Carla Braitenberg

Spatial variations of flexure parameters over the Tibet-Quinghai plateau

We investigate the Tibet–Quinghai plateau and the Tarim basin in terms of spatial variations of the elastic thickness (Te) in the frame of the thin plate flexure model. The method of investigation makes use of a convolutive method, which allows high spatial resolution of the flexure properties and overcomes some of the problems tied to the spectral admittance/coherence methodologies. We study the relation between the topographic and subsurface loads and the observed crust–mantle interface (CMI) undulations, the latter having been obtained from gravity inversion. The gravity data used for the inversion are a unique set of high quality data available over the Chinese part of the plateau, and constitute the highest resolution grid today available in this impervious area. The gravity inversion is constrained by results from the study of the propagation of seismic waves. The two extensive sedimentary basins, the Tarim and the Qaidam basins, are modeled by forward gravity modeling. The oscillations of the CMI obtained from the gravity inversion agree well with those expected by loading the thin plate model of spatially variably elastic thickness with the surface and subsurface loads. It is found that the modeling of the sedimentary basins is essential in the flexure analysis. The spatial variations of elastic thickness correlate with the extensions of the different terrains that constitute the plateau. Most of the Tibet plateau has low Te, varying in the bounds 10–30 km, with lower values in the Qiangtang terrain, where the Te reaches 8 km. The Tarim and the Qaidam basins, Precambrian platforms overlain by sediments, are rigid and have a Te of up to 110 km and 70 km, respectively. The flexural analysis distinctly discerns the Tibet plateau, with thick crust, part of which is molten, from the cratonic areas, the Tarim and Qaidam basins, which though of thinner crust, act as undeformable rigid blocks.

The gravity and isostatic Moho undulations in Qinghai- Tibet plateau

Abstract It is our interest to study the Moho depths in the Qinghai–Tibet. An iterative hybrid spectral–classical methodology is applied to invert the gravity data and obtain the 3D variation in Moho depth. The gravity inversion is constrained by results from deep seismic sounding and seismological investigations. The Moho is found between 70 and 75 km depth over most of Tibet. Maximum depths of up to 80 km are found along the margins of the plateau, and shallower depths of 65 km correlate with an important suture running along central Tibet (Bangong Nujiang). At Moho level most of Tibet is isostatically compensated at 90–110%, according to the Airy isostatic model. The Qaidam basin in North-Eastern Tibet and the Tarim basin to the North-West are found to be over-compensated.

Inverse modelling of elastic thickness by convolution method – the eastern Alps as a case example

An unconventional scheme is used to estimate the flexural rigidity, or equivalently the elastic thickness of the lithosphere, given the topography and gravity data. The flexural rigidity is the parameter that governs the flexural response of the lithosphere in the frame of the thin plate flexure model. The scheme is an alternative to the widely used calculation of admittance of topography (sea-floor or continental topography) and gravity, bearing some advantages which are explained in the paper. The scheme involves the inversion of the gravity data in order to formulate a model of the crust^mantle interface (CMI) undulations. In a second step the flexure parameter is then evaluated from the relation between topography and CMI variations. Instead of calculating the admittance function using a spectral analysis, a set of point-load response functions are used in order to retrieve the optimal flexure parameter. This has two main advantages: instabilities of the numerical admittance evaluation at wavenumbers with low spectral energy in the topography are overcome and the analysis can be made over an area which is not necessarily rectangular, as required for the spectral analysis. The proposed method allows a higher space resolution of elastic thickness than any spectral method. For validation, the numerical strategy is applied to the situation of a realistic synthetic model, where all inputs and outputs are known a priori. Finally the spatial variations of the elastic thickness are studied in an area across the Eastern Alps. 4 2002 Elsevier Science B.V. All rights reserved.

Measurements and interpretations of tilt–strain gauges in seismically active areas

Abstract An overview of different aspects of deformation measurements is given. An introduction to the general theory of point-crustal deformations with an application to the stress–strain relations at the earth surface is made. The instrumentation used in crustal deformation measurements is briefly explained, emphasizing different constructions of tilt and strainmeters. The problems connected with the housing of the instruments, i.e., the cavity and topography effects, and with the ambient noise factors, as the hydrological and pressure effects, are discussed. The last part of the paper is concerned with the relation between deformation measurements and seismic events, both from a theoretical and observational standpoint. A number of examples are shown that report on pre-, co-, and post-seismic and the secular term deformation in different areas of the world.

The enigmatic Chad lineament revisited with global gravity and gravity-gradient fields

Abstract The crustal structure of northern Africa is puzzling, large areas being of difficult access and concealed by the Sahara. The new global gravity models are of unprecedented precision and spatial resolution and offer a new possibility to reveal the structure of the lithosphere beneath the Sahara. The gravity gradients correlate better than gravity with geological features such as rifts, fold belts and magmatic deposits and intrusions. They are an ideal tool to follow geological units (e.g. basement units) below a stratigraphic layer of varying density (e.g. sediments). We focus on the Chad lineament, a 1300 km arcuate feature located between the west and central African rift system. The gravity fields show differences between the lineament and the west and central African rift system. Along the centre of the lineament high-density rocks must be present, which relate to either magmatic or metamorphic rocks. This is very different to the lineaments of the western and central-west African rift system which are filled with sediments. Considering present models of rifting and the absence of topography, the lineament cannot be coeval to the west and central African rift system and is most likely older. We suggest that the lineament is a structural element of the Saharan Metacraton.

Spectral and classical methods in the evaluation of Moho undulations from gravity data: The NE Italian Alps and isostasy

Abstract The mapping of the crust-mantle boundary surface is an important geophysical task, which the method of seismic profiling has dealt with profitably. There are, however, areas where the crustal structure is not known up to the present, and where the Moho has as yet not been determined by geophysical sounding. In such areas the isostatic theory may be applied to give a first estimate of the depths of the crust-mantle boundary. However, young orogenic regions are not necessarily in isostatic equilibrium. Therefore the isostatically calculated crustmantle boundary must be corrected. In our method, the long wavelength observed gravity anomalies are inverted in an iterative process to model the crust-mantle boundary, assuming thus that the mass responsible for the observed gravity anomalies is located at the level of the crust-mantle boundary. After illustrating the proposed method in different model situations, it is applied to two N-S-oriented profiles in the NE/Italian Alps, an area only scarcely studied with seismic deep sounding. We obtain a maximum crustal thickening of 60 km in the western profile, beneath the Hohe Tauern, whereas to the east a lower value of 50 km (Niedere Tauern), is retrieved. The eastern profile shows a secondary crustal thickening further to the south, below the Dinarides. The analysis shows that the Moho depth beneath the crest of the Alps is in the order of 10 km in excess compared to what we would expect for the Airy-Heiskanen (AH) isostatic model.

Three-dimensional fold structure of the Tibetan Moho from GRACE gravity data

[1] Although the prevailing wavelength of the Moho fold has been estimated from the spectral analysis of gravity and topography, there has not been a suggested method developed to reveal its structure. Here we present a threedimensional (3D) Moho fold structure beneath Tibet which clearly reflects the continental collision. For the structure estimation a new method has been introduced based on the gravity inversion and flexural model. The estimated direction and wavelength of the Moho fold are consistent with the velocities calculated from Global Positioning System (GPS) and with an elastic plate model under horizontal compression. The prevailing wavelength of the Moho fold is estimated to be 300 to 420 km, which corresponds to an elastic plate with effective elastic thickness (EET) of about 35 km, and much smaller than the prior estimates of 500 to 700 km. Citation: Shin, Y. H., C.-K. Shum, C. Braitenberg, S. M. Lee, H. Xu, K. S. Choi, J. H. Baek, and J. U. Park (2009), Three-dimensional fold structure of the Tibetan Moho from GRACE gravity data, Geophys. Res. Lett., 36, L01302, doi:10.1029/2008GL036068.

Gravity inversion in Qinghai-Tibet plateau

Abstract Due to its high elevation and high seismicity the Qinghai-Tibet plateau takes a primary position on the earth surface. The inaccessibility of the region makes geophysical studies difficult. Active seismic sounding is available along essentially one line crossing the eastern part of the plateau. In such a situation gravity is a powerful method to obtain information on the crustal structure. We apply an inversion of the gravity field throughout the entire plateau. The inversion is limited to the long-wavelength band of the field, which has been shown by spectral analysis to be generated at lower crustal levels. The field is inverted in terms of the oscillation of a boundary layer with strong density contrast. This boundary is identified with the crust-mantle discontinuity (Moho). A map of the 3D oscillations of the Moho is presented and the properties discussed along 4 profiles cutting the plateau longitudinally and transversally.

Estimating the hydrologic induced signal in geodetic measurements with predictive filtering methods

The high precision achieved with continuous geodetic instruments makes it necessary to take ambient factors into account. Among these, one of the most disturbing is the hydrologic agent. After giving a characterization of the induced signals in the specific case of subsurface tilt and extensometric measurements, the techniques of predictive filtering are shown to solve the problem of modeling the induced signals. The results obtained here may be applied also to other continuous geodetic and gravity measurements.

A Comparative Analysis of Seismological and Gravimetric Crustal Thicknesses below the Andean Region with Flat Subduction of the Nazca Plate

A gravimetric study was carried out in a region of the Central Andean Range between and south latitudes and from and west longitudes. The seismological and gravimetrical Moho models were compared in a sector which coincides with the seismological stations of the CHARGE project. The comparison reveals discrepancies between the gravity Moho depths and those obtained from seismological investigations (CHARGE project), the latter giving deeper values than those resulting from the gravimetric inversion. These discrepancies are attenuated when the positive gravimetric effect of the Nazca plate is considered. Nonetheless, a small residuum of about 5 km remains beneath the Cuyania terrane region, to the east of the main Andean chain. This residuum could be gravimetrically justified if the existence of a high density or eclogitized portion of the lower crust is considered. This result differed from the interpretations from Project “CHARGE” which revealed that the entire inferior crust extending from the Precordillera to the occidental “Sierras Pampeanas” could be “eclogitized”. In this same sector, we calculated the effective elastic thickness (Te) of the crust. These results indicated an anomalous value of Te = 30 km below the Cuyania terrane. This is further conclusive evidence of the fact that the Cuyania terrane is allochthonous, for which also geological evidences exist.