Block Motion in Geological Media
Regularities of the Block Structure Motion Due to Man-Made and Natural Influence
Tech Area / Field
- ENV-SEM/Seismic Monitoring/Environment
- OBS-NAT/Natural Resources and Earth Sciences/Other Basic Sciences
3 Approved without Funding
Institute of Dynamics of the Geosphere, Russia, Moscow
- Lawrence Livermore National Laboratory, USA, CA, Livermore
Project summaryThe effects of block movements are of great importance in solving many engineering problems – choosing locations and designing important underground and surface constructions, atomic power plants, radioactive waste depots, etc. These problems are of primary concern in seismically active regions. Besides that, developing the mechanics of block motion has a fundamental value as well, for example, to understand the physics of earthquake sources.
The aim of the project being proposed is further development of the mechanics of blocky rock mass deformation under natural and man-made dynamic effects and development of a methodology for block motion in designing engineering objects.
Our approach begins with the hypothesis that it is the specific properties and behavior of the boundaries between blocks of different hierarchical levels that must be understood to understand dynamic block motion. At the same time there are no models that adequately describe the effects of dynamic deformations on rock discontinuities under conditions found in the Earth’s crust. Most mechanical models of discontinuities are based on laboratory tests of small samples containing artificial or natural fractures. Although such investigations certainly are useful, it is obvious that application of the results of laboratory experiments to the scales, conditions, and complexity of natural events may not always be suitable.
Therefore, the first part of the research we propose focuses on understanding the strength and deformation properties of rock discontinuities in situ, and how they respond and evolve under dynamic loading.
We propose applying a new seismic technique to estimate deformation parameters of rock discontinuities that was developed in the Institute for Dynamics of Geospheres of the Russian Academy of Sciences (IDG RAS). The new technique we have developed is based on registration of the dynamic parameters of seismic waves in the vicinity of a fault or a fracture and is supported by a Russian patent. Analysis of the amplitude ratio and dominant periods of different types of waves interacting with a fault in situ allows us to estimate the normal and shear stiffnesses of a discontinuity. We have tested this thoroughly over the past few years both in the laboratory and in in situ experiments. We propose to use these data to develop physically reasonable models of the dynamic deformation of rock discontinuities.
The second part of the research being proposed is devoted to the analysis of block movements themselves, induced by dynamic effects.
We propose analyzing a unique data set of direct observations of interblock movements at different hierarchical levels – from tens of centimeters to kilometres – that result in the response to the dynamic loading from a wide range of explosion yields (TNT energy equivalents from several grams up to 150 kilotons). The IDG RAS is perhaps the only institution in the world that for many years has conducted systematic instrumental observations of interblock movements produced by large-scale underground explosions (including nuclear explosions). In addition to our own analyses of these unique data, we will make the data available to the seismological community, as they undoubtedly will be of use in other studies besides our own. Our interest is earthquake triggering and the dynamic loads from explosions are in most ways identical to those earthquakes produce. Moreover, the range of sizes and the controlled nature of explosion sources (e.g. their locations, sizes, etc. are known precisely) actually make them more useful than earthquake signals in many ways. The available data include measurements of several types: (i) residual displacements at base lengths ranging from 5 mm to 30 m; (ii) relative block displacements obtained by integrating the waveforms of particle velocity; (iii) displacements of marker points mounted both in tunnels and at the free surface using geodetic methods; (iv) inclinations of reference platforms; and (v) visual examination of tunnels.
A methodology of forecasting the parameters of block movements in a rock massif subjected to dynamic effects will be elaborated as a result of processing and summarizing the experimental data on the deformation characteristics of discontinuities.
The third part of the research contemplates studying the relaxation processes in blocky structures. We propose to complement the above explosion analyses with the study of an extensive set of data acquired from aftershocks triggered by underground nuclear explosions. In addition to quantifying the temporal and spatial patterns of explosion aftershocks, and performing analyses to infer characteristics such as localized stress fields from them and their relationship to local structure, we propose numerical simulations of the mechanical effects of an explosion in a heterogeneous medium with fault-bounded blocks.
The results of measuring the parameters of microseismic noise will be also used in the investigations of relaxation processes in the vicinity of fault zones.
Measurements of the parameters of microseisms in different frequency ranges that we have made show that both the quasi-stationary vibrations of blocks different in size – ranging from hundreds of meters to tens of kilometers – and pulse vibrations corresponding to relaxation processes of different hierarchical levels show up in the structure of seismic noise. Links between the characteristics of registered pulses and such parameters as specific block size, stress release, etc., can be established using a numerical model of the process of relaxation of a blocky structure deformed under constrained conditions.
We also plan to perform long-term measurements of microseismic noise near the deep tectonic structure of Nelidovo-Ryazan' in the vicinity of its section along the Oka river in the Moscow District, where the geophysical observatory "Mikhnevo" (MHV) of IDG RAS is located.
Implementation of the project will permit us to achieve the following results:
– Extensive experimental data on interblock dynamic motion and the effects produced by pulsed and quasi-static loads will be systematized, analyzed and published.
– Analytical and numerical models of dynamic deformation of interblock contacts and the formation of interblock movements will be developed.
– The effect of the variation of geophysical fields on regularities of accumulation and release of elastic energy in a blocky medium will be investigated.
– A methodology of forecasting the parameters of block movements at different hierarchical levels will be elaborated.
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