Creeping subduction zones are weaker than locked subduction zones

Faults that are fully or partially locked pose the greatest seismic hazard because they accumulate stress that can then be released in large earthquakes. In contrast, other faults continuously creep. The creeping versus locked behaviour is probably related to the frictional properties of the fault and the effective normal stress on the fault, but it is unclear whether locked faults are weaker or stronger than creeping faults. Here we use stress orientations in subduction zones from inversion of earthquake moment tensors, and find that geodetically determined creeping versus locked behaviour is correlated with the orientation of the subduction zone plate boundary fault relative to the principal stress axes. Globally, locked subduction zones appear well-oriented for failure, assuming a typical laboratory friction coefficient. Creeping subduction zones are more poorly oriented, implying a lower apparent friction coefficient, due to either low intrinsic friction or reduced effective normal stress. The spatial variations of stress orientation on the Japan Trench are similarly correlated with spatial variations in coupling, with creeping regions having a lower apparent friction coefficient than locked regions. The absolute strength of faults is influenced by the ambient fluid pressure, which is often elevated in subduction zones. This suggests low overall strength for locked subduction zone faults, and additional strength reduction in creeping zones that may be due to transient elevated fluid pressures. The faults in creeping segments of subduction zones are weaker than those in locked segments, according to analyses of stress orientations and GPS data from subduction zones globally.