Solid Earth Deformations Induced by the Sumatra Earth- quakes of 2004-2005: GPS Detection of Co-Seismic Displace- ments and Tsunami-Induced Loading

The two great Sumatra earthquakes of December 26, 2004, and March 28, 2005, are associated with a number of geodynamic processes affecting Earth’s shape, gravity field and rotation on a wide range of spatial and temporal scales. These include co-seismic strong motion, permanent co-seismic displacements, surface loading due to the barotropic tsunami, normal modes, and transient post-seismic deformations lasting from days to years. The December 26, 2004, earthquake is the first one with observed with spacegeodetic techniques. Thus, new insight into the earthquake processes and associated phenomena can be expected from studies of space-geodetic, and particularly GPS observations. Here we focus on the permanent co-seismic offsets and the tsunami loading signal. The global patterns of the permanent co-seismic offsets induced by the earthquakes are determined from time series of daily displacements resulting from an analysis of the GPS data from selected stations of the global IGS network. Our estimates are compared to similar estimates computed in other studies. The differences between the independent estimates are mainly noise, and the uncertainties of the offsets are of the order of 2 to 4 mm. The global pattern of the computed offsets is in agreement with the spatial fingerprint predicted by a reasonable rupture model. Using the predicted fingerprint in the determination of the offsets from the GPS time series is expected to improve the estimates of the offsets. Initial model predictions of the tsunami-induced loading signal show that the peak amplitudes are of the order of 10 to 20 mm. If detectable in GPS time series with low latency, the loading signal could be utilized in a tsunami early warning system.