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Abstract

Through the combination of rare historical sea level observations collected during Sir Douglas
Mawson’s 1911–1914 Australasian Antarctic Expedition (AAE), together with modern
sea level data, space geodetic estimates of crustal displacement and modelling of coseismic
and post-seismic earthquake deformation, we present a contemporary analysis to constrain
sea level and land level change over the twentieth century at Macquarie Island (54◦30S,
158◦57E).We combine 9 months of 1912–1913 sea level data with intermediate observations
in 1969–1971, 1982 and 1998–2007 to estimate sea level rise relative to the land at +4.8
± 0.6 mm yr−1. Combined with estimates of global mean sea level rise, this value supports
the geologically surprising notion of land subsidence, conflicting with longer term geological
evidence that suggests uplift at ∼0.8 mm yr−1 over the last 400–300 Kyr. We investigate the
current tectonic evolution of the Island through analysis of Global Positioning System (GPS)
solutions that utilize data over the period 2000–2009. Importantly, this provides an opportunity
to refine the source parameters of the Mw ∼8.0 great earthquake of 2004 December 23 using
estimates of coseismic displacements at regional GPS sites. We use the estimated earthquake
source and GPS observations of four years of post-seismic deformation at Macquarie Island
to infer the rheology of the oceanic upper mantle. We find that an asthenosphere bounded
by stronger material above and below is required to produce the observed post-seismic deformation,
particularly in the vertical component. Assuming a Maxwell rheology, the best fit
is given by an asthenospheric viscosity of 3×1019 Pa s. The inferred rheology determined
from the 2004 earthquake is used to model long period post-seismic deformation from Mw
∼8.0 earthquakes of 1989 and 1924. The 1924 earthquake is the closest of the three great
earthquakes to Macquarie Island, and our modelling suggests that the majority of the vertical
deformation at the tide gauge over the subsequent 80 years is related to ongoing viscoelastic
relaxation from this thrust earthquake that ruptured south from an epicentre south ofMacquarie
Island. Assimilated time-series of land level change from the earthquake modelling (suggesting
ongoing subsidence) constrained by the GPS estimate of vertical velocity of −2.46 ± 0.64
mm yr−1 combine with the relative sea level time-series to yield an estimate of absolute sea
level change of +2.0 ± 0.8 mm yr−1 over the twentieth century.We conclude this is consistent
with the upper bound of the global average rate of absolute sea level rise over the same period.
This represents one of few estimates of observed sea level change in the Southern Ocean,
re-emphasizing the importance of historical data and continued geodetic and oceanographic
observation in remote areas.
Key words: Sea level change; Space geodetic surveys; Seismic cycle; Transient deformation;
Oceanic transform and fracture zone processes; Rheology: mantle.

Item Type:	Article
Authors/Creators:	Watson, CS and Burgette, RJ and Tregoning, P and White, P and Hunter, J and Coleman, R and Handsworth, R and Brolsma, H
Keywords:	Sea level change; Space geodetic surveys; Seismic cycle; Transient deformation; Oceanic transform and fracture zone processes; Rheology: mantle.
Journal or Publication Title:	Geophysical Journal International
ISSN:	0956-540X
DOI / ID Number:	https://doi.org/10.1111/j.1365-246X.2010.04640.x
Additional Information:	The original publication is available at http://www.interscience.wiley.com/
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