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.

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.

Estimating Field Source Parameters of Gravity Change in North China Based on the Euler Deconvolution Method

Based on the absolute and relative gravity observations in North China from 2009 to 2014,spatial dynamic variations of the regional gravity field are obtained. We employed the Euler deconvolution method and the theoretical model to get the best estimates of parameters. Gravity field change caused by the depth and distribution in North China is calculated by back analysis. The results show the structural index that equals 1 is suitable for inversion of the gravity variation data. The inversion results indicate that the depths of anomaly field sources are spread over the Hetao fault. The research method of this paper can be used in the quantitative study on the field source and may shed new light on the interpretations of gravity change, and also provide quantitative basis for earthquake prediction index criterions based on the gravity change.

Dependence of Gravity Induced Absorption Changes on the Earth’s Magnetic Field as Measured during Parabolic Flight Campaigns

Various spectroscopic experiments performed on the AIRBUS ZERO G—located in Bordeaux, France—in the years 2002 to 2012 exhibit minute optical reflection/absorption changes (GIACs) as a result of gravitational changes between 0 and 1.8 g in various biological species such as maize, oats, Arabidopsis and particularly Phycomyces sporangiophores. During a flight day, the AIRBUS ZERO G conducts 31 parabolas, each of which lasts about three minutes including a period of 22 s of weightlessness. So far, we participated in 11 parabolic flight campaigns including more than 1000 parabolas performing various kinds of experiments. During our campaigns, we observed an unexplainable variability of the measuring signals (GIACs). Using GPS-positioning systems and three dimensional magnetic field sensors, these finally were traced back to the changing earth’s magnetic field associated with the various flight directions. This is the first time that the interaction of gravity and the Earth’ magnetic field in the primary induction process in living system has been observed.

Time-varying gravity field model of Sichuan-Yunnan region based on the equivalent mass source model

High-precision time-varying gravity field is an effective way to study the internal mass movement and understanding the spatio-temporal evolution process of the geodynamic system.Compared to the satellite gravity measurement,the repeated terrestrial gravity observation can provide a more high-order signal related to the shallow crust and subsurface.However,the suitable and unified method for gravity model estimation is a key problem for further applications.In this study,we introduce the spherical hexahedron element to simulate the field source mass and forward model the change of gravity field located at the Sichuan-Yunnan region(99—104°E,23—29°N)in the four epochs from 2015 to 2017.Compared to the experimental results based on Slepian or spherical harmonics frequency domain method,this alternative approach is suitable for constructing the equivalent mass source model of regional-scale gravity data,by introducing the first-order smooth prior condition of gravity time-varying signal to suppress the high-frequency component of the signal.The results can provide a higher spatial resolution reference for regional gravity field modeling in the Sichuan-Yunnan region.

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.

Study of the Gravity Changes before and after the Jingtai M_S5.9 Earthquake

By systematically analyzing the data of gravity reiteration in the Hexi region and taking a dynamic viewpoint, we have studied the evolution characters of gravity field during the preparation-occurrence of the Jingtai M S5.9 earthquake of June 6, 2000. The patterns of dynamic change of the gravity field clearly reflected how the gravity field evolved from the quasi-homogeneous state to non-homogeneous state for earthquake preparation and then the earthquake occurred. Besides, we have also studied the relation between the characteristic gravity change and strong earthquake activity.

Co-seismic slip distribution of the 2011 Tohoku(MW 9.0)earthquake inverted from GPS and space-borne gravimetric data

Data obtained by GRACE(Gravity Recovery and Climate Experiment) have been used to invert for the seismic source parameters of megathrust earthquakes under the assumption of either uniform slip over an entire fault or a point-like seismic source.Herein, we further extend the inversion of GRACE long-wavelength gravity changes to heterogeneous slip distributions during the 2011 Tohoku earthquake using three fault models:(Ⅰ) a constant-strike and constant-dip fault,(Ⅱ) a variable dip fault, and(Ⅲ) a realistically varying strike fault. By removing the post-seismic signal from the time series, and taking the effect of ocean water redistribution into account, we invert for slip models I, II, and III using co-seismic gravity changes measured by GRACE, de-striped by DDK3 decorrelation filter. The total seismic moments of our slip models, with respective values of 4.9×10^(22) Nm, 5.1×10^(22) Nm, and 5.0×10^(22) Nm, are smaller than those obtained by other studies relying on GRACE data. The resulting centroids are also located at greater depths(20 km, 19.8 km,and 17.4 km, respectively). By combining onshore GPS, GPS-Acoustic, and GRACE data, we obtain a jointly inverted slip model with a seismic moment of 4.8×10^(22) Nm, which is larger than the seismic moment obtained using only the GPS displacements. We show that the slip inverted from low degree space-borne gravimetric data, which contains information at the ocean region, is affected by the strike of the arcuate trench. The space-borne gravimetric data help us constrain the source parameters of a megathrust earthquake within the frame of heterogeneous slip models.

Monthly gravity field solution from GRACE range measurements using modified short arc approach

In this paper we present a series of monthly gravity field solutions from Gravity Recovery and Climate Experiment（GRACE） range measurements using modified short arc approach,in which the ambiguity of range measurements is eliminated via differentiating two adjacent range measurements.The data used for developing our monthly gravity field model are same as Tongji-GRACEOl model except that the range measurements are used to replace the range rate measurements,and our model is truncated to degree and order 60,spanning Jan.2004 to Dec.2010 also same as Tongji-GRACE01 model.Based on the comparison results of the C_（2,0）,C_（2,1）,S_（2,1）,and C_（15,15）,S_（15,15）,time series and the global mass change signals as well as the mass change time series in Amazon area of our model with those of Tongji-GRACE01 model,we can conclude that our monthly gravity field model is comparable with Tongji-GRACE01 monthly model.

Monthly gravity field recovery from GRACE orbits and K-band measurements using variational equations approach

The Gravity Recovery and Climate Experiment（GRACE） mission can significantly improve our knowledge of the temporal variability of the Earth＇s gravity field.We obtained monthly gravity field solutions based on variational equations approach from GPS-derived positions of GRACE satellites and K-band range-rate measurements.The impact of different fixed data weighting ratios in temporal gravity field recovery while combining the two types of data was investigated for the purpose of deriving the best combined solution.The monthly gravity field solution obtained through above procedures was named as the Institute of Geodesy and Geophysics（IGG） temporal gravity field models.IGG temporal gravity field models were compared with GRACE Release05（RL05） products in following aspects：（i） the trend of the mass anomaly in China and its nearby regions within 2005-2010; （ii） the root mean squares of the global mass anomaly during 2005-2010; （iii） time-series changes in the mean water storage in the region of the Amazon Basin and the Sahara Desert between 2005 and 2010.The results showed that IGG solutions were almost consistent with GRACE RL05 products in above aspects（i）-（iii）.Changes in the annual amplitude of mean water storage in the Amazon Basin were 14.7 ± 1.2 cm for IGG,17.1 ± 1.3 cm for the Centre for Space Research（CSR）,16.4 ± 0.9 cm for the GeoForschungsZentrum（GFZ） and 16.9 ± 1.2 cm for the Jet Propulsion Laboratory（JPL） in terms of equivalent water height（EWH）,respectively.The root mean squares of the mean mass anomaly in Sahara were 1.2 cm,0.9 cm,0.9 cm and 1.2 cm for temporal gravity field models of IGG,CSR,GFZ and JPL,respectively.Comparison suggested that IGG temporal gravity field solutions were at the same accuracy level with the latest temporal gravity field solutions published by CSR,GFZ and JPL.

Analyses on gravity variation before and after the Lijiang earthquake based on a finite rectangular dislocation model

The methods were discussed to calculate the gravity variation due to crustal deformation based on a model of dis-location on a finite rectangular plane. Taking the Lijiang MS=7.0 earthquake as an example the calculating princi-ple of fault parameters were determined, and the results were given. Of particular interests were the characteristics of the gravity variations in different dislocation types. With comparison between the calculated results and the practical measurements, it was found that the model could to some extent account for the observations. But it failed to give explanations to the more far spatial gravity variation.

Far-field deformations caused by the 2004 Sumatra earthquake

Theoretical horizontal displacements caused by the 2004 Sumatra earthquake in the Sichuan-Yunnan area have been calculated according to a spherical dislocation theory and an earthquake-fault model. The results show that the theoretical displacements are basically consistent with the GPS observations in situ. On this basis,we have calculated the co-seismic displacements, strains, changes of gravity and geoid of the whole Earth, including China mainland and vicinity, caused by this earthquake. Key wards：

三亚M_(S)4.2地震前后海南岛重力场变化及其小波多尺度分解

利用海南岛流动重力测网2016~2022年共13期复测资料,分析研究区域差分重力场和累积重力场动态变化,并着重分析2019-08-20海南三亚M_(S)4.2地震前后海南岛西南地区重力场时空变化。结果表明,三亚M_(S)4.2地震震中附近地区在震前2 a和3 a出现重力场累积变化梯度带,梯度带的异常范围约60 km,重力场变化最大约为60μGal;三亚M_(S)4.2地震之后,震中附近地区重力场继续上升,至2020年下半年才开始协同下降,在下降过程中,发生以2021-08-08乐东MS3.2地震为最大地震的震群事件。对2016-09~2019-08海南岛累积重力场动态变化小波多尺度分解表明,海南岛西南地区重力场变化在3阶、4阶和5阶小波细节图中均出现异常。

SWARM卫星时变重力场反演及其精度

基于SWARM卫星的精密轨道数据,利用短弧积分法解算2015-01~2021-12共84个月的40阶次TVG-SWARM月时变重力场模型,并与ASU、COST-G、IGG和ITSG等机构的月时变重力场模型进行比较。结果表明:1)从大地水准面阶误差与模型位系数误差谱看,不同SWARM模型的低阶位系数精度相当,特别是前10阶均与ITSG-GRACE/GRACE-FO接近;2)不同SWARM模型与ITSG-GRACE/GRACE-FO模型全球陆地水储量变化趋势空间分布具有较好的一致性,在亚马孙流域、格陵兰岛、密西西比河流域和西西伯利亚等区域,TVG-SWARM与ITSG-GRACE/GRACE-FO模型的趋势差值分别为0.23 cm/a、0.27 cm/a、0.57 cm/a和0.47 cm/a,相关系数均达到0.85以上,并与IGG-SWARM模型结果最为接近。本文研究结果证明了TVG-SWARM模型精度可靠,可以用于监测大尺度陆地水储量变化。

重力场动态变化与汶川M_s8.0地震孕育过程

基于中国大陆1998-2007年(复测周期2-3年)流动重力观测数据,结合GPS、水准观测成果和区域地质构造动力环境,分析研究了汶川8.0级地震区域重力场动态变化演化特征和孕震机理.结果表明:区域重力场动态演化大体反映了青藏高原物质东流的动态效应和汶川大震孕育的中长期(2-10年)信息;汶川大震孕育的显著重力标志为震中西南持续多年的正重力变化(上升)和出现较大规模的重力变化梯级带,前者有利于地震能量的不断积累,后者有利于地震剪切破裂的发生;与地震孕育相关重力场变化总体呈增大—加速增大—减速增大—发震的过程;8年累积重力变化幅差最大约200×10^(-8)m·s^(-2);2001年昆仑山口8.1级地震孕育发生和震后恢复调整,对区域重力场动态变化和汶川大震的孕育发展具有重要影响;松潘—甘孜块体一般呈现负重力变化,可能反映深部壳幔局部上隆、壳内温度较高而膨胀,有利于逆冲或推覆体运动的形成和大震的发生.

2008年于田Ms7.3地震前重力场动态变化特征分析

基于1998年以来的中国大陆流动重力网观测数据,分析2008年3月新疆于田7.3级地震前区域重力场动态变化的演化特征。结果表明:区域重力场动态演化图像反映了该地震孕育的中长期(2～10年)基本信息;该地震孕育时的显著重力标志为持续多年的正重力变化(上升)和较大规模的重力变化梯级带,前者有利于地震能量的不断积累,后者有利于地震破裂的发生;该地震孕育过程中相关重力场变化呈增大-加速增大-减速增大的过程,与1976年唐山7.8级地震具类似特征;与该地震有关的多年(8年)最大累积重力变化达200×10-8ms-2。

地面重力时空变化向卫星高度的解析延拓

为研究卫星在不同高度对地面重力变化的检测能力,利用地面获得的重力场时间和空间变化,采用空间域向上解析延拓原理,将地面数据向上解析延拓到数百千米的不同高度。计算分析表明,如果地表存在93×10-8ms-2的重力变化量,在450km高度可观测到20×10-8ms-2的重力变化。所以利用卫星重力观测技术进行地震监测,其观测精度应优于20×10-8ms-2。

2016年青海门源M_S6.4地震前重力变化

利用青藏高原东北缘2011—2015年期间的流动重力观测资料,系统分析了区域重力场变化及其与2016年1月21日青海门源MS6.4地震发生的关系,结合GNSS、水准观测成果和区域地质构造动力环境,进一步研究了区域重力场变化的时空分布特征及其机理.结果表明:(1)测区内重力场异常变化与祁连山断裂带在空间上关系密切,反映沿祁连山断裂带(段)在2011—2015年期间发生了引起地表重力变化效应的构造活动或变形.(2)门源MS6.4地震前,测区内先出现了较大空间范围的区域性重力异常,到临近发震前显示出相对闭锁的现象,且围绕震中区周围出现四象限分布特征的局部重力变化,地震发生在重力反向变化过程中,并出现显著的四象限分布特征的重力异常变化,其中,青海门源与甘肃天祝一带重力差异变化达100×10-8 m·s-2以上.(3)区域重力场动态演化大体反映了青藏高原东北缘物质北东流的动态效应,门源震中附近区域地壳受挤压变形显著、面压缩率和重力剧烈变化的特征最为显著.(4)重力场的空间分布及其随时间变化与地壳垂直与水平运动及地质构造活动等观测结果有一定的对应关系,强震易发生在重力变化四象限分布中心地带或正、负异常区过渡的高梯度带上.

数值模拟估算低低卫-卫跟踪观测技术反演地球重力场的空间分辨率

从两个方面模拟研究了低低卫-卫跟踪观测技术恢复地球重力场的空间分辨率.利用重力位系数作为扰动量,积分30天的轨道,研究重力位系数变化引起低低卫-卫跟踪星间距离和速率变化,结果表明,对于地球重力场模型EGM96的前12 0阶,99 8%和97%的位系数扰动引起星间距离和速率变化的均方差大于1×10 - 5m和1×10 - 7m s ,并且星间距离观测值对地球重力场的反应更为敏感.不考虑非保守力误差的影响,用随机误差为1×10 - 5m和1×10 - 6 m s的星间距离和速率变化作模拟观测量,恢复了78阶地球重力场位系数,结果表明,采用随机误差为1×10 - 5m的星间距离恢复地球重力场的精度明显高于1×10 - 6 m s的星间速率结果,但是如果考虑非保守力误差影响,则星间测速的优越性大大增强.

2009年姚安Ms6.0地震重力场前兆变化机理

利用2009年姚安Ms6.0地震前滇西实验场流动重力网2005—2009年9期复测数据,给出了区域重力场差分和累积动态变化图像。分析研究表明:1)重力场动态演化图像大体反映出震前地壳物质运动状态,累积动态变化更能反映区域构造运动与断裂构造作用,差分动态变化有利于突出短期局部效应;2)姚安地震显著重力"前兆"标志为:震前约2年穿过震中的近南北向正负过渡重力梯级带和约半年的以震中区为中心重力场变化呈现局部相对上升与下降的对称四象限分布;3)震中西部震前约3年持续的正重力变化应有利于震源能量的积累;4)震中区相对变化四象限分布图像反映出孕震体先存剪应力,为此提出了"闭锁剪力"前兆模式。