Case 18: Theoretical Modeling on Characteristics of Fault Creep and Its Propagation Along the Xianshuine Fault in the Interval Between two Strong Earthquakes

Using both elasticity and viscoelasticity theory of fault dislocation, this paper studies the characteristics of displacement field near a fault zone due to fault slip, its development and propagation. The variations of -both short baseline and levelling measurements on the ground surface are analysed. From the measured dalta of short baseline and levelling across the Xianshuihe fault in southwestern China, we have inversed the fault slip and its change with time and space during the period between the 1973 Luhuo earthquake of magnitude 7.9 and the 1981 Daofu earthquake of magnitude 6.9, and have estimated the relevant parameters. The main conclusions are as follows:Bight after the Luhuo earthquake, a left-lateral fault creep with a small dip-slip component occurred along the Xianshuihe fault zone in a range of 70 km, which nearly coincided with the surface fracture belt produced by the earthquake. The slip angle φ=-10°and the time function of the slip ΔV=B (1-exp (-t/T)), in which B=11.3 cm and T=3 yrs. Since the middle of 1979, a southeastward propagation of left-lateral fault creep took place in the southeastern part of the above creeping segment, with an average velocity V=150 m/day. The propagation stopped in the northwest of Daofu.Taking into consideration the studies on the fault movements related to the 1976 Tangshan earthquake and the results of large scale friction experiments, the authors have investigated the relation between the Xianshuihe fault movement and the above two strong earthquakes. The results imply that there are two types of fault motion, one is the post-seismic continuously decelerated fault slip in, the case of the Iuhuo earthquake, and the other is the pre-seismic preccusory slip with propagation of fault creep in the ease of the Daofu earthquake. Both characteristics of the fault movements reflect some regularity of pre- and post-seismic fault movements.