Physical properties and mechanics of fault sliding at the Alpine Fault, New Zealand

Abstract

The Alpine fault is an active plate-boundary fault in New Zealand, which has the potential to cause large earthquakes every 200 to 400 years. In 2011, the Alpine Fault Deep Fault Drilling Project drilled a borehole with the and which aim is to investigate the physical, chemical, and hydrological properties of fault-related rocks. The cataclasites samples in this study are from DFDP-1A and -1B wells, which are measured under dry and water-saturated conditions using the ultrasonic transducer assembly. To estimate the P- and S-wave velocities accurately, Dynamic Time Warping algorithm is applied to use all the characteristics of the waveforms when picking travel times. Elastic moduli are calculated based on the experimentially estimated seismic wave speeds. Furthermore, Gassmann’s equation is a useful tool to estimate the elastic moduli of water-saturated rocks using the elastic parameters on the dry rocks, and the theoretical values are compared with the laboratory results to test applicability. Understanding the elastic and geomechanical properties of cataclasites is essential to study the motion of rocks during the fault nucleation and rupture processes. Estimating the critical slip length is important to understand earthquake nucleation and the resulting earthquake moment magnitudes. Here I estimate the critical nucleation length with depth by combining experimentally derived elastic moduli and literature parameters. This research finds that there are general upward trends of P- and S- wave velocities with increasing effective pressure for both the dry and the water-saturated rocks. The mineral volumes and porosities are controlling parameters of elastic properties. Sample 1B112, which has the highest stiff mineral contents, shows the highest P-wave velocity under the water-staurated condition at 100 MPa. However, there is no observable relation between wave speeds and porosity in this study, probably the result of complex fracture networks. The Young’s modulus and the bulk modulus increase with effective pressure, the water-saturated results are higher than for the dry rocks. The shear modulus exhibits a gentle growing as the effective pressure rise, showing no differences between the dry and the water-saturated samples. Vp/Vs of water-saturated rocks is greater than that of dry rocks. Elastic waves velocity and moduli are insensitive to changes in pore fluid pressures for the same effective pressures indicating that it is possible that the effective stress coefficient for these rocks is equal to one. Additionally, the Gassmann’s equation shows good applicability for my samples, suggesting that using dry rocks and applying Gassmann’s relation to predict water-saturated wave speeds is a possibility. After comparing the critical slipping length with the earthquake radius, it can be concluded that earthquakes Mw < 1 will be less favourable to occur at depths less than 2 km in the Central Alpine fault.