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1 INTRODUCTION ABSTRACT: In the light of Theological property which crustal rocks have and some features of earthquake, a rheological model for faulted rocks is suggested in this paper, and the possibility for the suggested model's application to the study of earthquake mechanism is discussed. The results show that it is reasonable for faulted rocks to be simplified with the suggested model. By way of long-term observation on deformation of rocks, it is discovered that rocks have rheological properties, i.e., under the action of external force, stress and strain in the rocks are constantly changing with time. In this century, researchers have done purposefully a great deal of rheological experiments of rocks, and their studies showed that this kind of time-dependent change is not disorderly and unsystematic, but regular and with inherent laws and can be well simulated by use of some empirical and theoretical models. At present, though the rheological property simulated by these idealized model are not completely coincident with the actual situations and still have somewhat difference, this kind of simple physical idealization for the rheological properties of rocks has a bright future because the one-dimensional solutions deduced based on these simple rheological models can according to some particular conditions be spreaded to the three-dimensional problems which consider the time factor, and it plays a very important part in dealing with the practical problems (Jaeger & Cook, 1976). On the other hand, since H.F Reid (1910) advanced the elastic rebound theory, which he developed on the occasion of the great San Francisco earthquake of 1906, the theory which the reactivation and the formation of a fault causes an earthquake has been generally admitted and accepted, and a great deal of results achieved on this theory. Their studies obviously have played without doubts an important part in the study of earthquake mechanism. Especially in 1970's, W.P. Brace et al. advanced the stick-slip as a mechanism of shallow-focus earthquake (h=70 Km) based, on the laboratory studies, which further perfects the theory that the reactivation of a fault causes an earthquake, and opens up a new channel for combining the study on mechanical properties of faulted rocks with the study of earthquake mechanism. Nevertheless, their theory always are confined to the qualitative description of the earthquake mechanism. In order to achieve the idea of quantitative analysis, in this paper, we start with the consideration of the visco elastoplastic properties of faulted rocks, and advanced a mathematical model and analyze its application to the study of earthquake mechanism. 2 MODEL AND ITS STRESS-STRAIN RELATIONSHIP Natural rock is an uncontinuous medium with elasticity, plasticity, viscosity and a large number of fissures. According to the knowledge of the origin mechanism of tectonic earth- quake, the occurrence of a great earthquake and its after- shock sequence are the repeating of such a process that firstly, strain energy accumulated in the faulted rocks or the rocks surrounding the fault and then suddenly released due to the instability of the faulted rocks, at last, stress recovered.
ABSTRACT: The delimitation of zones of potential earthquakes and the evaluation of maximum possible intensity is based on the analysis of complex tectonical and geophysical data. Statistical methods of prediction were used in the case of rockbursts. For multichannel prediction were used wiener extrapolation, adaptive filtration and autoregresive method (Bayessan solution). The efficiency of statistical prediction was discussed for various lengths of prediction operators and various members of auxiliary data channels (convergency, acoustic emision) and for both predicted series - daily number of rockbursts and daily released seismic energy. 1 INTRODUCTION According to macroseismical evidence of historical earthquakes in the Bohemian Massif, intensity I did not exceed the value of 8 MSK-64 with a focal depth up to 30 km. For number N of these earthquakes the empirical relation of Krn, k V., Schenk V., Schenkov Z, 1981. With respect to this moderate seismicity prediction are important only in localities of structures with a high degree of seismic vulnerability, e.g. nuclear power plants. The assessment of seismic hazard is based on the estimate of maximum possible magnitudes in seismoactive seismotectonic zones and lineaments. For the small number of historical earthquakes the seismotectonic methods of prognosis are much more important than seismostatistical methods. 2 THEORETICAL BACKGROUND The rock massif in the vicinity of workings is strained by both primary and secondary states of stress induced by mining activities. As a rule, workings are situated in tectonically disturbed regions, i.e. in the regions of increased rock massif heterogeneity; the concentrations of the state of stress are locally increased here, particularly in the surroundings of tectonic disturbances and structural heterogeneities. As a result of the progress of mining activities, not only is the stress state distribution in the rock massif variable, but also the orientation and magnitude of the principal stress tensor components. In a massif thus loaded there occur creep deformations as well as mechanical instabilities, i.e. brittle fracturing of various dimensions, beginning from microfracturing to major ruptures - rockbursts. Rockbursts are thus the manifestation of a gradual compensation of localconcentrations of the state of stress. With respect to the distribution of the tectonic state of stress and the degree of the massif disturbance, rockburst foci need not be necessarily confined to the immediate surroundings of workings /Pribyl A., Rudajev V., 1969/. A rockburst can be physically described as a brittle failure of rock massif, under which the accumulated deformation energy is released. This is first of all transformed into the work of disturbing the massif continuity, then into the kinetic energy of irreversible displacements in the focus and finally, into the energy of seismic waves (less than 1 % of the total deformation energy). A prominent deviator stress in the proximity of the limit state is also manifest by changes of different physical characteristics of the massif. These are mainly- changes in effective elastic moduli, changes in mechanical anisotropy, changes in the density of magnetic susceptibility, electric conductivity and coefficients of inner thermal conductivity.
- Materials > Metals & Mining (1.00)
- Energy > Oil & Gas > Upstream (1.00)
- Energy > Power Industry > Utilities > Nuclear (0.54)
The Activity of Tectonic Fault And the Researches For Its Mechanism Related to Rock Mechanics
Zhen-Yu, Tao (Wuhan University of Hydraulic and Electric Engineering) | Li-Ming, Zhang (Wuhan University of Hydraulic and Electric Engineering) | Tie-Min, Chen (Wuhan University of Hydraulic and Electric Engineering)
1 INTRODUCTION ABSTRACT: In this paper, a self-lock model, which is used to explain the stick-slip mechanism theoretically for faulted rocks, is advanced, and offers a satisfactory explanation for the stick-slip as a possible mechanism of earthquakes. Based on this, a mathematical simulating method in which the visco-elastoplastic properties of faulted rocks are took into consideration is presented as well, with this method, an earthquake sequence can be well simulated and the fault activity can be well analyzed. There are a large number of structural defects randomly distributed in the actual crustal rocks, and these structural defects have been taken as fissures in different scale. As a result of shearing tectonic stress act on the crustal rocks, the number of defects and their scale in a certain direction will increase, and continuously be merged into larger scale fissures, which are usually called faults. Gradually, this procedure occurs mainly in a certain direction which is decided by tectonic stress. At last, it results in forming a principal fracture and causes an earthquake. In fact, the suggestion above is of from Reid's (1910) which the seismic focus of a tectonic earthquake could be defined as that the earth's materials are dynamically and continuously fractured in a certain direction under the action of the accumulated stress in the shearing tectonic deformation field. And the studies of a group of researchers (Benioff 1964; Brace et al. 1966; Dieterich 1972; Stesky et al. 1973; Mjachkin et al. 1975; Thatcher 1975a; 1975b; Langer et al. 1979) also proved that it is the reactivation of the large pre-existing fractures causes earthquakes, and consequently, earthquake produces new fractures. A great deal of observed data now-have show that the occurrence of the majority of intraplate earthquakes are coincident with the active fracture belts as well. However, the reactivation of a fault is usually showed as the dislocation of two sides of the fault, that is fault slip. It is a fact that the occurrence of an earthquake is a releasing process of the elastic strain energy accumulated in the crustal rocks by means of the fault slip. Usually, the fault slip can be grouped into stable sliding (creep- slip) and unstable sliding (stick-slip) and reckon that the creep-slip of the fault cannot cause an earthquake, but it is the stick-slip just causes an earthquake. Brace & Byerlee (1966) and Scholz (1968) have suggested the stick- slip mechanism as a principal pattern of the fault activation and a possible mechanism for earthquake occurrence based on the tests of rock specimens in the laboratory. Since the earthquake occurrence is closely relative to the fault activity, and the stick-slip is the dominant mechanism of the fault slip, it is of great significance for the earthquake forecast to make a further study on the stick-slip mechanism. 2 A MODEL UF STICK-SLIP FOR ROCK JOINTS Because stick-slip on pre-existing faults might be a possible source of shallow-focus earthquakes, it is important to know whether the sliding of a natural fault is stable or stick-slip.