The December 2004 Sumatra-Andaman earthquake was the second largest (Mw 9.3) and second deadliest (~280,000 casualties) earthquake ever recorded. It was also unusual in that it combined a normal ?fast? rupture with a more unusual slow fault slip event. This unique occurrence presents an important opportunity to examine two questions with potentially wide-ranging implications for global earthquake studies: (1) What processes and rheologies re-equilibrate stress following great earthquakes in island arc settings? And (2) What Earth properties control the speed of rupture in great earthquakes? The boundary between the Indian and Andaman tectonic plates is almost entirely submarine, but the Andaman-Nicobar Island group enables a revealing look at postseismic deformation in the segment that ruptured principally as a slow earthquake and hence generated relatively little ground shaking. The investigators on this project are continuing to capture postseismic deformation after the Sumatra-Andaman earthquake by operating a network of 14 Global Positioning System (GPS) sites in the Andaman-Nicobar Islands. The network currently consists of 4 continuous and 10 campaign sites. These ?ground-truth? geodetic data (from a locale where data acquisition is notoriously difficult) are being combined with satellite measurements of gravity and shoreline changes to examine key scientific questions about the deformation and stress cycle on a subduction thrust that exhibited unusual slow slip behavior during one of the largest earthquakes of the past century. Modeling of the first several years of GPS postseismic displacements indicates slip down-dip of the seismic rupture dominated near-field deformation during that period. Modeling suggests the early fault slip response is now completed and that mantle flow will dominate near-field deformation for the next several years. The network is designed to capture these viscoelastic flow signals, to fingerprint any continuing fault slip that may illuminate slip stability of the shallow plate-bounding thrust, and to assess whether time- and space-dependent nonlinear effects of power-law creep rheology are in evidence. Slip stability in particular is being assessed by examining the data for evidence of nonlinear fault slip (so-called ?slow slip events?) on the shallow plate boundary that ruptured during the earthquake. The rock flow process is also being examined for evidence of a different type of nonlinearity that is expected based on laboratory rock deformation experiments, but that has not been observed unequivocally in the real Earth. University of Memphis investigators are operating the GPS network in partnership with the Indian Institute of Science, Bangalore and the Society of Andaman & Nicobar Ecology (SANE). Modeling and interpretation combines the expertise of investigators at the University of Memphis and Utah State University (USU). Deformation processes that re-equilibrate stress following large earthquakes are intrinsically linked to the processes that accumulate stress leading up to large earthquakes, so this ongoing investigation will illuminate the physics of earthquake slip and the earthquake cycle globally
Seven years of raw GPS data has been collected at five continuous sites and 10 campaign sites from Andaman Islands which experienced the 2004 Sumatra - Andaman great earthquake and Tsunami. This data helped understand the postseismic deformation processes that follows great earthquakes. Also, this region is know for difficulties in getting raw scientific data out of it for the use of rest of the world. The collaborations PI has developed succeded in archiving seven years of GPS data in UNAVCO facility for the use of students and researchers world wide. More than six years after the great (Mw 9.2) Sumatra-Andaman earthquake, post-event processes responsible for relaxation of the coseismic stress change remain controversial. Modeling of Andaman Islands GPS displacements indicated early 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 modeling tools used. After subtracting a pre-2004 interseismic velocity, significant transient motion during the 2008.5-2010.5 epoch confirms that postseismic relaxation processes continue in Andaman. Modeling three-component velocities as viscoelastic flow yields a weighted root-mean-square (WRMS) misfit that always exceeds the WRMS 26.3 mm/yr of the measured signal. The best-fitting models are those that yield negligible deformation, indicating the model parameters have no real physical meaning. GPS velocities are well-fit (WRMS 4.0 mm/yr) by combining a viscoelastic flow model that best-fits the horizontal velocities with ~50 cm/yr thrust slip downdip of the coseismic rupture. Both deep slip and flow respond to stress changes, and each can significantly change stress in the realm of the other, so it is reasonable to expect that both transient deep slip and viscoelastic flow will influence surface deformation long after a great earthquake.
