Spatial Dependence of Dynamic Nonlinear Rock Weakening at the Alpine Fault, New Zealand

We perform laser ultrasonic measurements to investigate the spatial dependence of dynamic nonlinear weakening in rocks from the Alpine Fault, New Zealand. Rocks outside the damage zone display no nonlinear weakening. Within the damage zone (<30 m from the fault), cataclasites present a 3% reduction in shear modulus from wave amplitudes inducing 1-2 microstrain at atmospheric pressure. This nonlinear elasticity decreases with a characteristic pressure between 1 and 2.5 MPa. We show that rock weakening is therefore strongest in the near surface. However, this significant elastic nonlinearity in cataclasites at low strains confirms that rock weakening may play an important role in earthquake processes, such as fault weakening, triggering of slip, rupture propagation, and coseismic velocity decreases.

Automated summary

Laser ultrasonic measurements revealed a spatial dependence of dynamic nonlinear weakening in rocks from the Alpine Fault in New Zealand, with cataclasites within the damage zone showing significant reduction in shear modulus and increased nonlinear elasticity. The study suggests that rock weakening is strongest near the surface and may play a crucial role in earthquake processes such as fault weakening, triggering of slip, rupture propagation, and coseismic velocity decreases.