Characteristics and control factors of feldspar dissolution in gravity flow sandstone of Chang 7 Member,Triassic Yanchang Formation,Ordos Basin,NW China

To clarify the formation and distribution of feldspar dissolution pores and predict the distribution of high-quality reservoir in gravity flow sandstone of the 7^(th) member of Triassic Yanchang Formation(Chang 7 Member)in the Ordos Basin,thin sections,scanning electron microscopy,energy spectrum analysis,X-ray diffraction whole rock analysis,and dissolution experiments are employed in this study to investigate the characteristics and control factors of feldspar dissolution pores.The results show that:(1)Three types of diagenetic processes are observed in the feldspar of Chang 7 sandstone in the study area:secondary overgrowth of feldspar,replacement by clay and calcite,and dissolution of detrital feldspar.(2)The feldspar dissolution of Chang 7 tight sandstone is caused by organic acid,and is further affected by the type of feldspar,the degree of early feldspar alteration,and the buffering effect of mica debris on organic acid.(3)Feldspars have varying degrees of dissolution.Potassium feldspar is more susceptible to dissolution than plagioclase.Among potassium feldspar,orthoclase is more soluble than microcline,and unaltered feldspar is more soluble than early kaolinized or sericitized feldspar.(4)The dissolution experiment demonstrated that the presence of mica can hinder the dissolution of feldspar.Mica of the same mass has a significantly stronger capacity to consume organic acids than feldspar.(5)Dissolution pores in feldspar of Chang 7 Member are more abundant in areas with low mica content,and they improve the reservoir physical properties,while in areas with high mica content,the number of feldspar dissolution pores decreases significantly.

GRACE time-variable gravity and its application to geoscience:Quantitative analysis of relevant literature

The Gravity Recovery and Climate Experiment(GRACE)is the most important gravity satellite to date in human history.Since its launch in 2002,GRACE time-varying gravity has had an unprecedented impact on earth science and has generated revolutionary changes.Because of natural phenomena such as climate warming,glacial melting,sea level rise,and earthquakes,earth science research has become an increasingly popular discipline in recent years.This article summarizes the importance of GRACE time-varying gravity,its application to geoscience,and its development.We analyzed the historical development and current status of GRACE time-varying gravity as well as research hotspots by searching the literature in the core collection databases of the China National Knowledge Infrastructure and the Web of Science over the past 20 years.The CiteSpace and VOSviewer software packages were applied with reference to the principle of literature metrology.Our investigation and analysis of characteristic indexes,such as the numbers of publications,co-occurrence of keywords,and co-citation of documents,uncovered the wide application and promotion of gravity satellites,especially GRACE time-varying gravity,in earth science.The results showed that the number of publications addressing GRACE data and time-varying gravity theory is increasing annually and that the USA,China,and Germany are the main producers.The Chinese Academy of Sciences,the National Aeronautics and Space Administration of the United States,and the Helmholtz Association of German Research Centres rank among the top three institutions in the world in terms of producing the most publications on this topic.We found that GRACE time-varying gravity plays unique roles in measuring changes in terrestrial water storage changes,ice and snow melting and sea level changes,and(co)seismic gravity changes,as well as in promoting other disciplines.

Relationship between gravity change and Yangbi M_(S)6.4 earthquake

Based on the relative and absolute gravity measurements in the southern South-North Seismic Belt since 2015,we analyzed the dynamic change of the regional gravity field and its relationship with the Yangbi M_(S)6.4 earthquake that occurred on May 21,2021.The results show that:(1)The regional gravity field changes are closely related to the Weixi-Qiaohou fault,which reflects the surface gravity field changes caused by the fault activity from 2015 to 2021;(2)The gravity field change related to the preparation of Yangbi earthquake has experienced the evolution process of"steady state-regional gravity anomaly-local gravity anomaly-four-quadrant distribution-large area positive anomaly-earthquake occurring in the reverse change process";(3)The cumulative and differential change images of the gravity field show that there were significant gravity changes in the two years preceding the Yangbi earthquake,and the earthquake occurred in the high-gradient belt of gravity variation,the center of the four-quadrant,and close to the zero contour turn;(4)The dynamic evolution image of the gravity field can well reflect the precursory phenomena during the preparation for the Yangbi MS6.4 earthquake.Based on the anomaly change of mobile gravity,a certain degree of medium-term prediction was made before the Yangbi M_(S)6.4 earthquake,especially the determination of strong earthquake location.

Recent gravity changes in China Mainland

Based on results of the mobile gravity measurements of the Crustal Movement Observation Network of China and Digital Earthquake Observation Network of China, this paper shows the pattern of temporal gravity changes in China mainland on a time scale of 2 - 3 years since 1998, and gives an analysis of the patterns. The result shows that the temporal gravity changes basically reflect the current mass movement and occurrence of strong earthquakes.

Determination of the degree 120 time-variable gravity field in the Sichuan-Yunnan region using Slepian functions and terrestrial measurements

The terrestrial time-variable gravity measurements are characterized by a high signal-to-noise ratio and sensitivity to the sources of mass change in the Earth's crust.These gravity data have many applications,such as surface deformation,groundwater storage changes,and mass migration before and after earthquakes.Based on repeated terrestrial gravity measurements at 198 gravity stations in the Sichuan-Yunnan region(SYR)from 2015 to 2017,we determine a time series of degree 120 gravity fields using the localized spherical harmonic(Slepian)basis functions.Our results show that adopting the first 6 Slepian basis functions is sufficient for effective localized Slepian modeling in the SYR.The differences between two gravity campaigns at the same time of year show an obvious correlation with tectonic features.The degree 120 timevariable gravity models presented in this paper will benefit the study of the regional mass migration inside the crust of the SYR and supplement the existing geophysical models for the China Seismic Experimental Site.

Spatial distribution characteristics and mechanism of nonhydrological time-variable gravity in China's Mainland

The purpose of this study is to explore nonhydrological mass transfer in China's Mainland.For this purpose,gravity recovery and climate experiment(GRACE)data were obtained to study the spatial distribution of time variant gravity signals in China's Mainland.Then,from auxiliary hydrological data processed according to the current hydrological model,a new more comprehensive hydrological model of China's Mainland was constructed.Finally,the time variant signals of this new hydrological model were removed from the time variant gravity field computed from GRACE data,thus obtaining a description of the nonhydrological mass transfer of China's Mainland.The physical sources and mechanisms of the resulting mass transfer are then discussed.The improved,more realistic,hydrological model used here was created by selecting the hydrological components with the best correlations in existing hydrological models,by use of correlation calculation,analysis,and comparison.This improved model includes water in soils and deeper strata,in the vegetation canopy,in lakes,snow,and glaciers,and in other water components(mainly reservoir storage,swamps,and rivers).The spatial distribution of the transfer signals due to nonhydrological mass in China's Mainland was obtained by subtracting the combined hydrological model from the GRACE time-variable gravity field.The results show that the nonhydrological signals in China's Mainland collected in GRACE data were mainly positive signals,and were distributed in the Bohai Rim and the northern and eastern parts of the Tibetan Plateau.The above nonhydrological mass transfer signals have been studied further and are discussed.The results show that the nonhydrological mass migration signals in the Bohai Rim region originate primarily from sea level change and marine sediment accumulation.The mass accumulation from Indian plate collision in the Tibetan Plateau appears to be the main reason for the increase in the residual gravity field in that region.

Spatio-temporal variability of terrestrial water storage in the Yangtze River Basin: Response to climate changes

The Yangtze River Basin(YRB)is an important region for China's economic development.However,it has a complex terrain layout,most of which is affected by monsoon weather,and the geographical and temporal distribution of water resources is severely unbalanced.Therefore,the detailed analysis of spatio-temporal water mass changes is helpful to the development and rational utilization of water resources in the YRB.In this study,the variation of terrestrial water storage(TWS)is monitored by Gravity Recovery and Climate Experiment(GRACE)satellite gravity.We find that the University of Texas Center for Space Research(CSR)solution shows a notable difference with the Jet Propulsion Laboratory(JPL)in space,but the general trend is consistent in time series.Then the GRACE inferred water mass variation reveals that the YRB has experienced several drought and flood events over the past two decades.Global Land Data Assimilation System(GLDAS)results are similar to GRACE.Furthermore,the overall precipitation trend tends to be stable in space,but it is greatly influenced by the strong El Nino-~Southern Oscillation(ENSO),which is the response to global climate change.The upper YRB is less affected by ENSO and shows a more stable water storage signal with respect to the lower YRB.

Far-field coseismic gravity changes related to the 2015 MW7.8 Nepal(Gorkha)earthquake observed by superconducting gravimeters in Chinese mainland

Using data from five SGs at four stations in Chinese mainland,obvious permanent gravity changes caused by the 2015 MW7.8 Nepal(Gorkha)earthquake were detected.We analyzed the gravity effects from ground vertical deformation(VD)using co-site continuous GPS(cGPS)data collocated at the Lijiang and the Wuhan station,and hydrological effects using GLDAS models and groundwater level records.After removing these effects,SG observations before and after the earthquake revealed obvious permanent gravity changes:−3.0μGal,7.3μGal and 8.0μGal at Lhasa,Lijiang and Wuhan station,respectively.We found that the gravity changes cannot be explained by the results of dislocation theory.

Influence of fault asymmetric dislocation on the gravity changes

A fault is a planar fracture or discontinuity in a volume of rock, across which there has been significant displacement along the fractures as a result of earth movement. Large faults within the Earth＇s crust result from the action of plate tectonic forces, with the largest forming the boundaries between the plates, energy release associated with rapid movement on active faults is the cause of most earthquakes. The relationship between unevenness dislocation and gravity changes was studied on the theoretical thought of differential fault. Simulated observation values were adopted to deduce the gravity changes with the model of asymmetric fault and the model of Okada, respectively. The characteristic of unevenness fault momentum distribution is from two end points to middle by 0 according to a certain continuous functional increase. However, the fault momentum distribution in the fault length range is a constant when the Okada model is adopted. Numerical simulation experiments for the activities of the strike-slip fault, dip-slip fault and extension fault were carried out, respectively, to find that both the gravity contours and the gravity variation values are consistent when either of the two models is adopted. The apparent difference lies in that the values at the end points are 17.97% for the strike-slip fault, 25.58% for the dip-slip fault, and 24.73% for the extension fault.

Gravity changes and crustal deformations before the Menyuan,Qinghai Ms6.4 earthquake of 2016

In this study, the relative gravity data(2012 e2015), GPS data-derived horizontal deformation(2011 e2014) and the background vertical deformation from the leveling measurements(1970 e2011) in the northeastern margin of Tibetan Plateau were processed to systematically analysis the mechanism of temporalespatial patterns and the relationship with Menyuan Ms6.4 earthquake. It can be summarized in the following: 1) The regional gravity changes, the GPS and the vertical deformational showed an intense spatial relationship: the gravity increased along with the direction of horizontal movement, and decreased with the crustal uplift and vice versa, which reflected the inherited characteristics of geotectonic activities. 2) The crustal deformations were closely related to the active faults. The contour lines of gravity changes and vertical deformation were generally along with the Qilian-Haiyuan fault(strike is NWW), and the crustal horizontal deformation showed left-lateral strike slip motion near the Qilian-Haiyuan fault. 3) Menyuan Ms6.4 earthquake occurred in the high negative gravity variation area and a high gradient formed in regions, positive and negative variation of gravity amount to 110 m Gal.Specifically, a borderline of positive and negative gravity located in the south of epicenter along the north edge of Qilianshan fault and Lenglongling fault, as well as the vertical and/or horizontal deformation is intensely. The extrusion deformation, surface compression rate and gravity changes were obvious near the epicenter of 2016 Menyuan Earthquake.

GRACE detection of the medium-to far-field coseismic gravity changes caused by the 2004 Mw9.3 Sumatra-Andaman earthquake

Large earthquakes cause observable changes in the Earth’s gravity field, which have been detected by the Gravity Recovery and Climate Experiment (GRACE). Since most previous studies focus on the detection of near-field gravity effects, this study provides the results from the medium- to far-field gravity changes caused by the 2004 Sumatra-Andaman earthquake that are recorded within GRACE monthly solutions. Utilizing a spherical-earth dislocation model we documented that large-scale signals predominate in the global field of the coseismic gravity changes caused by the earthquake. After removing the near-field effects, the coseismic gravity changes show a negative anomaly feature with an average magnitude of -0.18×10-8 m·s-2 in the region ranging ～40° from the epicenter, which is considered as the 'medium ffield' in this study. From the GRACE data released by Center for Space Research from August 2002 to December 2008, we retrieved the large-scale gravity changes smoothed with 3 000 km Gaussian ffilter. The results show that the coseismic gravity changes detected by GRACE in the medium field have an average of (-0.20±0.06)×10-8 m·s-2, which agrees with the model prediction. The detection confirms that GRACE is sensitive to large-scale medium-field coseismic gravitational effects of mega earthquakes, and also validates the spherical-earth dislocation model in the medium field from the perspective of satellite gravimetry.

Dynamic changes of gravity fields before and after the 2008 Wenchuan earthquake(Ms8.0)

The pattern evolution and dynamic mechanism of the dynamic changes of regional gravity fields occurring before and after the Wenchuan Ms8.0 earthquake are analyzed, based on five epochs of 1998 -2007 mobile gravity data from the middle-south section of the north-south seismic belt, and two epochs of field research data collected after the 2008 Wenchuan earthquake in combination with GPS data, leveling observations, and geotectonic environment data. The regional dynamic gravity changes demonstrate the effects of the eastward flow of solid matter in the Qinghai-Tibetan plateau and the preparation of the 2008 Wenchuan earthquake （2- 10 yr）. The two most meaningful gravity indicators of the Wcnchuan earthquake preparation are the positive （increasing） gravity changes occurring over many years in the southwest epicenter and the largescale gradient zone of gravity variation, with the cumulative difference between the two sides of the gradient zone of gravity exceeding 200 μGal. The positive gravity changes may facilitate a constant energy accumulation and the gradient belt may support seismic shear breakage. Overall, the gravity changes associated with the earthquake preparation indicate a pattern of accelerating increase-decelerating increase-earthquake occurrence. The Songpan-Ganzi block generally displays a negative gravity change, providing evidence for a local upwarp- ing of the deep crust-mantle and an interior expansion of the deep crust attributable to high temperatures. The viewpoint is consistent with the dilatant mechanism for earthquake preparation.

Coseismic gravity and displacement changes of Japan Tohoku earthquake(Mw 9.0)

The greatest earthquake in the modern history of Japan and probably the fourth greatest in the last 100 years in the world occurred on March 11, 2011 off the Pacific coast of Tohoku.Large tsunami and ground motions caused severe damage in wide areas, particularly many towns along the Pacific coast. So far, gravity change caused by such a great earthquake has been reported for the 1964 Alaska and the 2010 Maule events. However, the spatial-temporal resolution of the gravity data for these cases is insufficient to depict a co-seismic gravity field variation in a spatial scale of a plate subduction zone. Here, we report an unequivocal co-seismic gravity change over the Japanese Island, obtained from a hybrid gravity observation（combined absolute and relative gravity measurements）. The time interval of the observation before and after the earthquake is within 1 year at almost all the observed sites, including 13 absolute and 16 relative measurement sites, which deduced tectonic and environmental contributions to the gravity change. The observed gravity agrees well with the result calculated by a dislocation theory based on a self-gravitating and layered spherical earth model. In this computation, a co-seismic slip distribution is determined by an inversion of Global Positioning System（GPS） data. Of particular interest is that the observed gravity change in some area is negative where a remarkable subsidence is observed by GPS, which can not be explained by simple vertical movement of the crust. This indicated that the mass redistribution in the underground affects the gravity change. This result supports the result that Gravity Recovery and Climate Experiment（GRACE） satellites detected a crustal dilatation due to the 2004 Sumatra earthquake by the terrestrial observation with a higher spatial and temporal resolution.

Trends in gravity changes from 2009 to 2013 derived from ground-based gravimetry and GRACE data in North China

North China is a key region for studying geophysical progress. In this study, ground-based and Gravity Recovery and Climate Experiment（GRACE） gravity data from 2009 to 2013 are used to calculate the gravity change rate（GCR） using the polynomial fitting method. In general, the study area was divided into the Shanxi rift, Jing-Jin-Ji（Beijing-Tianjin-Hebei Province）, and Bohai Bay Basin（BBB） regions. Results of the distribution of the GCR determined from ground-based gravimetry show that the GCR appears to be ＂negativepositive-negative＂ from west to east, which indicates that different geophysical mechanisms are involved in the tectonic activities of these regions. However, GRACE solutions are conducted over a larger spatial scale and are able to show a difference between southern and northern areas and a mass redistribution of land water storage.

Seismologic applications of GRACE time-variable gravity measurements

The Gravity Recovery and Climate Experiment（GRACE） has been measuring temporal and spatial variations of mass redistribution within the Earth system since2002. As large earthquakes cause significant mass changes on and under the Earth＇s surface,GRACE provides a new means from space to observe mass redistribution due to earthquake deformations. GRACE serves as a good complement to other earthquake measurements because of its extensive spatial coverage and being free from terrestrial restriction. During its over 10 years mission,GRACE has successfully detected seismic gravitational changes of several giant earthquakes,which include the 2004 Sumatra–Andaman earthquake,2010 Maule（Chile） earthquake,and 2011 Tohoku-Oki（Japan） earthquake. In this review,we describe by examples how to process GRACE timevariable gravity data to retrieve seismic signals,and summarize the results of recent studies that apply GRACE observations to detect co- and post-seismic signals and constrain fault slip models and viscous lithospheric structures. We also discuss major problems and give an outlook in this field of GRACE application.

Water storage changes in North America retrieved from GRACE gravity and GPS data

As global warming continues,the monitoring of changes in terrestrial water storage becomes increasingly important since it plays a critical role in understanding global change and water resource management.In North America as elsewhere in the world,changes in water resources strongly impact agriculture and animal husbandry.From a combination of Gravity Recovery and Climate Experiment（GRACE） gravity and Global Positioning System（GPS） data,it is recently found that water storage from August,2002 to March,2011 recovered after the extreme Canadian Prairies drought between 1999 and 2005.In this paper,we use GRACE monthly gravity data of Release 5 to track the water storage change from August,2002 to June,2014.In Canadian Prairies and the Great Lakes areas,the total water storage is found to have increased during the last decade by a rate of 73.8 ± 14.5 Gt/a,which is larger than that found in the previous study due to the longer time span of GRACE observations used and the reduction of the leakage error.We also find a long term decrease of water storage at a rate of-12.0 ± 4.2 Gt/a in Ungava Peninsula,possibly due to permafrost degradation and less snow accumulation during the winter in the region.In addition,the effect of total mass gain in the surveyed area,on present-day sea level,amounts to-0.18 mm/a,and thus should be taken into account in studies of global sea level change.

Surface deformation and gravity changes caused by dilatancy in a layered elastic-vis-coelastic half space

Surface deformation and gravity changes caused by dilatancy with magmatic intrusion (Mogi model) in two elastic layers overlying a Maxwell viscoelastic half space were systematically studied. We focused on the numerical calculation technique and the discussion of the effects of rheological properties of Maxwell half spqce on vertical displacements and gravity changes. The results show that the rheological properties of materials has an apparent effect on the results, when the intrusion was placed under Moho discontinuity, and has smaller effect if the intrusion was placed inside the crust. Therefore the elastic model is valid for considering the gravity changes or deformation caused by dilatancy with magmatic intrusion within the crust. The model and numerical calculation technique in this study can be used to model and interpret the long term deformation and gravity observations in volcanic, seismic and geothermal regions.

Analysis of Gravity Changes before and after the Akto MS6.7 Earthquake in 2016

This paper analyzes the characteristics of time sequence changes of gravity points near the epicenter,different changes of measuring lines and gravity changes of measuring areas in point-line-area manner respectively with the 5-period mobile gravity data through densified observation by the South Xinjiang Observation Network after the 2015-2016 Akto earthquake in Xinjiang. The gravity observation results before the earthquake indicate that the Wuqia-Bulungkol area near the epicenter presented the trend of gravity value increasing since 2015,but the gravity value decreased half a year before the earthquake,and witnessed a high gradient zone of gravity changes during some periods before the earthquake. The gravity observation results after the earthquake show that there is a trend of opposite changes in gravity difference on the northern and southern sides of Bulunkou,and good correspondence exists between the characteristics of gravity field changes near the epicenter before and after the earthquake and the geologic structure distribution in the area.

Coulomb stress and gravity changes associated with the 2016 Mw 7.8 Kaikoura Earthquake, New Zealand: Application for aftershock triggering and fault interaction process analysis

The Kaikoura earthquake on November 14,2016 is one of the largest and most complex earthquakes in New Zealand since 1947.Despite the fact that it has ruptured about 12 separate faults,triggered 2132 aftershocks within one week of the mainshock and induced considerable stress changes,few studies have been conducted to comprensively investigate the characteristics.The current study examines the horizontal and vertical displacements as well as the stress and gravity changes,aftershock distributions and also find out whether these changes affect the surrounding regions along the complex fault systems.The study covers the entire area affected by the Kaikoura event,which includes the northern part of the South Island and the southern part of the North Island.The dislocation theory was employed to evaluate the coseismic slip model on the multiple faults.The displacement results revealed that the maximum horizontal displacement is about 6 m and the vertical about 2 m,which are reasonably consistent with earlier study findings.Besides,the stress and gravity changes are quite complicated and inhomogeneous as evidenced by our coseismic model,demonstrating the complexity of the Kaikoura earthquake as well.Almost all the aftershocks are distributed in places where the stress and gravity change are found to be significant.In order to investigate the stability of our stress change models,we applied different friction coefficients and receiver fault parameters.The results justify the friction coefficient(μ=0.4)and the receiver fault parameters(230°,70°,150°)are suitable to define good stress change estimates.According to the stress change results at 15 km depth,the northern parts of the mainshock region,Hundalee fault,Humps fault and Jordan thrust areas together with the Wellington area are closer to failure and situated in a seismic risk zone.The multidimensional analysis adopted in this paper is helpful for making decisions and applications of stress and gravity change models in assessing seismic hazards.

Gravity changes and surface deformations due to faults with different geometry

Based on the formulae of the gravity changes and surface deformations raised by the dislocation of a point source,the gravity changes and deformations caused by the dislocations of fault with arbitrary geometry are computed by using numerical method. The results show that both of the dislocation and the geometry of the fault plane are the basic elements that determine the gravity and deformation effects. Gravity changes, vertical deformations and apparent vertical deformations induced by the dislocation are alike in their characteristic patterns. The similarities and differences of these patterns provide us a probability in acquiring the gravity and deformation anomalies due to faulting from the observed data. Thus the geometric and kinematic features of the earthquake-generating faults can be appropriately distinguished and evaluated.