An Experimental Study On Evolution Processes Of Physical Field Of Bend Fault During Deformation

The planar and straight faults are rarely seen in the field. The strike changes or steps of faults often occur between the natural fault segments, forming bend faults, en echelon faults, etc. It is important to study regional stress distribution, slip rate, the distribution of foreshocks and aftershocks and rupture process resulted from the different combinations of faults. In this thesis, the evolution processes of acoustic emission (AE) and the strain field are investigated, which occurred on the single bend faults and double compression-bend faults during the loading deformation, by use of AE technique and strain measurement in the laboratory. And the significances of the experimental results are discussed. The main results are as follows:1. Time sequence of stick-slip eventsThe quasi-periodic stick-slip events occurred on single bend fault at a certain displacement loading rate. And the lager the loading rate is, the smaller the stick-slip periodicity and stress drop. However, for a double-bend fault, the quasi-periodic instability didn't occurred during loading. The big and small strike-slip events alternately occurred, and some small instability events were observed between the big stick-slip events.2. The spatial-temporal-strength distribution of AE events during the stick-slip processOne stick-slip process includes the strain energy accumulation stage and release stage. Many small magnitude AE events occurred in the former, while several large magnitude AE events took place in the latter.For the single bend fault, the AE events migrated reaptedly between upper and lower fault segments during strain accumulation; and the big AE events occurred on the lower fault segment ( the higher angle of fault against the direction of principle compression stress). For the double compression-bend fault, the AE events mainly occurred near the relaxation-type bend, a few near the restraint one during the strain accumulation stage. AE events alternately migrated between the middle fault segment and the restraint bend during strain release stage, and the high-magnitude AE events occurred on the middle fault segment.3. The Spatial-temporal evolution process of the strain fieldThe strain filed intensely changed near the fault bend for the single bend and double compression-bend faults. Mean strain near bend was intense either for the single bend fault or the pressured double bend fault. The high-value zone of the mean strain increment appeared alternately inside and outside the bend and the high-value zone of the maximum shear strain increment also appeared above and below the bend during fault slip.In the relatively stable stage during early strain accumulation, the mean strain was large on the inner side of the bend, and small on the external side. It is the fundamental mean strain picture of the double compression faults. And the high values of the maximum shear strain occurred in middle fault segment.In one fast fault slip, the doublet sub-events often occurred. The interval time between the doublet sub-events was short for the single bend fault, long for the double compression-bend fault. And for the later, the first strain release took place in the lower fault section, the second at the two bends.On pressured double bend fault: mean strain increased on the inner side of the bend and decreased on the external side bend outside. This phenomenon constituted the basic image. Maximum shear strain increment accumulated on the middle of the pressured fault.4. Possible precursory locationThe experimental result suggest that for bend faults, the strong and complicated strain change always take place in the bend region. So, the possible precursory information appears near the bend.