Research

Cleaning the 'Crusty lens'

The Earth’s deep interior is a complex place. Beneath the crust, the mantle is a mixture of subducted slabs, ambient mantle, and possibly some primordial rocks left over from Earth’s formation. The slow mixing of these materials is evidence of the ongoing mantle convection. Understanding Earth's physical evolution necessitates examining these small-scale structures of Earth. Using seismic waves measured at seismometers on the surface, we can identify the presence of these heterogeneities in the mantle as they scatter the seismic wavefield. Combining data from several seismometers (known as an array of seismometers) allows us to map the presence of such heterogeneous material (seismic scatterers). However, what is poorly understood is to what degree the near-surface structure affects the observed scattered waves. We are investigating the effects of dipping structures on arrays with aperture up to 500km. We correct for the near-surface structures and provide estimates of potential errors in the inferred locations of mantle scatterers due to crustal effects.

Earthquakes and Great Artesian Basin springs - a tryst

The Australian continent, being void of plate boundaries, is often perceived as seismically quiescent. However, earthquakes of moderate magnitude (M6+) occur on the continent around once per decade. Within Australia the distribution of intra-plate  seismicity is non-uniform, but instead tends to concentrate along certain weak zones of increased activity. One such region is the eastern margin of the Gawler Craton in South Australia, one of the oldest building blocks of the continent. Recent deployment of new seismic arrays has greatly improved data coverage, revealing previously undetected local earthquakes. These earthquakes align with the edge of the Gawler Craton and show a strong correlation with mound springs, which are discharge points for groundwater from the Great Artesian Basin. The seismic activity is likely caused by changes in fault permeability, leading to accumulated stress and hydrofracturing-induced earthquakes.

Crustal imaging of a sediment-overburdened region 

The western two-thirds of Australia contains some of the oldest rocks on Earth, while the eastern part is comparatively younger. In the northeast corner of South Australia, there is a significant transition zone between these ancient and more recent geological formations. However, this area has been relatively understudied due to a lack of seismic stations. To address this gap, we utilized data from two recent deployments in the region to investigate the depth of the Earth's crust using receiver functions. The presence of multiple overlapping sedimentary basins posed a challenge in interpreting the data. To overcome this, we implemented a method to minimize the impact of reverberations caused by the sedimentary rocks, allowing us to focus on signals originating from the crust-mantle boundary. By analyzing the corrected data, we discovered that the crust in the study region is up to 10 km thicker than previously believed, suggesting a shared tectonic history with the older parts of Australia. This research sheds new light on the geological characteristics of this region. 

For more detailed information, please refer to the Tectonophysics paper here.

Uncovering South Australia 

Sedimentary basins trap seismic energy increasing seismic hazard and generating noisy seismograms that make determining deeper crustal and lithospheric structure more challenging. The most fundamental question that can first be asked in addressing these challenges is how thick are the sediments? Borehole drilling and active seismic experiments provide excellent constraints, but they are limited in geographical coverage due to their expense, especially when operating in remote areas. On the other hand, passive-seismic deployments are relatively low-cost and portable, providing a practical alternative for initial surveys. We utilize receiver functions obtained for both temporary and permanent seismic stations in South Australia, covering regions with a diverse sediment distribution. We develop a straightforward method to determine the basement depth based on the arrival time of the P-converted-to-S phase generated at the boundary between the crustal basement and sedimentary strata above.

This research was published in GJI.

Depth dependent deformation of The Nazca slab

This work was published in Geophysical Research Letters in 2020. Link to paper. 

Few observations exist of how a tectonic plate deforms as it descends deep into the Earth's interior at a subduction zone. Carefully selected seismic waves that mostly travel through this subducting plate, or slab, provide some of the most direct measurements of how the slab behaves as it sinks through the upper mantle (0–410 km) and the mantle transition zone (410–660 km). Studying the polarization of seismic waves allows us to detect and infer the pattern of deformation within the Earth's interior. Using this technique, we find that the Nazca slab in the Andean subduction zone in South America has undergone internal deformation during the process of subduction, in particular where the slab's 3-D shape changes. Furthermore, we find that the deeper Nazca slab (≥660 km) appears to undergo further deformation as it interacts with the stiffer uppermost lower mantle.