Aseismic to seismic transition at the brittle to ductile transition: insights from hydrothermal ring shear experiments and a microphysical model
Abstract
Earthquakes typically nucleate at the base of the seismogenic crust at temperatures where the major rock constituents such as quartz (T=350 ºC) or feldspar (T=450 ºC) are expected to deform plastically at geological strain rates. However, the seismic cycle is associated with a wide range of strain rates, and it is unclear how this affects the depth extent of earthquake propagation. In this presentation, I will show results from hydrothermal ring shear experiments on a variety of fault gouges derived from carbonates, quartz, quartzofeldspathic Alpine Fault rocks (New Zealand) and gouges derived from felsic rocks from the East African Rift. The results consistently show an apparent frictional weakening at the most elevated temperature (up to 700 ºC), which evolves over significant displacement and is most severe at the lowest velocity. At the same time, there is only minor deviation from linearity in the normal stress dependence of shear stress, indicating that, even at the highest temperatures, deformation remains predominantly frictional over the velocities imposed (0.03 to 1 um/s). Shear stress estimates using flow laws obtained from compression experiments on low porosity intact rocks are consistently higher than those measured. Our microphysical model (CNS model, Chen and Spiers, JGR, 2016) is capable of reproducing some of the experimental results of monomineralic gouges. The model, based on a competition between porosity creation via granular flow and porosity reduction via time-dependent compaction highlights the crucial role porosity plays in governing the transition to velocity-weakening, potentially earthquake-generating slip with increased strain rate as well as in maintaining a linear dependence of shear stress on normal stress. Seismic cycle simulations based on the microphysical model show that a continued loading of creeping fault segments by nearby earthquakes can push these segments into the velocity-weakening regime. The crucial parameter controlling this transition is the rate of the time-dependent creep mechanism, which remains poorly constrained for most mineralogical compositions of interest. There exists ample scope for future experiments that focus on flow laws for polymineralic fault rocks at conditions that span the range of the seismic cycle under hydrothermal conditions.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.T42C..08N
- Keywords:
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- 1242 Seismic cycle related deformations;
- GEODESY AND GRAVITYDE: 7230 Seismicity and tectonics;
- SEISMOLOGYDE: 8118 Dynamics and mechanics of faulting;
- TECTONOPHYSICSDE: 8163 Rheology and friction of fault zones;
- TECTONOPHYSICS