What goes up must come down: Why is Australia sinking?
Channel: GeoscienceAustralia
Date: 2020-07-15

Presented by Anna Riddell

What goes up must come down and it appears that the Australian continent is sinking. Understanding how the surface of the earth changes has many important applications such as understanding climate and sea level change. This presentation will explore how the Australian plate is moving and what that means for applications of precise positioning.

Motion of the Australian continent has been well studied in the horizontal direction, but there are minimal studies considering the vertical motion. Recent measurements using the Global Positioning System (GPS) suggest that the Australian continent is sinking, but current understanding of geophysical processes suggests that the expected vertical motion of the plate should be close to zero or uplifting.

Understanding how the surface of the earth changes has many important applications like the response of the earth system to climate and sea level change, and resource availability. This presentation will explore how the Australian plate is moving and what that means for applications of precise positioning.

Response to questions

• Time series results are interesting. Do seismic effects show exponential decay?
There is an element of exponential/logarithmic decay in the GPS postseismic deformation that I observed, but I did not constrain my models to these decay functions. It is common to model postseismic deformation with an exponential/logarithmic decay function.
• Thanks Anna that was fantastic. Can VLBI contribute to the SLR vs GPS origin determination?
Great question and yes, I think it would be interesting to see if VLBI could contribute to estimates of geocentre motion and help to shed any light on the non-linearity’s (banana shape) present in the SLR data.
• Great talk Anna, really thought-provoking. Sandiford (2007) showed tilting of Australia about a NW-SE axis over geological time, sinking in the NE and uplifting in the SW, Did you not see any evidence of this?
Sandiford (2007) was able to draw on really long timescales to determine the NE/SW tilting of the Australian continent, and I am looking at relatively short (present-day) timescales. There was some evidence in the spatial filtering of the tilt patterning, but not enough to make a conclusive statement.
• Perhaps postglacial (last 18ky) melt that causes isostatic rebound in the N hemisphere and Antarctica, and elsewhere is being compensated by Australia sinking? The earth volume stays the same... so plausible?
There are many processes at work that cause surface movement and during my research I attempted to isolate the isostatic adjustment component in Australia (expecting it to be small). Digging into the detail of the GPS observations of vertical motion in Australia demonstrated that if the movement is less than ~0.5 mm/yr, we were unable to determine if the observation is a real signal due to geophysical processes or due to noise (i.e. within the uncertainty of the original GPS observations).
• Potential ANSWER to Richard’s question: it's been shown that Australia is tilting around a NW-SE axis, going down in the NE and up in the SW (Sandiford, 2007).
Thanks – this helps understand the long-term geologic motion of the Australian continent.
• How do the geodetic observations reconcile with long term geological estimates of uplift or subsidence?
Noting that my research focused on the present-day motion, I did not look at long-term geological estimates of uplift or subsidence across Australia. This would be an interesting future research project/question.
As noted by another viewer: it's been shown that Australia is tilting around a NW-SE axis, going down in the NE and up in the SW (Sandiford, 2007).
• El Nino influences relative level? How?
The plots of the El-Nino patterns of motion are derived from GPS which demonstrate the solid-earth response to movements of mass associated with the atmosphere, hydrology and ocean-loading as linked to ENSO and other climate oscillations.
• Would the changes over a decade in the geocentre over time be clearer if we fitted the seasonal changes first?
Yes – taking into account the seasonal changes will improve the trend (long-term motion) and uncertainty estimate. This is done for the current estimate of the geocentre motion.
• Would you call seasonality coloured noise if linear was white noise?
Seasonality (annual and semi-annual) can be considered in the presence of white and/or coloured noise, as can a linear trend. It is mostly at the user’s discretion as to how the uncertainty and noise model is defined. i.e. by taking coloured noise into consideration (or not). It is common to fit a linear trend without the consideration of any additional noise model, in which the assumption is made that the trend is estimated under white noise conditions (without temporal correlations).