Subduction zone earthquakes result from the plate convergence between two plates. The strain is built up on the fault along the plate interface and in areas near the plate margin. If the fault is locked over some width, the horizontal shortening and uplifting can be observed on the overriding plate before occurrences of great earthquakes. There are several ways to release accumulated strain, for instances, large earthquakes, slow earthquakes, transient slip events, or fault creep after large earthquakes.
The subduction zone earthquake cycle can be viewed as a combination of interseismic, coseismic and postseismic periods. Using geodetic data before occurrences of large earthquakes (interseismic period), the spatial extent of the deformation, the width and location of locked fault, and the fault geometry can be estimated. During an earthquake rupture (coseismic period), the accumulated strain of locked fault is released and resulting coseismic subsidence and horizontal extension in the region where show uplift and horizontal shortening in the interseismic period. The stress perturbation due to the mainshock would result in postseismic creep on the ruptured fault, or transient slip on nearby fault systems, or viscoelastic relaxation of the lower crust and upper mantle in the postseismic period.
The focus of this talk on a series of earthquakes after the December 26, 2004 Mw 9.1 Aceh-Andaman earthquake, occurred on the subduction megathrust offshore northwestern Sumatra, In this region, the Indo-Australian plate converges obliquely towards the Sunda Block, at a rate of about 57 mm/yr. This convergence is approximately partitioned into a 40 mm/yr trench-normal component on the megathrust and a 25 mm/yr component of dextral slip along the Sumatran Fault. The subduction megathrust offshore Sumatra has produced 4 earthquakes with magnitudes larger than 8 since 1797, including a M8.5 event near Nias in 1861 that occurred in the approximate region of the southern asperity of the 2005 Mw 8.7 Nias-Simeulue earthquake. Using paleogeodetic and recent continuous Global Positioning System data from the Sumatran Geodetic Array as well as survey-mode GPS data, we study fault slip behaviors along the Sunda megathrust. A heterogeneous coupling pattern in the interseismic period is revealed. The correlation of locked fault patches during the interseismic period and large seismic asperities suggests that they are persistent features. The geomorphologic features, subducting seafloor roughness, the thickness of sediments, temperature, as well as some other factors may affect the spatial extent of great subduction zone ruptures.