The earth tides is a powerful source of periodic stresses and deformations in the Earth's crust. Since the publication by Nishimura [1950], there were many attempts at using earth tides for the assessment of seismic hazards [Seymour, 1972; Mikumo et al., 1978; Burton, 1986; Yaramanci et al., 1988; Hartzell and Heaton, 1989; Nikolaev and Nikolaev, 1993; Xiang-chu Yin et al., 1994; et al.] The effect of mechanical vibrations in addition to the main load, however, may alter the destruction process itself. In their paper, Sadovski et al. [1981] have shown that this influence on the samples of different rocks and man-made materials provokes transition from brittle destruction to the release of elastic energy, accumulated in the sample, by plastic deformation.
A paper by Sobolev et al. [1996] establishes that additional vibration reduces the time period between the successive movements of the stick-slip type on the contact between the blocks of rock.
The present research is aimed at studying the effect of vibration on deformation, destruction, and acoustic regime of complex samples with a lower strength layer as a model in the first approximation of a fault zone.
The experiments were carried out on models composed of concrete.
Figure 1 shows the general view of the model.
The two lateral layers were stronger than the central layer owing to the
400-brand cement that was used to make them, whereas the 50-brand cement
was the binding component in the central layer. Besides quartz sand, the
central layer also contained rock chips of different size ranging from
2 to 20 mm. The length of the model on its side facet was 300 mm, the
width along the perpendicular was 220 mm. The size of the model in its
third dimension was 65 mm. The thickness of the lateral and central layers
was the same. Owing to the jutting out angles of the chips, the contacts
between the lateral and central layers were rough, and the average angle
of their dip towards the vertical axis coincided with the main load
F and was 25o.
The lateral compressing stress G was 3 tons and was maintained constant
throughout the experiment with the accuracy of 1% by roller padding between the
side facets of the model and the pistons of the press. The vertical load F,
also applied by roller padding, was gradually increased to its peak value and then
reduced in the post-peak area of deformation to correspond to the condition of the
constant rate of relative deformation
= 10-6. The experiments
were conducted on a press "Inova" with the servocontrol at the Borok Observatory
of the Institute of Physics of the Earth. The load F and the movement of
the pistons in the vertical direction D were recorded with sampling rate
0.2 s or 1 s on PC IBM. The conclusions are based on the results of ten experiments,
seven of which had F load modulated by additional vibration with periods 2, 10,
30 and 100 s. The amplitude of vibration was about 12% of the maximum load.
The choice of construction of the models and the type of loading have caused formation of fissures and their development in the weaker central layer. A system of echelonlike fissures appeared, the typical case of which is shown in Figure 1. The macrodestruction occurred as a rupture of the shear type cutting across the system of echelonlike fissures and through the middle and along the central layer.
The appearing acoustic signals were recorded on PC IBM by two systems. The first system had six piezoreceivers and was tuned on registration of all signals above noise level. The output parameters of the system, i.e., amplitude and duration of impulses, allowed to estimate their energy. The second system with eight piezoreceivers recorded the waveforms of signals, and was used to locate the strongest of them. The main energybearing frequencies of signals were confined to the range above 30 kHz. In the course of the experiment, usually lasting several hours, the model was periodically sounded in different directions to measure the elastic waves velocities. The initial average velocities of compressional waves in the lateral layers were 4 km s-1 and in the central layer 2 km s-1. As soon as a system of echelonike fissures was formed, the elastic waves velocity in the central layer was reduced to 1.3 - 1.5 km s-1, whereas in the lateral layers it remained practically unchanged.
Figure 2 represents a typical example of a comparison between
the experiments without vibration (a) and those with additional vibration (b). The
difference was that in the first case the destruction was more brittle with a
macrorupture near the peak load. The additional vibrations allowed the experiment to
reach far into the post-peak area with gradual reduction of the model's strength.
Cessation of vibration partially restored the strength at the conclusive stage of
the experiment
(Figure 2b).The location of AE events indicates that fissuring always started in the lower part of the central layer, near the mobile piston of the press, and migrated upwards. The arrows in Figure 2b show the moments of three acoustic signals corresponding to the location of their sources 1, 2, 3 in Figure 1.
The acoustic signals with large amplitudes (Figure 2)
were recorded only near the peak load and in the post-peak area of deformation.
The distribution of the weaker signals during vibration period was also studied.
For this purpose their numbers in the subsequent cycles of vibration-relative additional
load release - were summed up to obtain "seasonal variations". The spectral analysis
of the distribution in time of the number of acoustic signals distinctly reveals the
periodicity corresponding to the vibration period.
The amount of signals, however, does not strictly follow the sinusoidal law. Most
of them are grouped in the 20-30% interval before and near F maximum.
Figure 3 represents the mode of seasonal variations
as compared with the sinusoidal period of F variation.
One of the principal problems of prognostic importance is to determine the point
of maximum rheological curve F - D. The experience of destruction mechanics
implies that, on reaching this point, the loaded body passes to the stage of unstable
deformation that initiates macrodestruction without outer energy supply. In his paper,
Sobolev [1993] emphasized that the discovery of this point
in the focus of a pending earthquake should improve prediction methods, because at that
stage the short-term precursors become apparent. The difficulty is that we do not know
the absolute values of stresses and deformations under conditions in the Earth's interior,
though we easily obtain them in the laboratory. Therefore, an idea occurs to use cyclic
variations of stresses and deformations to find the point of F - D maximum.
In the present paper, we used the following procedures to study this problem. We
obtained a nonlinear low-frequency trend of F and D plots by averaging
data from ten vibration periods. The residual plots were then standardized to standard
deviation. The next step was to construct the F - D hysteresis from individual
cycles with vibration period T. Figure 4 shows the
loops of hysteresis averaged from ten cycles on the linear part of load (a) and near
the maximum F- D points (b). It is apparent that hysteresis in
Figure 4b has a peculiar feature in the part covering about
1/4 of the duration of the cycle at large F and D values. This feature
shows that the reduction of deformation lags against the reduction of load on the
descending arm of the F - D curve. This is an indication that the deformation
process is unstable. Proceeding from this result, an attempt was made to determine
the instability stages without the absolute F and D values
(Figure 5).
The values of 
F and
D cyclic changes, relative to the
lines of the nonlinear trend, were calculated. After standardization the
F and
D values,
we calculated the functions
ni =1[(Fi+n -
Di+n) -
(Fi-n -
Di-n)],
where Fi,
Di) are load and
displacement values at
the points of maximums of vibration cycles, while n changes from 1 to n and
corresponds to F and
D values within 1/4 of the
duration of the cycle, j corresponds to the amount of vibration cycles.
The FD(j) function demonstrates the deviation from the synchronous changes of the F and D curves on the 1/4 portions of the cycle, where the largest differences from the linear law were observed in the hysteresis loops. A correlation of the F loading plot with the F-D function plot in Figure 5 shows that in transition of the model to the instability stage (the peak load on F plot) the FD values increase. A prognostic aspect is consequently revealed. A tendency to deviation from the linear law on approach to instability is also apparent in the study of asymmetry in the F and D cycles separately. The evaluation of stability of this prognostic feature, however, requires supplementary experiments.
There is an apparent opportunity for studying the instability of crustal blocks in the course of an analysis of distortions in the earth tides or in the signals from vibrosources close to peak loads.
The application of vibration reduces the possible brittle destruction and promotes a smooth transition of the model into the post-peak state.
A distinct periodicity in the appearance of acoustic signals corresponding to the vibration period reveals the appearance of impulses primarily in the 20-30% interval near the cyclic load maximums.
The approach of the model to instability is manifested by the expansion of the hysteresis area in the load-displacement loop.
The distortion of the form of cyclic changes in the load and in the deformation is a prognostic symptom of pending instability that provides a possibility to reveal this stage without knowledge of the absolute force and deformation values.
There is an obvious opportunity to observe an instability in the generating earthquake focus by studying similar changes in the oscillations from earth tides or vibrosources.
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