During 2015 the Seismology and Mathematical Geophysics group carried out research in structural and source seismology from the lithosphere to the core, Lithospheric and mantle dynamics and inverse theory, Natural Hazards, Geodynamics and community outreach through the Seismometers in Schools program.
This year saw the start of new AuScope programs to provide maintenance to the Earth imaging and seismometers in Schools programs. The ARC supported research projects within the group in the Discovery, Linkage and Future Fellow programs. Ongoing external funding support for various programs was also received from international sources including the US Dept. of Energy, DFAT/AuSAID, and The United Nations Comprehensive Test-Ban Treaty Organization.
The group’s seismic instrument pool on both land and at sea has been deployed under the auspices of ANSIR, the national research facility for Earth sounding, in several field projects this year. These include the WASP (Western Australia Spiral Array) array shown as a blow up in Figure 1. An experimental style of configuration following the earlier SQsp array in Queensland. The array AQ3 in NSW/QLD was completed this year. AQ3 is a 50 kms spacing continuation of the rolling Wombat array used to explore the seismic structure of the Australian Continent. In WA the larger ALFREX (Albany-Fraser experiment) was deployed to image the edge of the WA craton in the Albany-Fraser Orogen. Figure 2 shows arrays deployed in Indonesia up to and including 2015 which have been used to study the amplification of seismic waves in basins in the context of natural hazard assessment. The new ANSIR community model, involving 11 institutions in Australia and New Zealand, has seen completion of activities in its first full year of operation.
Over the last 3 years the group has installed 44 research quality seismometers as part of a combined outreach and research program. Figure 3 shows the map of seismometer locations at the end of 2015. The seismometers send data back to the Research School of Earth Sciences, which is in turn sent in near real time to the Incorporated Research Institutions for Seismology (IRIS) data management center where it can be accessed by researchers around the world. The data is already being used for earthquake location (Geoscience Australia and the Geological Survey of South Australia) and for research within the school and within Australia. 2015 saw a much anticipated deployment in Alice Springs High School which improves the national coverage. Alongside the seismometers group members have also conducted workshops to assist teachers to incorporate seismology into the classroom. This year these include the National Youth Science Forum Teachers Summer School, the Australian Science Teachers Association annual conference and for Teacher Earth Science Education Programme (TESEP) professional development workshop. Social media has been used to maintain engagement with students, teachers and enthusiasts through our website, facebook page and youtube.
The Terrawulf computational facility, has seen significant hardware upgrades in 2015. All of the T2/T3 compute servers are operating and users are routinely running production code on the cluster. During the past year over 4 million CPU-hours have been consumed and almost 10 thousand jobs run on a wide range of earth science simulation and inversion problems. Average cluster utilisation was around 50% with frequent peaks of over 90%. 31 users have been active on the facility with 21 additional users through collaboration across 22 institutions in 8 countries. Group members also made extensive use of the National Computational Infrastructure facility and renewed its large time allocation grant on computational geophysics.
This year has seen work across a broad range of topics in seismology including the simultaneous exploitation of multiple seismic arrays and the nature of lithospheric heterogeneity. Methods using stacked station autocorrelograms to look directly at P wave reflectivity in the crust and uppermost mantle have been refined and applied to refining the structure of the Moho is southeastern Australia and an exploratory survey of structure in the mantle lithosphere and asthenosphere across the Australian continent. Recent collaborative work has also established the first Pn tomography across Australia and significantly improved definition of P wave structure in the upper mantle across the continent.
Other contributions in theoretical and global seismology were made toward the characterisation of complexities at the Earth’s inner core and its boundary and new methods were developed for the measurement of Earth free oscillations using the Neighbourhood Algorithm. In other studies a new structural seismic model for northeast Asia was developed using ambient noise measurements as well as new methodologies for joint Surface wave dispersion and Receiver functions within a Bayesian framework. A project on real time earthquake monitoring has involved exploitation of the reciprocity theorem for efficient calculation of 3-D structural Green’s functions in Australasia. This has the goal of rapid inversion for the realistic earthquake source parameters in complex earth models.
The group has also continued with the application and development of Bayesian inversion in global seismology, with several recent contributions, including the characterisation of ultra-low velocity zones in the lowermost mantle; earthquake source parameter estimation; spherical harmonics expansion; and lithospheric structure.
In Natural Hazards the focus is on research into earthquakes and tsunamis as well as their impacts on society. In 2015 the group made important contributions to the science of earthquake source inversion, using both seismic and tsunami data, in particular showing how reverse imaging techniques commonly employed in seismology can be applied to tsunami data, and how better modeling of tsunami physics combined with new data inference techniques resolve important details of the source. The group has also progressed research on Indonesian earthquakes, with a seismic hazard studies of Sulawesi and Sumatra (See Figure 2), and an imaging study of the Jakarta basin that shows how remarkably deep and soft the infill sediments are, implying strong amplification of seismic waves in one of the world’s most densely populated cities. During 2015 the group has worked with Indonesian collaborators on similar seismograph deployments in Palu and East Java that will be the focus of future work.
Trans-dimensional Bayesian inversion has been further developed in the context of 3-D imaging using the tree based algorithm reported on last year. Signal reconstruction of time dependent signals has become a focus in 2015. A new theory has been developed which extends earlier results in encompassing arbitrary noise contamination of observations in both the time and signal measurement. An application of the new technique has been the subject of a new collaboration with RSES staff in the Environmental area of the school in the reconstruction of relative sea level variations over the past 500 k.a.
In 2015, geodynamical research in the group focussed upon three key topics: (i) lowermost mantle dynamics and its seismic expression; (ii) the relationship between intra-plate volcanism and deep-mantle seismic structure; and (iii) the origin and expression of intra-plate volcanism on the Australian continent.
Together with international collaborators a study was carried out which reviewed a wide-range of seismological observations and their implications for deep mantle structure and dynamics. This study concluded that thermal heterogeneity alone can explain observed deep mantle seismic characteristics, which has important implications for Earth’s thermo-chemical, geological and tectonic evolution.
The statistical significance was examined of the proposed correlation between volcanic hotspot locations and the reconstructed eruption sites of large igneous provinces at Earth’s surface with deep mantle large low shear-wave velocity provinces (LLSVPs). These rigorous tests demonstrate that the observed distribution of African and Pacific hotspots/reconstructed LIPs is consistent with the hypothesis that they are drawn from a sample that is uniformly distributed across the entire areal extent of each LLSVP. This result differs to the predictions of previous studies and has significant implications for our understanding of lowermost mantle dynamics and its surface expression.
In a separate collaborative study a result was obtained which identified the longest intra-plate continental hotspot track on Earth, a ~2000 km long track in eastern Australia that extends from central Queensland, through New South Wales and into Victoria. This work was published in Nature and received widespread media attention. A key conclusion is that lithospheric thickness variations have the dominant influence on volcanic outcrop and magma composition along this track, thus providing the first observational constraint on the sub-continental melting depth of mantle plumes.
Staff changes include the departures of Drs. Josip Stipcevic and Giampiero Iaffaldano to positions in Europe. Ph.D. students Mr. Surya Pachhai, Jakir Hoseen and Stephen Sagar submitted their theses during the year, while honours students Manon Dalaison began a project on geodynamics and Mr. Jack Muir moved to Caltech to pursue a Ph.D. in seismology and Mr. Ingo Stoltz moved with Dr. Iaffaldano to pursue his Ph.D. studies. A newly arriving Ph.D. student in the group was Mr. Marcus Haynes. Mr. Abhinav Purelli joined the staff as a Technical Officer at the Warramunga Seismic Array in the Northern Territory. Weijia Sun has been a longer term visitor this year from the Institute of Geology and Geophysics, Chinese Academy of Sciences.