Earthquake nucleation in weak subducted carbonates

Ocean-floor carbonate-and clay-rich sediments form major inputs to subduction zones, especially at low-latitude convergent plate margins. Therefore, knowledge of their frictional behaviour is fundamental for understanding plate-boundary earthquakes. Here we report results of mechanical tests performed on simulated fault gouges prepared from ocean-floor carbonates and clays, cored during IODP drilling offshore Costa Rica. Clay-rich gouges show internal friction coefficients (that is, the slope of linearized shear stress versus normal stress data) of mu(int) = 0.44-0.56, irrespective of temperature and pore-fluid pressure (P-f). By contrast, mu(int) for the carbonate gouge strongly depends on temperature and pore-fluid pressure, with mu(int) decreasing dramatically from 0.84 at room temperature and P-f = 20MPa to 0.27 at T = 140 degrees C and P-f = 120MPa. This effect provides a fundamental mechanism of shear localization and earthquake generation in subduction zones, and makes carbonates likely nucleation sites for plate-boundary earthquakes. Our results imply that rupture nucleation is prompted by a combination of temperature-controlled frictional instability and temperature-and pore-pressure-dependent weakening of calcareous fault gouges.