Coulomb stress perturbation after great earthquakes in the Sumatran subduction zone: Potential impacts in the surrounding region

The Sumatran subduction zone has been seismically very active compared with other major subduction zones in the world. Since 2004, a sequence of large earthquakes ruptured along the trench, including the 2004 M_w 9.2 Sumatra-Andaman, the 2005 M_w 8.6 Nias-Simeulue, the 2007 M_w 8.4 Bengkulu, the 2010 M_w 7.8 Mentawai tsunami earthquakes and numerous moderate and small events. These earthquakes released stress over tens of seconds to minutes, disturbing the lithosphere and asthenosphere across a broad region. As earthquake-introduced stress disturbance can trigger tremendous aftershocks and sometimes precipitate large earthquakes in their vicinity, gaining a detailed picture of spatial and temporal stress evolution on the fault and its neighboring faults is crucial for seismic forecasting. Here, we have developed spatiotemporal Coulomb stress models for the Sumatran region, which includes the Sumatran subduction zone and the Sumatran fault, using a well-studied postseismic model. This postseismic model is constrained by decade-long time series of geodetic observations from the Sumatran GPS Array (SuGAr) and from tide gauge data in Southeast Asia. We show that Coulomb stress changes imparted by co- and post-seismic processes of previous great ruptures could well explain the temporal and spatial connection between seismicity and stress evolution. Most importantly, our results reveal that the stress in the Mentawai seismic gap of the Sumatran subduction zone was loaded by more than two bars. These stress perturbations could potentially trigger the rupture of the Mentawai seismic gap, which was already overdue in the last seismic cycle. Now, the likelihood for the failure of this gap is even higher than before 2004. We also find that along the Sumatran fault stress has increased to more than one bar, which may explain the surrounding seismicity. Consequently, we highlight the considerable seismic hazard and associated tsunami threat to the neighboring regions.