Rheological Properties and Induced‐Coulomb Stress in the Southern Sumatra Subduction Zone: Insights From the 2007 Mw8.4 Bengkulu Earthquake

Abstract The 2007 Mw8.4 earthquake offers insights into the rheological structure of the southern Sumatra subduction zone, especially the Mentawai gap. Here, we derived 3‐year postseismic Global Positioning System (GPS) observations to study deformation processes based on a three‐dimensional viscoelastic finite element model. Model results indicate that a heterogeneous shear zone is required to better reproduce GPS observations. The megathrust at Mentawai gap underwent a continuous afterslip of about 1 cm within 5 years after the earthquake with a shear zone viscosity of 5 × 10 17 Pa s. Farther south, we divide the southern shear zone into shallow (≤20 km) and deep (20–100 km) segments whose viscosities are determined to be 10 16 Pa s and 5 × 10 17 Pa s, respectively. Afterslip in the shallow shear zone is up to ∼3.3 m, while afterslip in the deep shear zone is up to approximately 1 m. The observed uplift in the vicinity of the rupture area is probably due to a cold forearc mantle. Both the Burgers and power‐law rheologies produce similar surface deformation due to viscoelastic relaxation. Distribution of combined coseismic and postseismic Coulomb stress over the megathrust shows that subsequent earthquakes with magnitudes exceeding 7 experienced enhanced stress loading, over 0.01 MPa, indicating that 2007 earthquake may play an important role in triggering these subsequent events. This study provides insights into the potential hazards assessments in this region, emphasizing the role of Coulomb stress in triggering subsequent events.