Effect of Overpressure Caused By Clay Dehydration on the Triggering of Fault Slip

Abstract:

Many investigations currently show that the pore fluid pressure has been observed to influence the thrust fault strength and slip behavior and updip limit of the seismogenic zone. Clay dehydration is one key control on overpressure generation under undrained condition in thermal pressurization processes. Increasing pressure and temperature with depth depending on the local geological setting and conditions can cause clay dehydration which has been proposed as an explanation for the generation of overpressure. However, study about the effect of excess pore pressure caused by clay dehydration on the triggering of earthquake is seldom addressed in Taiwan. The fault zones in Taiwan like the Chelungpu Fault, clay minerals are abundant in the fault gouge. Therefore, to quantify the effect of overpressure caused by clay dehydration on the triggering of earthquake under undrained condition, we adopt the chemical kinetic model to calculate the amount of water expelled from clay dehydration; derive the three-dimensional governing equation of groundwater flow with clay dehydration varied with pressure and temperature, and simplify a one-dimensional analytical solution of overpressure in fault zones; follow the Coulomb-Mohr frictional failure model of earthquake occurrence to evaluate the influence of the pore pressure on the change of effective Coulomb stress. The results show that the overpressure is greater than the value of 0.01 MPa in fault zones derived from one-dimensional analytical solution. However, Coulomb stress increases of ≥0.01 MPa have been shown to be associated with seismicity rate increase and in many cases triggering earthquakes. Therefore the result denotes that the triggering of earthquake will be progressed. The result could prove to be a feasible examining tool for evaluation of overpressure influence on triggering of earthquake, especially when considering faults with abundant clay minerals of smectite.