Mineralogical and frictional properties of fault rocks from the Laohushan creep segment of the Haiyuan fault, Northeastern Tibet

Shallow creep has been observed in the Laohushan segment which lies within a seismic gap between the coseismic ruptures of the 1920 Haiyuan M8.5 earthquake and the 1927 Gulang M8.0 earthquake. However, there has been a lack of geological and mechanical evidence to explain the mechanism behind this shallow creep. In this study, we carried out detailed fieldwork along the Laohushan fault, analyzed the mineral compositions and microstructures of the fault rocks, and performed frictional experiments on fault gouges under various temperature and pressure conditions in the laboratory. Our findings revealed that clay minerals are prevalent in the fault rocks along the Laohushan creep segment, especially in the fault gouge at the fault core. Microstructural observations showed that extensive development of clay foliation in the fault gouges, along with carbonate veins healing cracks and minerals dissolution in the fault rocks. The frictional experiments indicated that the clay-rich gouges showed low frictional coefficients of 0.46-0.58, with positive a-b values of 0.0035-0.0052, reducing to 0-0.0015 at 200 degrees C. The experimental samples exhibited microstructural evolution of pervasive well-developed clay foliation and fewer clast fractures, which controls the velocity-strengthening sliding behavior of fault gouges. Our results suggest that the enrichment of weak clay minerals in the fault zone generates a well-foliated microfabric, which, as demonstrated by frictional experiments, controls the Laohushan shallow creep along the Haiyuan fault at depths of less than 8 km. This finding is consistent with the shallow creep inferred from geodetic data and repeating earthquakes.