Ultralow-angle faults produce giant earthquakes

What controls the occurrence of giant earthquakes remains a central question in seismology. We reveal a notably simple-but previously overlooked-relationship between earthquake size and fault dip in subduction zones. Global earthquake statistics show a robust dependence of earthquake size on fault dip, with the probability of extreme events peaking on ultralow-angle faults. Through three case studies, we demonstrate that earthquakes tend to grow larger when the fault geometry is aligned with the regional stress field. We further discuss how such favorable stress configurations are generated along subduction interfaces and emphasize the critical importance of monitoring the temporal evolution of stress in subduction zones to better forecast future megaearthquakes.