Numerical Studies On Rock Fracture And Generation Mechanism Of Low-frequency Signals Before Earthquakes

Earthquake preparation and generation, this extremely complex process isdifficult to fully understand due to three aspects: impenetrability of the curst of theEarth, non-frequent of earthquakes and uncertainties, nonlinearity and complexity ofseismic physical process. So far, there are no scientific physical mechanisms toexplain various anomalies before earthquakes. But low frequency signals alwaysexist before large earthquakes through small scale rock compression experimentsand statistical results.This thesis simulates the mechanical properties of rock under compressionthrough extended finite element method（XFEM）provided by ABAQUS. And thenexplores earthquake preparation and generation process, predict earthquake risk bystudying fault dislocation.The main contents are as follows:(1) The mechanical properties of rock under compression experiment wereanalyzed by numerical method and further compared with experiment results.Morphologies of crack growth were analyzed and the influence of pre-exist cracklength and angle on the mechanical properties of rock were discussed. It is shownthat as the angle increases, the harder the rock destruction will be. The longer thelength is, the more easily the rock will be damaged. It is found that there is oscillateonly during the initial stage of crack growth. It is difficult to capture continuousfrequency information when crack does not grow.(2) The mechanical properties of rock under dynamic loading conditions: theapplicability of XFEM is validated after analyzing crack growth and rock fractureform. By applying impact load, vibration conditions and corresponding energychanges were researched. Compression stress waves induced by forced vibration ofnodes propagate along the media and reflect, transmit along the interface whenencountered cracks. The propagation paths are various and stress waves force crackcontinue to expand. Alternating waves and propagation cause oscillate of nodes.Wave shapes are similar to waves measured by actual sensor, this indicates thatdynamic impact loading during earthquake preparation can stimulate oscillate, thatis, low-frequency signal.(3) Fault models were simplified using seismic normal, reverse fault mechanism. Ground deformation, node force changes were studied by applyingimpact load on the boundary. It is found that the original crack length, crackinclination angle, change of loading directions have a great impact on grounddeformation and crack form. Thrust fault are more dangerous after comparison. Thederived acceleration, velocity and stress, displacement curves are different with eachother, so it is necessary to analyze their frequencies. Impact load and boundary largeenough are possible to make fault generate continual low-frequency signal.