Skip Navigation | ANU Home | Search ANU | Directories
The Australian National University
Research School of Earth Sciences
Printer Friendly Version of this Document
RSES SITE SEARCH
Untitled Document

Research Activities 2011

Earth Physics

 

 


cover

Introduction

The Research School of Earth Sciences includes substantial activities in geophysics. The main research themes are Geodynamics, Geodesy, Geophysical Fluid Dynamics, Mathematical Geophysics and Seismology. These span observational, theoretical, laboratory, computational and data oriented studies, all directed towards understanding the structure and physical processes in the earth’s interior, the crust or the earth’s fluid envelope.

This year saw the commencement of the ARC Centre of Excellence in Climate System Science, with one of its 5 university nodes in the Earth Physics area of RSES, focusing largely on ocean modeling. Two new Postdoctoral Fellows were appointed to the CoE, Drs. S. Downes and A. Klocker. Dr. G.O. Hughes started his ARC Future Fellowship during the year. Ph.D. studies K. Stewart, submitted his PhD thesis and moved to a Postdoctoral research position at Johns Hopkins University, USA. Three undergraduate students who had completed research projects in Earth Physics were awarded University Medals during the year. They are C. Shakespeare and X. Qin who both did their Honours projects in Geophysical Fluid Dynamics, and D. Leykam, a Ph.B. student in Physics who completed special research projects, one in Mathematical Geophysics and the other in Seismology. During the year Dr. N. Balfour joined Seismology to pursue research interests in earthquake sources and also take charge of the outreach program AuSIS, Australian Seismometers in Schools program. Earth Physics staff were successful in applications for ARC Discovery, Linkage, and LIEF during the year.

Seismology

The Seismology group at RSES has carried out a series of field experiments in the last year. The EAL2 experiment, which covers a large region of central northern New South Wales with an array of 53 stations, was installed in mid-2010 and removed in mid-2011. The goal of the experiment is to image the crust and upper mantle beneath the northern Lachlan Orogen by exploiting the Earth's natural seismicity. It also represents a single movement of the larger WOMBAT transportable array experiment, which seeks to cover much of southeast Australia with passive seismic arrays by moving a large array from place to place. The EAL3 experiment, which involved the deployment of 42 stations in northeastern New South Wales in November 2011, represents the next phase of WOMBAT, which to date has installed more than 15 arrays with a cumulative total of over 600 stations. In July 2011, 24 broadband stations were deployed in southern Victoria, northern Tasmania, and the Bass Strait Islands as part of the BASS experiment. This is a joint venture between RSES and the University of Tasmania, and aims to image the crust and lithosphere beneath Bass strait by exploiting ambient seismic noise. In Queensland, the 31 station MINQ-B array, which was deployed in September 2010 in the Cloncurry-Julia Creek Region, was retrieved in mid 2011. The goal of this experiment is to image lithospheric structure in the vicinity of the Mt. Isa Inlier.

In other seismological studies of the deep interior resulted in observational evidence that the complex rotational dynamics of the Earth's inner core appear to be in close relationship with the geomagnetic field. An analysis technique known as partition modeling, was applied to the analysis of newly observed collection of earthquake doublets found by the group. Results indicate that the Earth's inner core shuffles, exhibiting both prograde and retrograde rotation in the reference frame of the mantle. An exciting result is that all three time-intervals in which the inner core distinctively accelerates with respect to the rest of the planet are in agreement with known occurrences of geomagnetic jerks. Because there is also a documented correlation between the geomagnetic jerks and the Length of Day time series, this all points to the same source and works in favour of a differential rotation rather than processes at the inner core boundary.

In Geophysical Fluid Dynamics exciting new results revealed a substantial role for buoyancy in driving the ocean circulation, and show turbulent mixing contributes to governing the global rate of overturning in the oceans. The overturning continually ventilates the ocean depths with cold dense polar water from the surface, and also carries heat from equatorial to polar regions. Laboratory experiments demonstrated how overturning is likely to be faster for more rapid small-scale mixing of density (or heat), supporting earlier theoretical modelling in RSES. The experiments also showed that a well-known methodology previously used to infer the rate of mixing in the oceans over-estimates the actual mixing rate. In a closely related study, computer models of the circulation in the Southern Ocean showed that surface buoyancy fluxes play a major role in driving the flow. Globally, an analysis of the ocean's energy budget showed that surface buoyancy fluxes and surface wind stress provide comparable sources of energy to the circulation. These conclusions are in contrast with an existing school of thought in which the energy input by buoyancy fluxes is neglected. They show instead a subtle inter-dependence of buoyancy and wind forcing, and the work will lead to a better understanding of how the circulation, water conditions and heat transport will change with global warming.

In Mathematical Geophysics research has been ongoing in the area of nonlinear inverse problems and development of new ensemble based approaches for seismic imaging and more general inference problems. Establishment of the new inversion AuScope funded laboratory took place during the year. This is a venture whereby scientific computer software is developed for the geoscience community implementing advance algorithms for nonlinear inversion applied to various data types. During this year programmers have been seconded from the ANU supercomputing Facility to build the first suite of software for release in early 2012.

In lithosphere dynamics attention has been focused on applying new inversion methods, developed in the group, to the reconstruction of the Earth’s past plate motions from finite rotations of the Lithosphere. It has been found that the largest kinematic changes across ridges are actually due to the observational noise in finite rotations. Upon noise removal a formal analysis of the spectral content associated with spreading-rate records excludes contributions from periods shorter than 0.5 – 1 Myr since mid/late Cenozoic. This supports the notion that the figure of current plate motions from space geodesy remained stable, and may therefore be extended back in time, for longer than ever thought.

Geodynamics research activity increased in 2011 with the addition of three new staff, involved in the development of a web interface and software for the analysis of space gravity data and the assessment of GPS time series. In addition, two international students were hosted (from Toulouse and Princeton) for several months, both of whom participated in the analysis of GRACE observations. In-house software was developed to process raw GRACE observations in order to generate estimates of the temporal gravity field of the Earth, and were also made official members of NASA's GRACE Science Team. Terrestrial gravity surveys were conducted in Western Australia and Central Australia using both the absolute gravimeter and relative tide meters. Advances were made in the software used for modelling of the response of the Earth to changes in continental ice loads, in particular the inclusion of degree-1 deformation and higher resolution coastlines and bathymetry models.


Professor Malcolm Sambridge
Associate Director, Earth Physics


Research Projects

EP: Seis & Math Geophysics   
Extending the Global Database of Geomagnetic Excursions Elizabeth Ingham
Varying mechanical coupling along the Andean margin: Implications for trench curvature, shortening and topography Giampiero Iaffaldano
Crustal Structure of Australia from Ambient Seismic Noise Tomography Erdinc Saygin
Self Adaptive Surface Reconstruction from Multiple Datasets Thomas Bodin
AuSREM – Australian Seismological Reference Earth Model Michelle Salmon
Building a Dual Purpose Network: The Australian Seismometers in Schools Program (AuSIS) Natalie Balfour
Seismic structure of the southeast Australian lithosphere from surface and body wave tomography Nick Rawlinson
3-D Structure of Flinders Ranges from Local Earthquake Tomography Simone Pilia
AusMoho: the variation of Moho depth across Australia B.L.N. Kennett
Signals from Noise: High-Resolution Maps of the Southeast Australian Crust Mallory Young
The Shuffling Rotation of the Earth's Inner Core  Hrvoje Tkalčić
Structural controls on the Mw 9.0 2011 Offshore-Tohoku earthquake B.L.N. Kennett
EP: GFD   
A New Conceptual Model for the Antarctic Circumpolar Current Callum J. Shakespeare
The Effects of Turbulent Mixing on the Global Ocean Overturning Kial D. Stewart
Isothermal dynamics of channelled viscoplastic lava flows Jesse Robertson
How quickly does the ocean adjust to changes in surface forcing? Ross W. Griffiths
EP: Geodynamics   
Leaky LMS Algorithm and Fractional  Brownian Motion Model for GNSS Receiver Position Estimation Jean-Philippe Montillet
The anatomy of interglacial sea levels  Kurt Lambeck
Reconstruction of the Laurentide Ice Sheet Evan James Gowan
Developing temporal gravity fields from GRACE observations E-K Potter, S. McClusky, P. Tregoning