Interseismic deformation and creep along the Sumatran fault, Indonesia from InSAR time-series analysis

Abstract:

The fast subduction (~60 mm/yr) of the Indo-Australia plate underneath the Sunda microplate in Southeast Asia causes major seismic and tsunami hazards to surrounding regions. Previous land-based GPS studies have revealed that the plate motion composes of ~45 mm/yr of subduction directed normal to the Sunda trench and ~23 mm/yr of right-lateral strike-slip along the 1000km long Sumatran fault. The decoupling of the forearc blocks from the overriding plate leads to slip partitioning at this highly oblique subduction zone. Geological and geodetic studies have suggested that the long-term fault slip rate along the Sumatran fault increases significantly from southern to northern Sumatra. We analyzed the SAR data from the L-band ALOS-1 satellite to image the interseismic deformation along the Sumatran fault.

To extract the deformation signal from the interferometric phase observations, we have improved the conventional SBAS methodology by including the decorrelation information of the interferograms into the inverse problem. Instead of discarding the noisy data present in a fraction of the interferograms, we keep all the pixels in the processing chain and weight the observed phase data based on the correlation using the covariance matrix. This new InSAR technique is less prone to the unwrapping errors and it results in a spatially coherent signal with dense coverage and high precision.

The InSAR time-series analysis has detected up to ~20 mm/yr of fault creep on the Aceh segment along the Northern Sumatran fault. The spatial extent of the aseismic creep reaches at least 100 km, measured from N5.2, E95.7 to N4.5, E96.5. The along-strike variation of the aseismic creep has an inverse “U” shape. We will present new line-of-sight interseismic velocity and along-strike variation of fault creep rate along the Northern Sumatran fault in Southeast Asia.