Ongoing postseismic deformation of the Australian continent from far-field plate boundary earthquakes

Abstract:

Vertical deformation of Australia due to earthquakes has previously been considered to be negligible or undetectable using modern geodetic techniques. We investigate the magnitude, duration and extent of coseismic and postseismic deformation of Australia using GPS time series and compare these with elastic and viscoelastic model outputs. We observe and model vertical surface deformation in Australia caused by far-field plate boundary events, including: 2004 M_w 8.1 Macquarie Ridge, 2004 M_w 9.3 Sumatra-Anderman, 2005 M_w 8.6 in northern Sumatra, the 2007 series of M_w 8.5 and 7.9 in southern Sumatra, two events in 2012 of M_w 8.6 and 8.2 in northern Sumatra, and the 2009 M_w 7.8 south of New Zealand. In the vertical component, we find coseismic offsets of up to 3 mm more than 5000 km from the earthquake event, and postseismic signals reaching several mm/yr sustained over several years. In particular, the Sumatran sequence produces observed subsidence in north-western Australia of up to 4 mm/yr over 2004.9-2010.0 where predictions based on one-dimensional viscoelastic Earth models replicate the subsidence but underpredict the vertical rate by at least a factor of two. While viscoelastic models of postseismic deformation can explain the observed changes in velocity in one or two coordinate components, we were not able to reproduce them in all three coordinate components with a single model. We suggest that this is due to limitations in using viscoelastic deformation models with a 1D (radially varying) Earth model, whereas the Earth rheological structure between the offshore earthquakes and continental Australia is complex. A 3D laterally and radially varying model is needed to test these ideas. As the demand for an accurate and stable geodetic reference frame increases toward the stated international goals, care needs to be taken assuming geodetic velocities of land motion are representative of centennial or long-term geophysical processes. Given that ITRF2014 did not parameterise postseismic deformation at the sites studied on mainland Australia, there is scope for improvement in future realisations of the frame across this region.