A decade of horizontal deformation from great earthquakes

Abstract

The 21st Century has seen the occurrence of 17 great earthquakes (Mw >8), including some of the largest earthquakes ever recorded. Numerical modeling of the earthquakes shows that nearly half of the Earth's surface has undergone horizontal coseismic deformation >1 mm, with the 2004 Sumatra‐Andaman earthquake dominating the global deformation field. This has important implications for both the realization of a terrestrial reference frame and in the interpretation of regional tectonic studies based on GPS velocities. We show that far‐field coseismic deformations from great earthquakes will, if unaccounted for, introduce errors in estimates of linear site velocities of at least 0.1‐0.3 mm/yr across most of the surface of the Earth. The accumulated global deformation field shows that two regions, Australia and the north Atlantic/Arctic Ocean, have been largely undeformed by these great earthquakes, with accumulated deformations generally <0.5 mm. Using GPS estimates of surface deformation, we show that the majority of the Australian continent is deforming at <0.2 mm/yr, the northern part of New Zealand is rotating clockwise relative to the Australian Plate with relative horizontal velocities of ∼2 mm/yr, while the southeastern coast of Australia is undergoing post‐seismic relaxation caused by the 2004 Mw = 8.1 Macquarie Ridge earthquake. The presence of ongoing post‐seismic relaxation thousands of kilometers from plate margins violates the secular/linear assumption made in current terrestrial reference frame definitions. These effects have significant ramifications for regional tectonic interpretations and global studies such as sea level rise that require reference frame accuracy greater than this level.