Mantle Wedge Heterogeneities and Afterslip Distribution Following the 2004 Mw 9.2 and 2005 Mw 8.6 Sumatra Earthquakes

Abstract

Viscoelastic postseismic deformation following the 2004 Mw 9.2 Sumatra and 2005 Mw 8.6 Nias earthquakes extend thousands and hundreds of kilometers from the rupture area, respectively, providing an opportunity to better understand the rheology of the northern Sumatra subduction zone. We have developed a 3D viscoelastic finite-element model to study the postseismic deformation of the 2004 and 2005 events. The time-dependent and stress-driven afterslip is simulated by a 2-km-thick shear zone. Model results indicate that the viscosity of the shear zone of the two events is different, and their boundary is the southern edge of the 2004 rupture area, which is also consistent with the southern edge of the Andaman microplate. The viscosity of the shear zone is determined to be 2×1017  Pa·s in the northern segment, 1016  Pa·s at shallow depths (≤20 km) and 2×1018  Pa·s at greater depths (>20 km) in the southern segment. Afterslip of the 2004 event takes place mostly surrounding the rupture area and is up to 3.2 m within 10 yr after the earthquake. Afterslip of the 2005 event takes place mostly up-dip of the rupture and is up to 4.3 m. The viscosity of the weakened areas in the Andaman spreading center and Toba volcano is determined to be 1018  Pa·s and 3×1018  Pa·s, respectively. A test model with the oceanic asthenosphere extending to depths up to 110 km better explains the vertical motion in the near field.