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EXPERIMENTAL ROCK MECHANICS AND TECTONOPHYSICS

MA Shengli and MA Jin

Institute of Geology, China Seismological Bureau, Beijing 100029, China

This report briefly reviews the progress during the last four years (1999-2002) in the studies of experimental rock mechanics and tectonophysics in China. Here we only mention the related studies in seismology and physics of the Earth's interior, and those studies in engineering rock mechanics, structural geology and other fields are not covered.

I.  BRITTLE FRACTURE OF ROCKS AND ASSOCIATED PHYSICAL PHENOMENA

The brittle fracture of rocks and the associated physical phenomena are concerned with the mechanisms of earthquake and its precursors. A lot of studies have been done in this field in the last four years, and most of them focus on the deformation and failure process of rocks or analogue materials and their implications in earthquake process and precursor by using acoustic emission, strain measurement and other techniques. Liu et al. (1999a, b) studied the characteristics of acoustic emissions during the deformation of two kinds of granite under triaxial compression, and suggested that rock structure had important effects on the spatial and temporal distribution of microfractures and the statistic characteristics. Jiang et al. (2000a, b, c) studied the temporal characteristics of acoustic emission sequence during the deformation of granite under different temperatures and confining pressures, and discussed some seismic phenomena before strong earthquakes (Jiang, et al., 2002a). Liu et al. (2001) analyzed the temporal characteristics of b-value and spectrum of acoustic emissions during the deformation of samples with different geometric textures, and suggested that the changes of the two parameters were related to the predominant deformation mode (fracturing or frictional sliding). Jiang et al. (2002b) studied the spatial and temporal characteristics of acoustic emissions during the deformation of samples with en-echelon faults. Ma et al. (1999) discussed the relationship between the fault geometry and the departure of precursors from the epicenter based on the experimental observations of acoustic emissions, strain and fault slip on samples containing different fault complexes. Li et al. (2000) and Teng et al. (2002) studied the evolution and nucleation of microcracks in marble samples with typical tectonics by using the method of optical transmission, and discussed their significance in earthquake precursors. Wu et al. (2000) studied the micromechanics of compressive failure and spatial evolution of anisotropic damage in sandstone based on the observations of optical and electric microscopes. Xu et al. (2002) studied the different features of dynamic variation in stress field, plastic area and nucleation zone during loading and unloading by means of real-time holographic interferometry and shadow optical method of caustics. Wang et al. (2002) discussed the heterogeneous distribution of earthquake precursors based on the measurements of strain field of two samples with pre-setting faults. In addition, the numerical simulation has been used in rock mechanics to simulate the failure process of rocks and earthquake process, for example, studies made by Lin et al. (1999), Tang et al. (2001), and Jiao et al. (2002a).

Some experimental and theoretic studies were aiming at proving earthquake-generating models and earthquake-predicting theories. Focusing on the strong body earthquake-generating model, Cui et al. (1999) studied the stress field and rupturing features in compound model containing typical cracks system and hard inclusion. Xu et al. (1999) studied the precursory features of strain shaking and wave velocity anomaly during the fracturing of a combined strong body. Yin et al. (2000) performed a series of studies in the development of the earthquake prediction theory of load/unload response ratio. Xu et al. (2002) discussed the physical meaning and prediction efficiency of the load/unload response ratio in the weakening stage before the failure of the samples.

A lot of studies have been done in the changes of electric and electromagnetic properties during fracture of rocks. Guo et al. (1999) studied the electro-acoustic effect in rock fracturing, and suggested that there might exist some non-fracturing mechanism responsible for electromagnetic emissions in addition to the mechanism of microfracturing. Guo and Guo (1999) proposed a model of electromagnetic emission of multi-crack simultaneous extension based on the experimental results, and calculated the corresponding field of electromagnetic emission. Chen et al. (2000) studied the changing anisotropy of resistivity and the propagation of microcracks in rocks and suggested a method to explore the precursors of rock fracture and to determine the propagation direction of the main fracture. Hao et al. (2002a) studied the mechanism of change in resistivity during rock fracturing, and suggested that the existence of cracks in the rock and the saturation state of the contained liquid were the most important factors controlling the change of resistivity before rock failure.

Some studies were concerned with the new technique and method in earthquake prediction, which enlarged the field of the experimental study. Gao et al. (1999, 2000) studied the response of shear-wave splitting to differential stress variation, and suggested that shear wave splitting was more sensitive than the change of microfracture state and the time decay of shear wave splitting could indicate the coming of the critical state of rock failure. Wu et al. (2000, 2002) reviewed the experimental results in remote sensing rock mechanics and performed some new experiments. Yin et al. (2000) discussed the possibility of the application of infrared remote sensing in earthquake prediction and its physical mechanism based on experimental results. Fang et al. (2000) performed experiments on the application of passive microwave remote sensing technology to the earthquake prediction for different rocks and proposed a method to infer stress state from the passive microwave remote sensing detection. Feng et al. (2000) and Hao et al. (2000) developed a new method to study resistivity tomography of samples by using an electrode array.

 

II.  ROCK FRICTION AND FAULT INTERACTION

Study on rock friction can provide physical insights on fault behavior and earthquake source mechanics. Ma and He (2001) studied the frictional behavior of the homogeneous and non-homogeneous faults, and suggested that the heterogeneity of friction was an important reason to cause the nonlinear behavior in sliding and systematically discussed the nonlinear phenomenon of period doubling bifurcation. Ma et al. (2002) analyzed the waveforms of acoustic emission and fault slip corresponding to stick-slip events and suggested that there existed two types of nucleation phases for stick-slip events occurring in non-homogeneous faults. There were some theoretic and numerical simulation studies in rock friction. He (1999) discussed the similarities and differences of the two rate and state friction laws and their significance in faulting mechanics. He (2000) simulated the earthquake nucleation process and seismic precursors on faults based on rate and state friction law. Liu et al. (2000, 2001) proposed a time-dependent frictional cellular automation model to simulate seismicity along a fault, and suggested that the events with small and large magnitude obeyed quite different magnitude-frequency relationships.

Considering the complexity of the fault system in the natural crust, studying on fault interaction is useful for understanding earthquake mechanism. Ma et al. (2000) experimentally studied the activity of the intersecting faults, and suggested that the alternate slips of the faults were controlled by the block movement. Jiao et al. (2001, 2002) studied the interaction between parallel faults containing barriers with the same and the opposite slip directions. Ma et al. (2002) proposed four patterns of fault interaction based on a series of experiments and discussed the corresponding tectonic and mechanical conditions and the possible effects on seismicity.

 

III.  BRITTLE-PLASTIC TRANSITION AND PLASTIC FLOW OF ROCKS

The brittle-plastic transition of rock deformation is concerned with many important problems, such as the strength of the lithosphere, the condition for earthquake occurrence and the earthquake mechanism, and the property of plastic flow is an important parameter for geodynamics. Liu and Shimada (2000) systematically analyzed the mechanical behavior and the deformation mechanism and their changes with temperature and pressure for granite, the predominant rock in the upper and middle crust, and discussed the strength and deformation mechanism of the continental lithosphere and proposed a new model of fault zone. Sang et al. (2001) studied the brittle-plastic transition and its affecting factors for gabbro, one of the predominant rocks in the lower crust. He et al. (2002) studied the brittle-plastic rheology of gabbro at high temperatures and pressures, and obtained systematic rheological parameters and discussed their significance in the strength of lithosphere and seismicity. Jin et al. (2001) studied the rheology of eclogite and harzburgite, the mantle rocks, and obtained the rheological parameters of eclogite and discussed the rheological properties of the deeply subducted ocean lithosphere (Jin et al., 2001). Zhao et al. (2001) studied the high temperature dislocation creep of Ni2GeO4 spinel, one of the predominant rock-forming minerals in the upper mantle. Wang et al. (2001) simulated the ductile layer in the lithosphere using analogue materials and studied the propagation of the plastic-flow waves in the lithosphere under the driving of plate boundary. Wang et al. (2000) experimentally studied the mantle convection and simulated the non-columnar plume and the mantle vortex.

 

IV.  PHYSICAL PROPERTIES OF MATERIAL IN THE EARTH'S INTERIOR

The experimental study on physical properties of rocks can provide necessary constrains on the state and property of material, structure and dynamics of the Earth' interior. In the last four years, there were many studies in this field, and most of them focused on the factors affecting rock physical properties. Liu et al. (1999) studied the effects of water saturated cracks on seismic velocity and anisotropy in crustal rocks, and suggested that the S-wave velocity and polarization could be used to determine the spatial orientation of oriented cracks in the rock samples and the ratio Vp/Vs might give evidence for fluids at grain boundaries and an estimate of crack densities in rock samples. Ge et al. (2001) discussed the effect of effective stress on rock elastic wave velocity based on a series of experimental results. Zhu et al. (1999) studied the electrical conductivity of serpentine at high temperature and pressure and showed that the electrical conductivity rapidly increased after the dehydration of serpentine, and suggested that this mechanism could form the high-conductivity layer. Zhou et al. (1999) studied the compressional wave velocity of trachybasalt at high temperature and pressure and suggested that the phase transition was the predominant factor affecting wave velocity. Xie et al. (2000) studied the elastic characteristics of serpentinite dehydration at high temperature and pressure and showed that the velocity suddenly dropped and the amplitude increased accompanying the dehydration. Liu et al (2001) studied the electrical conductivity of granite, basalt and pyroxene peridotite under high temperature and pressure and discussed the change of electric conductivity with temperature, indicating that the conductivity could increase 3-5 orders with increasing temperature.

There were some studies focusing on the deep-lying material and structure in some areas. Zhang and Sun (1999) studied the characters of wave velocity and constitution structure of rocks of the craton crust in north part of North China. Sun et al. (2000) discussed the seismic wave velocity of Archaeozoic felsic rocks from North China and its existing location in the crust. Lin et al. (2001) measured the elastic wave velocities of lower crustal xenoliths selected from the north part of North China and discussed their geological implications. Wang et al. (2001) measured the electrical conductivity of dunite at high temperature and pressure and discussed the existence of the cold mantle beneath Gaize-Lugu area in the Qinghai-Xizang (Tibet) Plateau. Zhu et al. (2001) studied the electrical conductivity of the Dabie ultrahigh-pressure eclogites at high pressures and temperatures and discussed the formation of the high conductivity layer in Dabie area.

 

V. CONCLUDING REMARKS

In the last four years, the progress in the experimental rock mechanics and tectonophysics concerning seismology and physics of the Earth' interior are presented in the following aspects. (1) A lot of results of experiment and numerical simulation enrich our knowledge of the brittle fracturing process under the condition with heterogeneity in material and structure, and provide useful data for deeply understanding earthquake mechanism and precursors. (2) Some new results on frictional behavior of non-homogeneous faults and earthquake nucleation are obtained, which will be helpful in understanding the complexity of earthquake dynamics. (3) Some new results on the brittle-plastic transition and plastic flow are obtained, especially the important progresses are obtained on rheological properties of rocks in the lower crust and the upper mantle. These results provide valuable data for study of earthquake genesis and geodynamics. (4) A lot of experimental results are obtained on rock physics at high temperature and pressure, which will provide constrains on material composition and state, structure and dynamics of the Earth' interior.

As any experiment cannot completely duplicate the natural conditions, the main role of the laboratory study is to establish physical models and to validate theories. Even so, the force driving the laboratory study to develop comes from the field observation and study. The recent studies indicate that the heterogeneity in material and structure and the complexity of time process will still be one of key problems in the study of seismology and physics of the Earth' interior. Therefore, to study the complexity of earthquake and geodynamics based on the geological and geophysical observations and to explore the predominant factor from the complexity and establish physical models should be one topic of the experimental rock mechanics and tectonophysics in future.

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