V. O. Mikhailov

Deep structure of the Baikal rift zone revealed by joint inversion of gravity and seismology

[1] The question of plate boundary forces and deep versus shallow asthenospheric uplift has long been debated in intracontinental rift areas, particularly in the Baikal rift zone, Asia, which is colder than other continental rifts. As previous gravity and teleseismic studies support the dominance of opposing mechanisms in the Baikal rift, we reconsidered both data sets and jointly inverted them. This more effective approach brings insight into location of the perturbing bodies related to the extension in this region. Our new joint inversion method allows for inverting the velocity-density relationship with independent model parametrization. We obtain velocity and density models that consistently show (1) crustal heterogeneities that coincide with the main tectonic features at the surface, (2) a faster and denser cratonic mantle NW of Lake Baikal that we relate to the thermal contrast between old and depleted Archean (Siberian platform) and Paleozoic orogenic belt (Sayan-Baikal belt), (3) three-dimensional topographic variations of the crust-mantle boundary with well-located upwarpings, and (4) the lithosphere-asthenosphere boundary uplift up to 70 km depth with a NW dip. Our resulting velocity and density models support the idea of a combined influence of lithospheric extension and inherited lithospheric heterogeneities for the origin of the Baikal rift zone. INDEX TERMS: 1234 Geodesy and Gravity: Regional and global gravity anomalies and Earth structure; 7218 Seismology: Lithosphere and upper mantle; 8122 Citation: Tiberi, C., M. Diament, J. Deverchere, C. Petit-Mariani, V. Mikhailov, S. Tikhotsky, and U. Achauer, Deep structure of the Baikal rift zone revealed by joint inversion of gravity and seismology,

Tensor deconvolution: A method to locate equivalent sources from full tensor gravity data

We present a method dedicated to the interpretation of full tensor gravity gradiometry FTG data called tensor deconvolution. It is especially designed to benefit from the simultaneous use of all the FTG components and of the gravity field. In particular, it uses tensor scalar invariants as a basis for source location. The invariant expressions involve all of the independent components of the tensor. This method is a tensor analog of Euler deconvolution, but has the following advantages compared to the conventional Euler deconvolution method: 1 It provides a solution at every observation point, without the use of a sliding window. 2 It determines the structural index automatically; as a consequence, the structural index follows the variations of the field morphology. 3 It uses all components of the measured full gradient tensor and gravity field, thus reducing errors caused by random noise. It is based on scalar invariants that are by nature insensitive to the orientation of the measuring device. We tested our method on both noise-free and noise-contaminated data. These tests show that tensor solutions cluster in the vicinity of the center of causative bodies, whereas Euler solutions better outline their edges. Hence, these methods should be combined for improved contouring and depth estimation. In addition, we use a clustering method to improve the selection of solutions, which proves advantageous when data are noisy or when signals from close causative bodies interfere. INTRODUCTION

Noise reduction through joint processing of gravity and gravity gradient data

In mineral and oil exploration, gravity gradient data can help to delineate small-scale features that cannot be retrieved from gravity measurements. Removing high-frequency noise while preserving the high-frequency real signal is one of the most challenging tasks associated with gravity gradiometry data processing. We present a method to reduce gravity and gravity gradient data noise when both are measured in the same area, based on a least-squares simultaneous inversion of observations and physical constraints, inferred from the gravity gradient tensor definition and its mathematical properties. Instead of handling profiles individually, our noise-reduction method uses simultaneously measured values of the tensor components and of gravity in the whole survey area, benefiting from all available information. Synthetic examples show that more than half of the random noise can be removed from all tensor components and nearly all the noise from the gravity anomaly without altering the high-frequency information. We apply our method to a set of marine gravity gradiometry data acquired by Bell Geospace in the Faroe-Shetland Basin to demonstrate its power to resolve small-scale features.

Numerical modelling of regional neotectonic movements in the northern Black Sea

Abstract The paper examines the manifestation of tectonic processes related to the Neogene and recent subsidence of the Black Sea margin and uplift of the Crimea mountains as well as current seismicity in the southern Crimea. A 2-D finite-element model, based on a seismic line across the northern Black Sea and the Crimean mountains, that predicts the present-day tectonic velocity field, using the available neotectonic data as boundary conditions, has been developed. The model results indicate that the observed structure and tectonic motions cannot be explained by forces imposed on the external side, boundaries of the model only. Rather, movements consisting of upwelling beneath the Black Sea and subsidence under the Crimea, interpreted as the effects of mantle processes, are necessary to explain the observed topographic evolution of the area. Zones where the ratio of the calculated shear stress to calculated pressure is greatest correspond to the position of the Southern Coast fault that controls the seismicity of the northern Black Sea margin.

Structure and evolution of the Molucca Sea area: constraints based on interpretation of a combined sea-surface and satellite gravity dataset

The paper presents an interpretation of the complete Bouguer gravity anomaly for the Molucca Sea area (northeast of Indonesia) in order to investigate the structure and interrelation of the main tectonic units of the region. Data on the gravity field and topography incorporate all available shipboard and satellite-derived data, including data collected during a 1994 R/V L’Atalante cruise in the Molucca Sea (MODEC). These data were compiled by weighted interpolation of surface and satellite data. The anomalous gravity field of the area contains components of different wavelengths, which we separated into regional and local anomalies using a spherical analogue of Kolmogorov^Wiener optimal (mean-square) filtering. Position and depth of the shallow lithospheric gravity sources were then estimated from the local field component by applying a new approach to Euler solution selection based on a recently developed fuzzy logic clustering method, called RODIN. The spatial distribution and depth of Euler solutions provide new information on the tectonic structure of the upper lithosphere resulting from the convergence of the Philippine Sea, Eurasian and Australian plates. The local Bouguer anomalies and dense clusters of Euler solutions make it easy to trace the Sangihe Trench further north, up to 5.5‡N, joining it to the Pujada and Miangas ridges and to trace the Miangas Ridge southwards to its junction with the Central Ridge. Seismic data revealing compressive structure and dense shallow clusters of Euler solutions suggest that the Pujada Ridge overthrusts the Miangas Ridge from the west. Clusters of Euler solutions also clearly outline an ophiolite body of the Talaud Archipelago, show main thrust zones bounding it, and trace the southern termination of the Philippine Fault horsetail structure up to 5.5^6‡N in the area southeast of Mindanao Island. Our results support the hypothesis that the Talaud Archipelago was formed in situ as an uplifted Central Ridge block. We suggest that the structure of the Archipelago and of the area to the east

Geophysical and geomechanical aspects of the study of meteorite structures

Abstract Two mechanical models of the evolution of meteorite craters are considered: the model of evolution in the lithosphere-asthenosphere, and the model of the origin of the central rise. The solution of the inverse problem of gravimetry is given within the framework of the second model. The results of the numerical simulations are consistent with the experimental data.

Crustal control on the Terek-Caspian trough evolution: constraints based on a new paleotectonic analysis method

Abstract The evolution of the Terek-Caspian trough is investigated using a numerical paleotectonic analysis method based on expanding the tectonic velocity into a series of functions that depend on spatial coordinates and time. The method relies on the same kind of data as backstripping analysis, including the thicknesses of the sedimentary strata, their formation periods and facies composition (depth of deposition). It allows investigating the rate of motion in greater detail than conventional methods, and under certain conditions may be used to estimate the rate of movements not only for periods of sedimentation, but for periods of erosion as well, and also to determine the position of deep-seated faults, their periods of activity and amount of slip. The analysis has shown that, for the Terek-Caspian trough, it is possible to represent the rate of motion as a sum of two components: the overall motion of the entire profile proportional to the square root of time, and the local component as a function of time and distance along the profile. With regard to the local component, the profile subdivides into two parts, northern and southern, which have tended to be involved in vertical motion of the respectively opposite senses. These parts are separated by a deep-seated fault. This fault has controlled the trough evolution for at least 180 Myr. The antiphase movement of its sides may be explained by assuming that the fault plane is inclined: a regional compression would then drive one side up and the other down. As the compression is removed or changed over by an extension, the sense of movement will be the opposite. Under the assumption that the fault plane plunges beneath the Caucasus, the phases of compression appear to have occurred 120–140 Myr and 50–75 Myr ago.

Joint Inversion of the Differential Satellite Interferometry and GPS Data: a Case Study of Altai (Chuia) Earthquake of September 27, 2003

Based on the data of differential satellite interferometry, the field of displacements of the Earth’s surface in the line-of-sight direction is determined for the region of the Altai Earthquake that struck on September 27, 2003. The displacements are estimated for unforested areas of Chuia and the Kurai depressions, and for a part of their mountainous surroundings. In that part of the region where unwrapping of the data was possible, the amplitude of displacements amounts up to 150 cm for Chuia and 100 cm for the Kurai depressions. In order to locate the surface of the seismic rupture and to find the field of displacements on this surface, the method for the combined inversion of the displacements data, provided by satellite interferometry (the present work) and geodesy [Gol’din et al., 2005], is suggested and applied. The admissible range of the parameters of the rupture was specified from the seismology and seismotectonics data.The combined use of geodetic and satellite interferometry data makes the solution of the inverse problem more stable and yields a seismic momentum estimate, which is consistent with the seismological determinations. We discuss the possible contributions of various postseismic processes; in particular, based on analyzing the energy of the aftershocks, we assess the contribution of the postseismic creep to the displacements, determined from the interferometry and geodesy data, for different coseismic and postseismic time intervals.

Reservoir induced seismicity in the Koyna–Warna region, India: Overview of the recent results and hypotheses

The state of the art in the geological and geophysical study of the region of Koyna and Warna water reservoirs is reviewed. The probable geodynamical factors of induced seismicity are discussed. The detailed geophysical surveys, satellite geodetic data, and time history of the seismicity in the region reveal a complicated pattern of the structure and recent geodynamics of the region. The existing data suggest that the induced seismicity is here most likely to be caused by the regional (intraplate) stresses driving the displacements along the orthogonal network of the faults whose strength has dropped and continues decreasing due to the reservoir impoundment and operation processes. The evolution of the seismicity which started immediately after the rapid filling of the Koyna reservoir in the region of the dam, then rapidly expanded southwards and eventually became concentrated in the region of the subsequently constructed Warna reservoir shows that seismic events can be initiated by a number of factors whose contributions may vary with time. The key ones among them include reservoir loading and its seasonal variations; water saturation of the faults which guide the propagation of the front of fracture, increased permeability, and, probably, mineral transformations (hydrolysis) under the water level fluctuations in the reservoirs; and displacement of the front of the high pore pressure down to the main source zone of the earthquakes at a depth of 6–8 km. Based on the analysis presented in the paper, we outline the directions of the future research aimed at studying the nature and dynamics of induced seismicity in the region of large water reservoirs.

Some problems of landslide monitoring using satellite radar imagery with different wavelengths: Case study of two landslides in the region of Greater Sochi

The problems of processing and interpreting the data provided by radar satellite interferometry for the conditions of landslides covered by vegetation are analyzed in two case studies of landslides in the Northern Caucasus in the region of Kepsha and Mamaika villages in the vicinity of the railway tunnels. The estimates of the displacement fields are obtained by the method of persistent scatterers using the StaMPS program package. The five-year experience of landslide monitoring shows that in the unfavorable conditions of satellite radar interferometry, proper selection of the strategy of satellite image processing is vital. In the present paper, we discuss, in particular, the crop selection, the selection of the master image, reference area, and digital elevation model. For the landslide located in the sparsely populated region near Kepsha village, we used the data from the ascending and descending tracks of the long-wavelength ALOS and shorter-wavelength ENVISAT satellites. For the landslide in the region of Mamaika village with a large number of different buildings serving as good scatterers for radar signals, we used the images from the ENVISAT and from TerraSAR satellite, which transmits even shorter waves. The average line-of-sight (LOS) displacement velocities VLOS for the landslide near Kepsha village measure at most 10 mm per annum, which means that this landslide has remained stable at least since 2004. The landslide in Mamaika village is significantly more active. The average LOS displacement velocities in the active part of this landslide attain 60 mm per annum. The artificial corner reflector installed on the segment of the landslide where natural scatterers of radar signal are absent made it possible to estimate the LOS displacement velocity on this segment of the slope at 49 mm per annum.