Measuring and modeling postseismic deformation processes of Andaman Islands

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Nearly a decade after the great (Mw 9.3) Sumatra-Andaman earthquake, the understanding of the post-event processes responsible for relaxation of the co-seismic stress change still remains incomplete. Modeling of early Andaman Islands GPS displacements indicated near-field motions were dominated by slip down-dip of the rupture, but various researchers ascribe elements of relaxation to dominantly poroelastic, dominantly viscoelastic and dominantly fault slip processes, depending primarily on their measurement sampling and the modeling tools used. Intriguingly, uplift and right-lateral horizontal shear deformation, observed in the Andaman data, is exactly opposite the predictions of radially-symmetric viscoelastic relaxation models. We hypothesized then that, either radially-symmetric models neglect heterogeneities in flow properties that are important to understanding the earthquake cycle, or transient slip still dominates the near-field signals years after the event. Modeling of GPS displacements suggests that the twin (M8.6 and M8.2) ruptures of 2012 intra-plate oceanic earthquakes could have possibly accelerated the ongoing slow slip, along the lower portion of the thrust fault beneath the Islands with a month long slip of 4-10 cm. Thus, the continuing postseismic deformation excited by the December 2004 Sumatra-Andaman (Mw = 9.2) co-seismic stress change and interfered by 2012 M8.6 and M8.2 twin earthquake, is the primary objective of this continued work. There is also the need for better models for the near field deformation along the Andaman Islands. Currently there are models that either explain the far field deformation or near field deformation but not both combined.Measurements will continue at five continuous and twelve campaign stations in the Andaman Islands. UNAVCO facility has loaned GPS receivers that are operating in this region. Data from these sites along with surrounding IGS sites will be processed using GAMIT/GLOBK software. PYLITH finite element software will be used to model the post seismic deformation combining both after slip and viscoelastic relaxation due to 2004 co-seismic change and that of 2012 Sumatra twin earthquakes. Data collected will be archived at the UNAVCO facility and is open for all. This endeavor is in collaboration with Jawaharlal Nehru Centre for Advanced Scientific Research Bangalore, India. In the coming year we will initiate the processes to hand over the complete technology transfer to this institute in main land India and to locals in the Andaman Islands to maintain the network of GPS stations. This includes training of the locals and those in JNCASR in Bangalore.