5:15 PM - 7:15 PM
[SCG45-P01] Interaction between fluid phase transition induced by frictional heat, formation of off-fault damage, and styles of on-fault slip
Keywords:Hydrothermal condition, High velocity friction, Fluid phase transition, Off-fault process, Rupture dynamics
Frictional behavior of fault zone including fault core and surrounding wall rock under hydrothermal conditions (i.e., high temperature and high pressure with fluid) plays a critical role in seismogenesis at various settings such as subduction zones and geothermal systems. The HDR (hydrothermal high-velocity rotary shear apparatus) at Kochi Institute for Core Sample Research, JAMSTEC, is a newly developed apparatus that has a capability of imposing high-velocity slip (~2 m/s) at up to 600 ℃ with pore fluid pressure of 120 MPa at most. High velocity friction experiments under hydrothermal conditions have been successfully performed on various materials including gabbro, granite, olivine, and clay minerals. In this presentation we focus on the experimental results on gabbro.
We firstly performed experiments at 1.8 m/s under effective normal stress of 5 and 10 MPa with pore fluid pressure of 5-20 MPa at temperature range from room temperature to 300 ℃. Dynamic slip weakening behavior toward the dynamic friction coefficient of ~0.2 following peak friction coefficient of ~0.6 was observed at all temperature conditions. Slip weakening distances were also similar among temperature conditions, However, the shear displacement to reach peak friction after the beginning of deformation shortened with increasing temperature. This temperature dependence may suggest that the style of coseismic rupture at a given location on a fault can be different among temperature conditions. Generation of frictional heat is critically important at the location which rupture propagates through and starts to slip because of high stress level.
To explore the influence of frictional heat at the initial stage of coseismic slip under hydrothermal conditions, we carried out intermediate velocity experiments with 10 mm/s of slip velocity at 200 ℃ with pore fluid pressure of 5 MPa and effective normal stress of 10 MPa. Friction coefficient remained ~0.7, which means that the fault was not ready dynamic slip weakening. Instead of dynamic slip weakening, we observed abrupt shortening of gabbro specimen after slipping a certain shear displacement without showing significant change in frictional strength. This shortening behavior was not observed at an experiment at the same experimental condition but at dry condition. As the frictional strengths of both the wet and dry experiments were almost the same, generated frictional heat is also the same, and therefore, fluid is related to the shortening. Using finite element method modeling of heat diffusion, we found that the temperature on the fault plane exceeded the boiling point of 5 MPa-water around the shear displacement where the shortening happened. Recovered samples were extensively fractured showing racks in the specimen and a lot of fragments with sharp edges and large surface areas. Therefore, the abrupt shortening may be caused by extensive fragmentation after volume increase due to fluid phase transition from liquid to gas following generation of frictional heat.
Such behavior may contribute to forming an off-fault damage zone as it can break up cohesive wall rock as observed in the experiments. Following the formation of damage zone, if shear strain distributes within the entire damage zone, the on-fault strain rate becomes less, and further heat generation gets less effective. This scenario may hinder on-fault slip acceleration and thermal weakening. On the other hand, if the damage zone does not contribute to shear deformation, the on-fault strain rate does not decrease, and frictional heat will be further generated. In this scenario, as the fault temperature continuously increases, the fault effectively weakens and keeps slipping until the energy supplied by dynamic stress concentration is totally consumed.
Although it is still unclear which scenario is likely to happen in natural systems as the experimental conditions cannot confine the damaged material near fault, shear localization vs shear distribution and on-fault vs off-fault energy distribution at the initial stage of coseismic slip would play important roles on fault slip behavior such as pulse-like or crack-like rupture. Also, it is also an interesting topic if off-fault damage would be formed under higher pore fluid pressure where phase transition from liquid to supercritical fluid does not have a sharp boundary in terms of fluid properties such as viscosity, which we are now challenging to do. Interaction between dynamic change in fluid properties and fault slip behavior as well as off-fault damage would need to be further clarified to understand coseismic energy release and fault motion.
We firstly performed experiments at 1.8 m/s under effective normal stress of 5 and 10 MPa with pore fluid pressure of 5-20 MPa at temperature range from room temperature to 300 ℃. Dynamic slip weakening behavior toward the dynamic friction coefficient of ~0.2 following peak friction coefficient of ~0.6 was observed at all temperature conditions. Slip weakening distances were also similar among temperature conditions, However, the shear displacement to reach peak friction after the beginning of deformation shortened with increasing temperature. This temperature dependence may suggest that the style of coseismic rupture at a given location on a fault can be different among temperature conditions. Generation of frictional heat is critically important at the location which rupture propagates through and starts to slip because of high stress level.
To explore the influence of frictional heat at the initial stage of coseismic slip under hydrothermal conditions, we carried out intermediate velocity experiments with 10 mm/s of slip velocity at 200 ℃ with pore fluid pressure of 5 MPa and effective normal stress of 10 MPa. Friction coefficient remained ~0.7, which means that the fault was not ready dynamic slip weakening. Instead of dynamic slip weakening, we observed abrupt shortening of gabbro specimen after slipping a certain shear displacement without showing significant change in frictional strength. This shortening behavior was not observed at an experiment at the same experimental condition but at dry condition. As the frictional strengths of both the wet and dry experiments were almost the same, generated frictional heat is also the same, and therefore, fluid is related to the shortening. Using finite element method modeling of heat diffusion, we found that the temperature on the fault plane exceeded the boiling point of 5 MPa-water around the shear displacement where the shortening happened. Recovered samples were extensively fractured showing racks in the specimen and a lot of fragments with sharp edges and large surface areas. Therefore, the abrupt shortening may be caused by extensive fragmentation after volume increase due to fluid phase transition from liquid to gas following generation of frictional heat.
Such behavior may contribute to forming an off-fault damage zone as it can break up cohesive wall rock as observed in the experiments. Following the formation of damage zone, if shear strain distributes within the entire damage zone, the on-fault strain rate becomes less, and further heat generation gets less effective. This scenario may hinder on-fault slip acceleration and thermal weakening. On the other hand, if the damage zone does not contribute to shear deformation, the on-fault strain rate does not decrease, and frictional heat will be further generated. In this scenario, as the fault temperature continuously increases, the fault effectively weakens and keeps slipping until the energy supplied by dynamic stress concentration is totally consumed.
Although it is still unclear which scenario is likely to happen in natural systems as the experimental conditions cannot confine the damaged material near fault, shear localization vs shear distribution and on-fault vs off-fault energy distribution at the initial stage of coseismic slip would play important roles on fault slip behavior such as pulse-like or crack-like rupture. Also, it is also an interesting topic if off-fault damage would be formed under higher pore fluid pressure where phase transition from liquid to supercritical fluid does not have a sharp boundary in terms of fluid properties such as viscosity, which we are now challenging to do. Interaction between dynamic change in fluid properties and fault slip behavior as well as off-fault damage would need to be further clarified to understand coseismic energy release and fault motion.