Fluid Expulsion at the Forearc Mantle Corner and the Generation of Cascadia Episodic Tremor and Slip (ETS)

English Abstract:

There is not a continuous change downdip from Cascadia seismic behaviour to episodic slow slip and tremor (ETS). ETS is located 70 km downdip from the rupture zone of great subduction thrust earthquakes and coincides very well with the forearc mantle corner at 35-40 km depth. Fluids from dehydration of the downgoing oceanic plate may be released vertically upward into the overlying forearc at the corner. Fluids generated downdip of the corner are blocked from moving vertically by overlying impermeable serpentinized mantle. They are channelled updip in the permeable oceanic crust and subduction shear zone and released into the more permeable overlying forearc crust at the corner. Low Poisson’s Ratio above the corner provides evidence of quartz deposited from these silica saturated fluids as the temperature and pressure decrease upward. Several questions that must be addressed are: (1) Is the shallower tremor as observed elsewhere (not in Cascadia)where temperatures are much lower, produced by the a similar mechanism? (2). If the inferred amounts of quartz in the crust over the corner are correct, a very large amount of fluid is required. Where does it come from? (2) Why does the fluid only produce tremor when it reaches the corner? (3) How is tremor produced at the very high temperatures of 450-500 C where there is usually ductile or semi-ductile behaviour. Does the formation of quartz play a role in generating tremor? Two models may be relevant to these questions: (1) The fault valve and quartz deposition model of Sibson, in which pore pressure and shear stress increase until rupture failure, at which time both are released. (2) The mobile hydrofracture fluid transport and quartz deposition model of Bons, in which fluid moves in lenses with a fracture opening ahead and closing behind. Quartz veins are built up of thin layers deposited from multiple high-velocity fluid squirts.