most zircons from both types of granulite have core-rim structure in their CL images. All the magmatic zircon cores yield essentially identical ∼2.5Ga ages, whereas metamorphic zircons yield ages of ∼1.85Ga and 120–140Ma, respectively. Most the ∼1.85Ga metamorphic zircons have Th/U ratios lower than 0.4 with a range of 0.01 to 0.63. However, the range of Th/U ratios of Mesozoic (120–140Ma) metamorphic zircons (0.12–3.48) overlaps that of the magmatic zircons (0.12–2.44). The Mesozoic and ∼1.85Ga metamorphic zircons have REE patterns consistent with magmatic zircons, except that the ∼1.85Ga ones show less enrichments and much less negative Eu anomalies than those of magmatic zircons. Therefore, Th/U ratios and the REE patterns cannot be used to distinguish metamorphic zircons from magmatic ones. The ∼1.85Ga metamorphic zircons, however, have lower formation temperatures, oxygen fugacity and 176Lu/177Hf ratios than the Mesozoic ones. We speculate that the ∼1.85Ga metamorphic zircons represent newly grown zircons coexisting with garnets and melt/fluid while the Mesozoic metamorphic zircons were formed quickly without the presence of garnet. The Mesozoic metamorphic zircons vary in their origins; some of these zircons were metamorphic newly grown while the others experienced recrystallization from a U-depleted melt/fluid. Our results suggest that P-T conditions, the melt/fluid and oxygen fugacity play important roles in controlling the trace element distribution and Hf isotope compositions of metamorphic zircons in the granulites.

A Bayesian Method for Water Resources Vulnerability Assessment: A Case Study of the Zhangjiakou Region, North China

Water resources vulnerability (WRV) assessment is an important basis for maintaining water resources security in a basin. In this paper, considering the complexity of the water resources system and the uncertainty of the assessment information, a method based on the Bayesian theory was developed for performing WRV assessments while using the constructed indicator system. This system includes four subsystems, the hydrological subsystem, the socioeconomic subsystem, the ecoenvironmental subsystem and the hydraulic engineering subsystem. The WRV degree for each subsystem and the integrated water resources system were assessed. Finally, the assessment results and the characteristics of the Bayesian method were compared with those of the grey relational analysis method and the parametric-system method. The results showed the following. (1) The WRV of the integrated water resources system of the entire Zhangjiakou region was very high; Zhangjiakou City and Xuanhua County have tendencies to belong to Extreme WRV, with probabilities of 26.8% and 25%, respectively, while the other seven administrative counties have tendencies to belong to High WRV, with probabilities ranging from 24.6% to 27%. (2) Compared with the parametric-system method and the grey relational analysis method, the Bayesian method is simple and can effectively address the uncertainty issues with the reliable WRV assessment results.

Zhangjiakou a Residential Building Envelope Structure Optimization Design

The energy consumption of a residential building Zhangjiakou area in different envelope was numerically simulated by using DeST software, and the influence factors on building energy consumption was analyzed, which provides a certain reference for optimization design of building structure Zhangjiakou region.

Test Research on HIMS-Reverse Flotation of Oolitic Hematite of Longyan Iron Mine

In order to better exploit and utilize the oolitic hematite ore resource in Zhangjiakou region, staged grinding-separation process consisting of high intensity magnetic separation(HIMS) and reverse floatation was adopted in the beneficiation test on the regionally representative oolitic hematite ore of Longyan Iron Mine, Xuan Stee1. The test results indicate that，with Slong pulsating high gradient magnetic separation as HIMS equipment，with NaOH，starch，CaO and TS as flotation reagents，and at a grind of 65％ -200 mesh for the primary grinding and 95％-200 mesh for the secondary grinding，an iron concentrate grading 62.34％ and having an iron recovery of 53.07％ can be achieved after two stage HIMS and one roughing—one cleaning reverse flotation.

An optimal water allocation model based on water resources security assessment and its application in Zhangjiakou Region, northern China

Problems with water resources security (WRS) resulting from water scarcity are occurring worldwide, and the most feasible solution to such problems is to increase the productivity of limited water resources through rational water use allocation within the affected region. In this study, an optimal water allocation model (OWAM) based on WRS assessment was developed and applied to improve the water use benefits in Zhangjiakou Region of northern China in 2020. The OWAM was an optimal linear programming model that comprehensively considered eco-environmental and socio-economic benefits. Some WRS assessment indicators that can be easily adjusted, including the ratio of ecological water to total water demand, surface water pollution index, exploitation ratio of water resources, water supply per capita, industrial water scarcity, and agricultural water scarcity, were incorporated into the OWAM as constraining conditions. The model had different solutions corresponding to four WRS statuses (quite safe, basically safe, unsafe, and extremely unsafe). The results indicated that optimal water allocation determined using this model met the domestic and environmental water demand, and led to increased water use benefits within the region when compared with before allocation. The benefits of basically safe status were bigger than those of unsafe status. In contrast to the conventional OWAM, the developed model could help to assure sustainable water use at the regional scale when used in combination with WRS assessment.

Links between Mesozoic intermediate–felsic igneous rocks and lower crustal granulite xenoliths, North China Craton: O and Pb isotopic evidence

We present and compare whole-rock and zircon O and Pb isotopic compositions for the Hannuoba granulite xenoliths and Mesozoic intermediate-to-felsic igneous rocks from the Zhangjiakou region, northern margin of the North China Craton, northeast China. The xenoliths have an overall Pb isotopic range similar to rocks from the regionally exposed Neoarchaean granulite terrain. Mesozoic zircons from different types of granulite xenoliths have a narrow range of δ18O values (6.0–7.7‰) higher than normal mantle δ18O values (∼5.7‰). Mesozoic intermediate–felsic igneous rocks have O and Pb isotopic compositions indistinguishable from the Hannuoba intermediate–mafic granulite xenoliths. Our new data suggest that the Mesozoic igneous rocks and granulite xenoliths are genetically linked and that both were derived from the late Neoarchaean lower crust. This argues against previous proposals that the granulite xenoliths are either products of Mesozoic basaltic underplating or formed by mixing between mantle-derived and pre-existing crustal magmas.

Water Resources Vulnerability Assessment based on the Parametric-system Method: a Case Study of the Zhangjiakou Region of Guanting Reservoir Basin, North China

Abstract A water resources vulnerability (WRV) assessment is important to maintain water resources safety in a basin. In this paper, an index system, including four subsystems -- the hydrological subsystem, the socioeconomic subsystem, the eco-environment subsystem and the hydraulic engineering subsystem, is constructed for an integrated WRV assessment of the Zhangjiakou region of the Guanting Reservoir Basin, North China. The parametric-system (PS) method based on background value is used for the quantitative WRV assessment of each subsystem and of the integrated water resources system. The results of the calculations show that the degree of vulnerability of Guanting Reservoir Basin is quite serious, with Zhangjiakou City being extremely vulnerable, and Yanqing County and Zhuolu County mildly vulnerable. The assessment process and results, as well as the characteristics of the method used, have been compared with those of the fuzzy optimization (FO) method and the grey relational analysis (GRA) method. The parametric-system (PS) method is appropriate for level classification and rank analysis of many samples in the system, with the obvious advantage of employing the simple process of linear calculation for both the assessment and reference systems. In practical calculation, multiple methods should be used comprehensively, so as to provide a more rational decision-making basis for water resources management.

Source of Mesozoic intermediate-felsic igneous rocks in the North China craton: Granulite xenolith evidence

Four intermediate to felsic igneous rocks from the Zhangjiakou region, along the northern margin of the North China craton, have magmatic zircon U–Pb ages from 122 to 144Ma. Two of these samples have inherited zircon U–Pb ages of ~2.5Ga, similar to the zircon ages of rocks from the surrounding granulite terrain. Zircons from two intermediate composition granulite xenoliths (JN0811 and JN0919) in the nearby Cenozoic Hannuoba basalts yield two groups of ages. The rims have concordant Mesozoic ages mostly between 120 and 145Ma, coeval with the Mesozoic intermediate-felsic magmatism in the region, while the cores have discordant U–Pb ages with upper-intercepts of ~2.5Ga, overlapping the zircon ages of granulite terrain rocks, and lower-intercept ages of ~130Ma, approximating the ages of the Mesozoic intermediate-felsic magmatism. The Sr–Nd isotopic compositions of the Mesozoic intermediate-felsic igneous rocks are completely different from those expected for basaltic melts from either the lithospheric mantle or the asthenospheric mantle, precluding a derivation by extensive fractional crystallization of mantle-derived magmas. The lack of correlation between (86Sr/87Sr)i, εNd(t) and SiO2 for the Mesozoic igneous rocks, the very narrow range of zircon εHf(t) for individual intermediate-felsic igneous rocks, and simple binary mixing calculations argue against them being formed by mixing between mantle-derived magma and preexisting crust that has extremely evolved Sr–Nd isotopic compositions like granulite xenoliths JN0811 and JN0919. Hf isotopic compositions of the Mesozoic zircons and whole-rock geochemistry show that the granulite xenoliths with extremely evolved Sr–Nd isotopic compositions have not undergone partial melting during the Mesozoic and thus do not contribute to the Mesozoic intermediate-felsic magmas. Further comparisons show that the source rocks for the Mesozoic intermediate-felsic magmas likely were late Archean lower crustal rocks similar in chemical and isotopic compositions to rocks with SiO2<62wt.% from the granulite terrain. If so, pyroxene-rich mafic granulite xenoliths found in the Hannuoba basalts could be restites left behind after the partial melting of the late Archean lower crust. The results shed new light on the origin for Mesozoic zircons in granulite xenoliths from Nushan, at the southern margin of the North China craton. We suggest that the widespread Mesozoic intermediate-felsic igneous rocks in the eastern North China craton are most likely derived from partial melting of the intermediate-mafic rocks of the late Archean lower crust.

LA-ICP-MS dating of Zhangjiakou Formation volcanic rocks in the Zhangjiakou region and its geological significance

The geological cross section of the Zhangjiakou Formation is originally established in the Zhangjiakou region. From the dating of four samples from typical cross sections of the Zhangjiakou Formation in the Zhangjiakou region, the age range is found to be mainly from 143.0 ± 3.7 Myr to 136.1 ± 1.4 Myr, which almost equals the sum of the age range of the Zhangjiakou Formation (136–135 Myr) and that of the Tuchengzi Formation (143–136 Myr) in the Luanping region, and the Zhangjiakou and Tuchengzi Formations in Zhangjiakou-Luanping of North Hebei are found to be conformable. The evidence above indicates that the Zhangjiakou and Tuchengzi Formations, which are distributed in Zhangjiakou-Luanping, were formed under the same geological setting. Volcanic activity was earlier and longer (143–136 Myr) in the western region (Zhangjiakou) than that in the eastern region (Luanping). The “Zhangjiakou Formation” in Chengde and Pingquan formed in the period in which the Yixian Formation in West Liaoning formed, so should be renamed the Yixian Formation.

Determination of earthquake epicentroids by inversion of gravity variation data in the BTTZ region, China

During the last decade and a half, precise measurements of gravity variations have been carried out in the Beijing–Tianjin–Tangshan–Zhangjiakou (BTTZ) region in China. A possible seismogenic mechanism of a combined dilatancy model (CDM) for the BTTZ region has been proposed by Kuo and Sun (1993). The model consists of the dilatancy–diffusion, the dilatancy–instability, and the fault-zone dilatancy models. Gravity variations in space and time associated with the model involve a tectonically stressed volume surrounding a small volume of an impending rupture (fault) zone. They refer to the center of effective mass of the total stress volume which is defined as `hypocentroid', the projection of which on the earth's surface is defined as `epicentroid'. This paper attempts to confirm the above stated concept of `epicentroid' associated with the fourteen earthquakes of magnitude mainly 4–5 which occurred in 1981–95 in the BTTZ region by means of a proposed least-squares inversion method, using observed gravity variation data. On the basis of the 1976 Tangshan earthquake, the 1989 Datong earthquake, and these fourteen earthquakes, an empirical relationship between the maximum variations of gravity and magnitude was first established. Iterative inversions of observed gravity variation data were then carried out. Inversion results show that (1) the separation of the epicentroids and epicenters range from 0 to about 40 km, (2) the epicenters of these fourteen earthquakes are generally located at the terminations of the faults or at the intersections of the faults, whereas the epicentroids are inevitably located within intact tectonic blocks, and (3) the epicentroid of an earthquake at least in the BTTZ region as attested appears to be determinable.

Seismogenesis and occurrence of earthquakes as observed by temporally continuous gravity variations in China

In the last decade and a half, a number of earthquakes of magnitude 4-5 have occurred in the Beijing_Tianjin_Tangshan_Zhangjiakou (BTTZ) region. On the basis of the analysis of the temporally continuous gravity variation data principally from the Baijiatuan (BJTN) semi_permanent gravity base station, a general picture of gravity variation related to the seismogenesis and occurrence of earthquakes has emerged.\;As gravity variation is generally observed on the earths surface, the predominant influence is that of the near_surface groundwater. The subsurface fluids are distributed throughout all depths in the crust and respond to the seismogenic processes of earthquakes as well. The influence of the subsurface fluid on gravity variation is, therefore, of equal importance. The fluids, which include the near_surface groundwater and the subsurface fluids distributed throughout all depths in the crust, play a more important role in the gravity variations in terms of the seismogenesis and occurrence of earthquakes than previously realized.\;The abundance of accumulated data shows that the dilatancy instability (IPE) model seems not applicable at least to the seismogenesis and occurrence of earthquakes in the BTTZ region. In order to reflect the physical reality, the earlier proposed combined dilatancy model requires modification. The seismogenic area in the BTTZ region may be modelled as a large pre_stressed volume of a fluid_filled poroelastic medium, including not only the pre_stressed volume surrounding the impending rupture zone but also the volume containing the rupture of the fault zone itself. The pre_stressed volume outside the impending rupture zone is under a state of relatively small change of the pre_existing regional tectonic stress, while the volume containing the impending rupture zone is an induced region of very high local stress concentration, and/or pore over_pressure.\;The calculated gravity variations based on the modified combined dilatancy model (MCDM) with the known physical parameters of the region resemble the observed residual gravity variations. Apparently the residual gravity variations, in addition to responding to the deep_seated seismogenesis and occurrence of earthquakes, predominantly respond to the near_surface groundwater, and the subsurface fluids, which themselves also respond to the seismogenesis and occurrence of earthquakes. On the basis of comparison between the calculated MCDM gravity variations and the observed residual gravity variations, the change of the regional tectonic stress field for the earthquakes of magnitude 4-5 in the BTTZ region could be approximately estimated to be in the neighbourhood of 5%-7%.\;It is apparent that simultaneously monitoring the temporally continuous variations of the near_surface groundwater, subsurface fluids, and gravity coupled with modelling would provide vital information on the history and evolution of the seismogenic processes about 10 months to 1 year prior to the occurrence of an earthquake of magnitude 4-5 and tens of years prior to that of an earthquake of magnitude 7-8 such as the Haicheng earthquake in 1975 and the Tangshan earthquake in 1976 in the BTTZ region. These earthquakes of magnitude 4-5, which so far have occurred in the BTTZ region, may well be the precursory events to a larger earthquake.

Gravity variation before and after the Tangshan earthquake

Since 1967, a gravity observing network and profiles have been set up respectively in Beijing, Tianjin, Tangshan and Zhanjiakou region. On July 28, 1976, the Tangshan earthquake occurred having a magnitude of 7.8, and valuable data from repeated gravity measurements across the epicentral area have been obtained. In this paper, a statistical analysis has been made on the data of repeated gravity measurements of the period 1970 to 1980 in Beijing, Tianjin, Tangshan and Zhangjiakou region. The method of generalized inversion is used in square error calculation, and the method of mean square error is used to test whether there is a trend anomaly. We use the ideal formula of non-systematic interference to evaluate the practical precision of the dynamic problems, and adopt the steeply regression analysis method to determine factors such as the correlations between atmospheric pressure, air temperature, groundwater level and gravity. The primary and secondary variables factors that affect the gravity value should be analysed and separated. The statistical result shows that, before the Tangshan earthquake, an increased trend in gravity variation appeared for 2 1 2 years, having an epicentral area of 100 km in radius with maximum amplitude of 0.1 mGal; a medium-short-term variation (3-1 months before the quake) also appeared. It can be seen that the gravity value, corrected by the crustal deformation data, still remains high after the shock; it has not returned to the original basic value. Through contrast analysis of the gravity data with the data of deformation resistivity and total intensity of geomagnetism, it appears that before the Tangshan earthquake some materials of high temperature, high pressure and high density intruded upward along fissures and remained at a certain depth. As a consequence, this caused the trend of increased gravity value variation measured before the quake and also the large fluctuations measured after the quake.