Frictional strength and stability of greywacke fault zones
Lead Investigator: Dr Carolyn Boulton, Te Herenga Waka Victoria University of Wellington
Research Team: Associate Professor André Niemeijer, Utrecht University, the Netherlands
Project number: 1975
Abstract:
Aotearoa New Zealand sits astride the Australian-Pacific plate boundary. As the Australian and Pacific plates are forced to slide past each other, strain energy accumulates in the rocks on either side of locked faults. If the locked faults slide suddenly in earthquakes, the stored strain energy results in movement on the faults and radiated seismic waves, which can cause damaging ground motions. In Aotearoa, faults are most commonly located in basement sandstone and siltstone rocks of the Torlesse Composite Terrane. These rocks are colloquially termed “greywacke”. The frictional strength of basement sandstone and siltstone, as well as the propensity of these rocks to host earthquakes, are important input parameters in the seismic hazard models that are used to forecast earthquakes and their effects. By performing friction experiments on sandstone and siltstone samples collected from a fault that ruptured in the 2016 Kaikōura earthquake, we found that at shallow depths (typically < 10 km depth), the siltstone is frictionally weaker than the sandstone. However, at depths > 10 km, both rock types have approximately the same frictional strength and this strength is consistent with a majority of rock types studied worldwide.
At depths > 10 km, both rock types are also frictionally unstable, meaning that they are likely to nucleate earthquakes if there is enough driving stress to overcome the frictional resistance to sliding. As temperature increases, our experiments show that both rocks become frictionally stable again, and the temperature range that corresponds to the change from frictionally unstable to frictionally stable is called the seismic-to-aseismic transition. Earthquakes are unlikely to nucleate below the seismic-to-aseismic transition, so this depth fundamentally limits the area of a fault that can rupture in an earthquake and thus the maximum moment magnitude able to be generated. In Torlesse Composite Terrane sandstone and siltstone, the seismic-to-aseismic transition temperature depends on rock type and sliding velocity. For the slowest sliding velocities tested, the transition happens at temperatures above 350C in the siltstone and 400C in the sandstone. Correlating these temperatures with absolute depths requires additional constraints on how temperature changes with depth, which varies in different regions across Aotearoa New Zealand.