Coulomb-like creeping segment acts as a stress sensor in Northern Sumatra

Understanding the spatial and temporal evolution of creep along continental faults is key to identifying where stress is released aseismically and where it may accumulate, potentially leading to future seismic events. Typically, creeping segments exhibit rate-strengthening behaviour, where frictional resistance increases with sliding velocity, resulting in stable sliding. However, the northern Aceh segment of the Sumatran Fault Zone (SFZ), a right-lateral strike-slip fault, presents a notable exception. Here, we observe creep signals using satellite radar interferometry, capturing the temporal evolution of creep, which decreased by ~60% from the 2007–2010 period to the 2017–2023 period. Numerical modelling constrained by these observations identifies two distinct creep events. The first was triggered by stress transfer from Mw 9.2 2004 earthquake more than 150 km away, and a second localized re-acceleration due to nearby continental earthquakes. These results reveal that the fault behaviour is more consistent with a brittle Coulomb surface and lacking the usual self-stabilizing influence of velocity- and slip history-dependent friction. This central creeping section, sandwiched between two large locked domains, instead exhibits nearly velocity-neutral behaviour at the fault segment scale, making it highly sensitive to both local and regional stress changes. These findings provide evidence of long-range fault interactions, where both the subducting megathrust and possibly the oceanic mantle drive creep on a continental strike-slip fault. Located within a densely populated region, the velocity-neutral Aceh fault may participate in future earthquakes nucleating in surrounding locked segments, highlighting elevated seismic hazard in northern Sumatra.