Relation Between Temporal Change Of Fault Rock Materials And Mechanical Properties

Materials of fault rocks are a critical factor that may control the mechanical behavior of faults. To examine whether the change in mineral composition of fault rocks may result in the change in frictional behavior of faults, we conducted an integrated study on some subsidiary faults of the Yangsan fault, Bogyeongsa area, Pohang, by combining field observation, materials and microstructure analyses and shear tests. From two fault outcrops, both fault gouges and fractured wall rocks were collected. Powders of the wall rocks were used to simulate gouges of immature faults. According to an analysis of the materials, the wall rock powders consist mainly of non-clay minerals (>80 wt%) such as quartz and feldspar; the fault gouges are rich in clay minerals (>50 wt%; illite and chlorite). Some shear tests on the materials at water-saturated conditions show that the gouges are weak (0.27 in terms of friction coefficient, mu) and velocity strengthening, and have a low frictional healing rate, while the wall rock powders are characterized by high frictional strength (mu similar to 0.70), velocity weakening behavior and a high frictional healing rate. Microstructural observations reveal that interconnected alignment of clay minerals with preferred orientation is developed in the fault gouges. The microstructure may be responsible for the weakness of the fault gouges. Given the results described above, it appears that the change in mineral composition from the gouges of immature faults (i.e., simply the product of crushing of wall rocks) to clay-rich gouges may be accompanied by the dramatic change in frictional properties of faults.