Data on the Earth Gravity Field
New Methods of Interpretation of New Satellite Data on the Earth Gravity Field and Its Temporal Variations: Development and Applications
Tech Area / Field
- OBS-NAT/Natural Resources and Earth Sciences/Other Basic Sciences
- ENV-SEM/Seismic Monitoring/Environment
- INF-SOF/Software/Information and Communications
8 Project completed
Senior Project Manager
Rudneva V Ya
Research Institute of Aviation Systems, Russia, Moscow
- Institute of Physics of the Earth, Russia, Moscow
- Ecole et Observatoire des Sciences de la Terre, France, Strasbourg\nInstitut de Physique du Globe, France, Paris
Project summaryDuring last years big progress has been achieved in the global gravity field measurements from satellites. New satellite system GRACE launched in 2002 increased by almost two orders of magnitude the accuracy of the first 90 harmonics in the spherical expansion of the global gravity field and for the first time permits registration of its temporal variations. New satellite GOCE (under preparation) will provide high accuracy gravity field models containing 250 spherical harmonics. Methods of the global gravity field interpretation develop considerably slower; there are almost no methods for interpretation of the temporal variations.
The main objective of the project is to develop new methods and software for interpretation of the global gravity field and its temporal variations. The main problem is that satellite data characterize mainly the gravity field of regional tectonic structures, which include the gravity effect of many density interfaces and causative bodies situated in the Earth crust and the upper mantle. The innovative idea of the project is in use of geodynamic models as a basis for geophysical data interpretation. To realize this approach it is necessary to construct new detailed geodynamic models of regional tectonic structures (orogenic belts, continental rifts, subduction zones). For this purpose high-efficiency software U-Way, designed in the weapon State Research Institute of Aviation Systems (SRIAS) will be developed and adjusted to geodynamic applications. U-Way is based on finite element method and is able to solve wide range of numerical problems. New analytical solutions of the theory of elasticity for domains containing singularities, which were developed in SRIAS recently, will be employed also to model lithospheric plate interactions. Within the frameworks of geodynamical models topography of all interfaces, distribution of the physical properties and as a consequence model gravity field and its temporal variations are functions of a number of parameters characterizing initial and boundary conditions (e.g. initial topography and distribution of physical properties; intraplate forces and mantle drag). These parameters are the target of interpretation. Considerable part of the model parameters can be obtained a priory from seismic, geothermal and other data (e.g. thickness of lithospheric layers, position of faults, distribution of the temperature and as a consequence, rheological profiles). It reduces dimension of the gravity inverse problem, makes its solution unique and stable and allows joint interpretation of all available geological and geophysical data.
The project will require resolving the following problems: (1) To develop new, high-accuracy methods of processing and interpretation of the global gravity field and its temporal variations, including new methods of joint interpretation based on geodynamic models and methods based on fuzzy logic and artificial intelligence. (2) To create new geodynamic models of formation and evolution of different tectonic structures. This work will largely rely on the new FEM software U-Way and the new analytical solutions for singular domains developed in SRIAS recently. (3) To study connections between gravity field variations and variations of the Earth crust state of stress. A method to recover of short-term variations of the state of stress is being developed in the Institute of Physics of the Earth (IPE) recently. (4) To create, on the basis of methods to be developed, new software for interpretation of the global gravity field and its variations. To apply it to data from test areas located in tectonically-active zones as well as to data for the Caucasus, Baikal and subduction zones of the Pacific Ocean.
At present 12 Earth gravity models covering the interval from August 2003 to August 2004 are available. These models include spherical harmonics coefficients as well as error estimates in the harmonics of different order. Using there data we investigated whether temporal variations of the gravity field caused by tectonic deformations in locked areas of subduction zones can be recognized in satellite gravity data. We considered the following problems: (1) to recognize temporal variations of the Earth gravity field caused by an earthquake assuming that all parameters of the fault plane are known except the modulus of the displacement vector; (2) to discriminate between several possible earthquake fault plane models using satellite gravity data; (3) to test whether temporal variations contain a linear in time trend caused by stain accumulation in a partly locked fragment of a subduction zone, assuming that a set of monthly gravity models is available. It is worth noting, that the first GRACE gravity models are almost two orders of magnitude more accurate than the gravity models based on data from all previous satellite missions, even GRACE did not achieved its target accuracy yet. It is very likely that the accuracy of the following GRACE models will increase; thus we stated a problem of gravity signals recognition assuming different levels of data accuracy, ranging from the first GRACE gravity model to its target accuracy. We developed and applied a statistical technique of signal recognition in noisy data. To model stress accumulation and release in locked areas of subduction zones we used a model of a dislocation in an elastic half-space. Parameters of a locked area or a fault plane were constrained using repeated surface geodetic and gravity measurements and seismological data, thus we used considered above approach to joint analysis of geophysical data based on geodynamic modelling. We concluded that using the developed technique, gravity field variations similar to those caused by Alaska-1964 earthquake should be recognizable in GRACE data at the accuracy level of the first GRACE models. Improvement of accuracy of forthcoming satellite gravity models to GRACE target accuracy permits recognition of considerably smaller earthquake-induced signals similar to Hokkaido-2003, which occurred after the launch of the GRACE mission. For many earthquakes there are several equivalent fault plane models, especially for those which epicentre is below the sea level. We demonstrated that even at the present level of the data accuracy it is possible to distinguish between different fault plains for big earthquakes such as Chile-1960 one. Gravity data corresponding to the GRACE target accuracy will allow the discrimination between fault plane models for smaller magnitude earthquakes, similar to Hokkaido-2003. We also demonstrated that if forthcoming gravity models are one order of magnitude more accurate compared to the first GRACE model (being one order of magnitude less accurate than the target accuracy) then GRACE data will allow recognition of time varying gravity signals associated with locked areas of subduction zones, i.e. monitoring of the process of earthquake preparation. Papers devoted to these results have been accepted for publication in EPSL and Physics of the Solid Earth (proceedings of Russian academy of Sciences). They also were presented at 2004 annual meeting of European Geosciences Union (Nice, April, 2004) и Joint CHAMP/GRACE Science Meeting (GFZ, Potsdam, July, 2004).
The project will allow achieving progress in resolving a number of fundamental problems, including: development of the theory of interpretation of global gravity field and its temporal variations; creation of new geodynamic models; studying of interrelations between temporal variations of the gravity field, the Earth crust state of stress and seismisity.
The main practically valuable results are: creation of new highly-efficient software for geodynamic modeling and interpretation of satellite gravity data; results on structure and geodynamics of the Caucasus, Baikal and subduction zones of the Pacific Ocean.
Project participants possess profound experience in scientific research in the fields related to the project. SRIAS developed new mathematical methods and software necessary for creation of new geodynamic models. IPE develops new methods and software for processing and interpretation of gravity data, monitoring of the Earth crust state of stress. At present GRACE satellites are in health condition. Our estimates showed that even target accuracy has not been achieved yet, first gravity data are sufficient for studying and monitoring of a number of tectonic processes.
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