Ongoing postseismic vertical deformation of the Australian continent
from far-field earthquakes
Abstract
We use GPS observations to investigate the magnitude and spatial
distribution of vertical coseismic and postseismic deformation of the
Australian continent and compare these with elastic and viscoelastic
model outputs. We observe and model surface deformation in Australia
caused by six recent large far-field events: 2004 Mw 8.1 Macquarie
Ridge, 2004 Mw 9.3 Sumatra-Anderman, 2005 Mw 8.6 in northern Sumatra,
the 2007 series of Mw 8.5 and 7.9 in southern Sumatra, two events in
2012 of Mw 8.6 and 8.2 in northern Sumatra, and the 2009 Mw 7.8 south of
New Zealand. Observed vertical coseismic deformation reaches 3 mm, with
the magnitude varying spatially and by earthquake in broad agreement
with modelling of coseismic deformation. Postseismic deformation is
observed in all three coordinate components at Australian GPS sites
nearest to these earthquakes, with deformations reaching several mm/yr
in the vertical over multiple 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 a factor of two. Across all earthquakes, the models
often fit one or two coordinate components of the observations, but
rarely all three. Unmodeled lateral rheological structure likely
contributes to this given the difference between the oceanic location of
the earthquakes and the Australian continental setting of the GPS sites.
The magnitude and spatial extent of these coseismic and postseismic
deformations warrant their consideration in future updates of the
geodetic terrestrial reference frame.