Frictional properties of JFAST core samples and implications for slow earthquakes at the Tohoku subduction zone

Slow earthquakes occur in the shallow (<20 km deep) part of the Tohoku subduction zone. To understand how frictional properties of the plate boundary fault affect the generation of these slow earthquakes, we conducted friction experiments using borehole samples retrieved from the plate boundary décollement and footwall during Integrated Ocean Drilling Program Expedition 343 (Japan Trench Fast Drilling Project (JFAST)). The plate boundary fault material exhibits a transition from velocity-strengthening to velocity-weakening behavior at temperatures between 50 and 150°C, which, in the framework of rate-and-state friction laws, is a necessary condition for the generation of slow earthquakes, whereas the footwall sample mainly shows velocity-strengthening behavior except at temperatures of <50°C. The downdip temperature limit of slow earthquakes in the Japan Trench also lies between 100 and 150°C. Our results suggest that the frictional properties of the plate boundary fault may play a key role in controlling the locations of observed slow earthquakes.