Australia: The Land Where Time Began

A biography of the Australian continent 

Low Friction and Fault Weakening Revealed by Increasing Sensitivity of Tremor to Tidal Stress

Stress accumulation and release which governs stress transfer to the shallower part of the fault that is locked, which can slip in a megathrust earthquake, is controlled at subduction zones by the level of friction on the deep part of the plate boundary fault. The deep fault slips slowly in some subduction zones, at speeds that are far slower than at shallower, regular earthquakes, and is often accompanied by tremor, which is a weak seismic wave. Small stress changes that are induced by ocean tides or tides in the solid Earth can induce tremor and slow slip. In this paper Houston¹ used seismic data combined with tidal stress calculations to determine the influence of tides on 31,000 tremors that have been generated by 6 large, slow-slip events in Cascadia between 2007 and 2012. Houston¹ concluded that during each slip event the sensitivity of tremor to tidal stress increases as slip at each spot on the fault accumulates. Specifically, the rate of tremor is an exponential function of tidal stress, and this exponential sensitivity increases for several days, which implies that the fault weakens during slip. The relationship between tidal stress and tremor was used by Houston¹ to calculate the coefficient of intrinsic friction, finding values of 0 to 0.1, which imply that the deep fault is inherently weak.

Sources & Further reading

1. Houston, H. (2015). "Low friction and fault weakening revealed by rising sensitivity of tremor to tidal stress." Nature Geosci 8(5): 409-415.