JSG T.25

JSG T.25: Combining geodetic and geophysical information for probing Earth’s inner structure and its dynamics

Chairs: Robert Tenzer (Hong Kong)
Affiliation: Commissions 2 and 3, GGOS

Introduction

The seismic tomography is primarily used to provide images of the Earth’s inner structure based on the analysis of seismic waves due to earthquakes and (controlled) explosions. This technique involves several different methods for processing P-, S- and surface waves on the principle of solving inverse problems for finding locations of reflection and refraction of wave pathways in order to create topographic models. In this way, 3D models of P- and S-wave seismic velocity anomalies are obtained which can be interpreted as structural, thermal or compositional variations inside the Earth. Focusing on the Earth’s density structure, the conversion between seismic velocities and mass densities are adopted to construct regional or global seismic density models of the crust and the mantle. Two major limiting aspects restrict possibilities of recovering Earth’s density structure realistically. The first one is practical. Since active seismic experiments are relatively expensive, large parts of the world are not yet covered sufficiently by seismic surveys, most remarkably most of world’s oceans as well as remote parts of Antarctica, Greenland, Africa and South America. The other aspect is of a theoretical nature. The determination of mass density from seismic data could be ambiguous while affected by many uncertainties, meaning that the relationship between seismic velocities and mass densities is not unique. Actually, the density structure inside the Earth is controlled by many factors such a thermal state or mineral composition.

Gravity data has been used to interpolate the information about the Earth’s density structure (or density interfaces) where seismic data coverage is uneven or sparse. The National Geospatial-Intelligence Agency in conjunction with its partners from around the world has begun to develop a new global gravitational model, EGM2020, which should be released to public in 2020. EGM2020 should significantly improve the accuracy (as well as the actual resolution) of the global Earth’s gravity field. This will be achieved by incorporating new data sources and procedures. Updated satellite gravity information from the GOCE and GRACE missions will better support the lower harmonics, globally. Multiple new acquisitions (terrestrial, airborne and shipborne) of gravimetric data over specific regions, will provide improved global coverage and resolution over the land as well as for coastal and some oceanic areas. Ongoing accumulation of satellite altimetry data will contribute to refinement and accuracy improvement of the marine gravity field, most notably in polar and near-coastal regions. A significant improvement is also anticipated over large remote regions in Africa, South America, Greenland and Antarctica. EGM2020 will provide opportunities to improve the current knowledge about the Earth’s inner structure and processes particularly in regions with a low seismic data coverage. Gravimetric interpretation of the Earth’s inner density structure is, however, a non-unique problem because infinity many density configurations could be attributed just to the one gravity field solution. Moreover, the gravity inversion is (in a broader mathematical context) an ill-posed problem.

To overcome partially theoretical deficiencies and practical restrictions of both, seismic and gravimetric methods for the recovery of the Earth’s inner density structure, techniques for a combined or constrained inversions of gravity and seismic data are optimally applied, while incorporating additional geophysical, geological and geodynamic constraints. Many such methods already exist or could be developed and further improved within the framework of scientific activities of members of this (multidisciplinary) study group over the next four years. This is achievable, given their expertise in the field of geodesy, geophysics, mathematics and to some extent also geology.

We expect that our research activities will substantially contribute to the current knowledge of the lithospheric structure, focusing on continental regions of Africa and South America and other continents where seismic data are sparse. Our ongoing research already involves Antarctica and central part of Eurasia. Moreover, a special attention will be given to study the lithospheric structure beneath the Indian Ocean, which is probably the most complex, but the least understood. Despite the lithosphere is the most heterogeneous layer inside the Earth, large lateral structural irregularities are still present even deeper within the mantle below the lithosphere-asthenosphere boundary that are mainly attributed to the mantle convection pattern. The combined gravity and seismic data will be exploited in order to improve existing global or continental-scale mantle density models. A further improvement of the knowledge on the Earth’s inner structure is important, among many other subjects, also for a better understanding of the response of the lithosphere to the mantle convection. This involves numerous study topic, including but not limited to the compensation stage of the crust/lithosphere, the lithospheric strength, mechanisms behind the oceanic subduction, the relation between the mantle convection pattern and the global tectonic configuration (and its spatio-temporal variations), the glacial isostatic adjustment, volcanic processes, or geo-hazard. The members of this study group will address some of these aspects within the following overall objectives.

Objectives

• Improvement of (regional and continental-scale) lithospheric density models based on combining geodetic and geophysical data and additional geological constraining information, focusing mainly on regions with insufficient seismic data coverage. Special emphasis will be given to Africa, Greenland and South America. Studies will involve also Indian and Pacific Oceans.
• Development of a preliminary global density model of the mantle bellow the lithosphere-asthenosphere boundary based on the combined analysis of seismic and gravity data, focusing on the seismic data conversion to mass densities within the gravimetric inversion scheme constrained by geothermal, geochemical, geodynamic and other information.
• Contribution to a better understanding of the interaction between the mantle dynamics and the lithospheric state and structure.

Program of Activities

• Presenting research findings at major international geodetic or geophysical conferences, meetings and workshops.
• Interacting with related IAG Commissions and GGOS.
• Monitoring research activities of the JSG members and of other scientists, whose research interests are relevant to the scopes of the JSG.
• Organizing a session at the Hotine-Marussi Symposium 2022.
• Providing bibliographic list of publications from different branches of science relevant to JSG scopes.

Members

Robert Tenzer (Hong Kong), chair
Aleksej Baranov (Russia)
Mohammad Bagherbandi (Sweden)
Carla Braitenberg (Italy)
Wenjin Chen (China)
Róbert Čunderlík (Slovakia)
Franck EK Ghomsi (Cameroon)
Mirko Reguzzoni (Italy)
Lars Sjöberg (Sweden)