Herbert McQueen

Estimation of current plate motions in Papua New Guinea from Global Positioning System observations

Plate tectonic motions have been estimated in Papua New Guinea from a 20 station network of Global Positioning System sites that has been observed over five campaigns from 1990 to 1996. The present velocities of the sites are consistent with geological models in which the South Bismarck, Woodlark, and Solomon Sea Plates form the principal tectonic elements between the Pacific and Australian Plates in this region. Active spreading is observed on the Woodlark Basin Spreading Centre but at a rate that is about half the rate determined from magnetic reversals. The other major motions observed are subduction on the New Britain Trench, seafloor spreading across the Bismarck Sea Seismic Lineation, convergence across the Ramu-Markham Fault and left-lateral strike slip across the Papuan Peninsula. These motions are consistent with a 8.2° Myr -1 clockwise rotation of the South Bismarck Plate about a pole in the Huon Gulf and a rotation of the Woodlark Plate away from the Australian Plate. Second order deformation may also be occurring; in particular, Manus Island and northern New Ireland may be moving northward relative to the Pacific Plate at ∼5-8 mm yr -1 (significant at the 95% but not at the 99% confidence level) which may suggest the existence of a North Bismarck Plate.

Glacial isostatic adjustment and nonstationary signals observed by GRACE

[1] Changes in hydrologic surface loads, glacier mass balance, and glacial isostatic adjustment (GIA) have been observed using data from the Gravity Recovery and Climate Experiment (GRACE) mission. In some cases, the estimates have been made by calculating a combination of the linear rate of change of the time series and periodic seasonal variations of GRACE estimates, yet the geophysical phenomena are often not stationary in nature or are dominated by other nonstationary signals. We investigate the variation in linear rate estimates that arise when selecting different time intervals of GRACE solutions and show that more accurate estimates of stationary signals such as GIA can be obtained after the removal of model-based hydrologic effects. We focus on North America, where numerical hydrological models exist, and East Antarctica, where such models are not readily available. The root mean square of vertical velocities in North America are reduced by ~20% in a comparison of GRACE- and GPS-derived uplift rates when the GRACE products are corrected for hydrological effects using the GLDAS model. The correlation between the rate estimates of the two techniques increases from 0.58 to 0.73. While acknowledging that the GLDAS model does not model all aspects of the hydrological cycle, it is sufficiently accurate to demonstrate the importance of accounting for hydrological effects before estimating linear trends from GRACE signals. We also show from a comparison of predicted GIA models and observed GPS uplift rates that the positive anomaly seen in Enderby Land, East Antarctica, is not a stationary signal related to GIA.

Motion of the South Bismarck Plate, Papua New Guinea

The absolute motion of the South Bismarck Plate was first estimated by Tregoning et al. [1998] from three site velocities estimated from Global Positioning System (GPS) observations. We report an improved estimate of the Euler vector for this plate using site velocities derived from new GPS data which include the velocity of a site located ∼25 km from the pole of rotation. The GPS velocities of Madang, Witu, Jacquinot Bay and Finschafen can be modelled to within ∼3 mm/yr using a single pole of rotation located at 6.75°S, 147.98°E with a clockwise rotation rate of 8.11°/My. The known tectonic features and available geophysical data surrounding the South Bismarck Plate can also be explained by a rotation of the South Bismarck Plate about this pole.

Incessant excitation of the Earth's free oscillations: global comparison of superconducting gravimeter records

Abstract Records of superconducting gravimeters (SGs) at Canberra (Australia), Esashi (Japan), Metsahovi (Finland) and Syowa Stations (Antarctica) were analyzed to search for further evidence of background free oscillations of the Earth. Spectrograms for 1-year period and averaged power spectra for seismically quiet periods were obtained for each of the stations. Anomalous features of the oscillations observed at Syowa Station, such as an apparent seasonal variation and a high intensity at frequencies between 3 and 4 mHz, were absent at the other SG stations. Among the SG stations used in this study, the background free oscillations were detected most consistently and distinctly at Canberra, where the noise level was comparable to that at the IDA quietest station, while that at Syowa Station was close to the critical limit for detecting the oscillations. The background free oscillations provide a good reference to evaluate the noise level in the milliHertz band.

The search for postglacial rebound near the Lambert Glacier, Antarctica

A GPS network has been installed to monitor vertical crustal movement in the Lambert Glacier region, East Antarctica. The program commenced in January 1998 with a solar-powered GPS system installed at Beaver Lake. Solar-powered observations were also made late in the Antarctic summer of 1999. In January 2000, two new solar-powered sites will be installed to expand the monitoring network. In addition, we will be installing a hydrogen fuel cell power system at Beaver Lake to enable the equipment to operate throughout the winter months when solar power is not available. In this paper we outline the equipment which has been developed in order to operate remote GPS equipment in Antarctica, provide predictions of the expected rate of rebound and comment on preliminary results from the data collected to date.

A simple kinematic model for crustal deformation along two- and three-dimensional listric normal faults derived from scaled laboratory experiments

Abstract We have derived a simple kinematic model of the deformation that results from extension accommodated by movement of a crustal block along two- and three-dimensional listric fault surfaces. The model accurately reproduces deformation observed in a series of scaled analogue models. The kinematic model is based on the simple assumption that lines within the hangingwall that are normal to the fault surface before deformation remain so following deformation. An additional constraint built into the model is that of incompressibility. Deformation in the hangingwall block as observed in the laboratory experiments and predicted by the kinematic model is characterized by: (1) a key-stone structure (or crestal-collapse graben) at some finite distance from the fault tip; and (2) pure solid-body rotation of the hangingwall head area near the tip of the fault. In three dimensions, the central region of the model undergoes extension in a direction normal to the direction of imposed displacement in such a way that the direction of dip of the upper surface of the hangingwall is aligned with the direction of extension. This result provides quantitative support for the use of dip analysis to infer tectonic transport direction. We also show how the distribution of extension within the hangingwall is affected when the constraint of constant displacement along the fault is relaxed.

The effect of melting land-based ice masses on sea-level around the Australian coastline

Changes in relative sea-level (RSL) are generally caused by variations in sea surface heights from steric effects (thermal expansion and salinity changes) and the mechanical response of the Earth to past and current redistributions of ice and water between land and oceans. This paper focuses on the latter, where we present scenario calculations of the spatial variability in present-day RSL change around the Australian coastline resulting from melting land-based ice masses. Three scenarios are investigated: (1) the ongoing effect of glacial isostatic adjustment (GIA) arising from ice- and water-load redistribution during the last glacial–interglacial transition; (2) the effect of present-day changes in the Greenland and West and East Antarctic ice sheets (GIS, WAIS and EAIS, respectively) and two regions of major mountain glaciation, Alaska and Patagonia; and (3) a hypothetical complete melting of the GIS, WAIS and EAIS occurring over 5000 years. The first scenario shows falling RSL around Australia of the order of 0.4 to 1.2 times the average value around the coast (equivalent to a RSL fall of between 0.2 and 0.6 mm/a). For the second scenario, the spatial variability is strongly dependent upon the location of each ice mass relative to Australia. For Greenland and Patagonia, the resulting changes to the Earths rotation strongly affect the spatial variability, while the direct gravitational effect is more important when considering the Antarctic ice sheets. The variability associated with the first two scenarios becomes clearer when examining RSL change estimates for the locations of tide-gauge stations around the Australian coast, especially for the ongoing GIA (a south-to-north increase in the simulated rate of RSL change), the WAIS (east-to-west increase) and the EAIS (south-to-north increase), with the melting of the EAIS potentially having the greatest influence on the variability of the melting land-based ice contribution to RSL change around Australia. The spatial variability associated with the third scenario is strongly influenced over century-length time-scales by the resulting changes in the Earths rotation and the direct gravitational attraction of the ice masses, while after several thousand years the uplift of the continent by mantle material displaced towards it by increased ocean loading becomes more prominent. It must, however, be kept in mind that the spatial variability associated with these scenarios is generally a small proportion of the total RSL change, and that the steric (especially thermosteric) contribution is not included in these results.

New Geodetic Infrastructure for Australia

In November 2006, the Australian Federal Government announced

Structure of the Mt Isa region from seismic ambient noise tomography

15.8M in funding for geospatial research infrastructure through the National Collaborative Research Infrastructure Strategy (NCRIS). NCRIS is an initiative under the Australian Governments Backing Australias Ability package with a number of key principles, including maximising the contributions of the R&D system to economic development, national security, social wellbeing and environmental sustainability. Here we outline why particular components of geospatial infrastructure are required in Australia to advance (equip) geospatial research over the next 20 years. We describe some of the scientific objectives that required an upgrade and densification of Australias geospatial infrastructure. This paper is the perspective from a subset of University researchers involved in the AuScope Geospatial component, and so does not necessarily encompass the opinions of all those involved in AuScope Geospatial.

Absolute Gravity Measurements in Australia and Syowa Station, Antarctica

We use seismic tomography, exploiting group velocities derived from ambient noise, to delineate the crustal structure beneath Mt Isa and the surrounding blocks and basins. The depth extent of the blocks can be traced into the mid-crust and the spatial extent of the associated velocity anomalies mapped over an area of approximately 500 km by 500 km. The Proterozoic Mt Isa block is imaged as a region of elevated seismic velocities comparable to the Yilgarn craton in Western Australia, while the surrounding basins have relatively low velocities. Seismic velocity anomalies display correlations with the regional Bouguer gravity data and with high crustal temperatures in the region. There are a number of isolated low-velocity anomalies under the Millungera basin that suggest either previously unknown thermal anomalies or zones with high permeability, which can also produce lowered velocities.