Post-seismic deformation of Andaman Islands since the 2004 earthquake

On 26 December 2004 the Andaman Islands near Port Blair, on the hanging wall of the Sumatra/Andaman mega thrust, sank 86 cm and shifted 3.1 m WSW. Port Blair since then has continued to move WSW at an exponentially decaying rate that by mid 2006 had attained 25 cm (i.e. 8% of coseismic slip). In contrast the harbour rose 15 cm, an 18% reversal in coseismic subsidence. We have installed ten additional GPS points since the earthquake (http://www.ceri.memphis.edu/people/jpuchkyl/) and these too show post seismic slip, but with spatial variations that require variations in the subsurface process along strike, in addition to downdip post-seismic adjustment processes. The coseismic hinge-line separating uplift from subsidence in the southern Andaman Islands appears to have shifted to the east by 10-20 km. We note that vertical movements in the islands in the past 150 years have been quite complex but are known largely from eyewitness descriptions in many cases, even including the 2004 earthquake. For example, war-time coastal bunkers constructed shortly after the 1941 Andaman earthquake on a raised wave-cut marine terrace had been partly submerged prior to 2004, and then incremented 80 cm lower in the 2004 event. This has led us to install a series of tidal bench marks linked to GPS measurements so that future investigations will be able to quantify these changes better. Pressure gauges linked via leveling to GPS control points define mean sea level at 2-3 points every 5 minutes, while secondary points using hydraulic averaging are surveyed along the coast during their operation. We are currently in the process of modeling these GPS measurements, both as viscoelastic relaxation in response to the coseismic strain change and postseismic slip on the plate boundary interface down-dip of the seismic rupture. Previous studies elsewhere suggest that using both the horizontal and vertical displacements enables the models to distinguish the relative importance of these two processes.