Australia Seismicity
The Australian continent is a unique natural laboratory for studying intraplate seismicity as it has hosted a significant number of intraplate earthquakes in the recorded history. The following maps show the geologic make up of the Australian continent (Betts et al., 2002, Aust. J. Earth. Sci., ) and the latest locations of significant earthquakes reported in the ISC-GEM Global Instrumental Earthquake Catalogue for the period 1904-2015 within the Australian continent.
Although seismicity looks sparse, an accurate picture of seismicity emerges when we map earthquakes smaller than those shown in the above map.
Left: Active seismic zones in Australia: South East Seismic Zone (SESZ), Flinders Range Seismic Zone (FRSZ), Southwest Seismic Zone (SWSZ), and Northwest Seismic Zone (NWSZ). The locations of earthquakes are illustrated by triangles. Right: A zoomed-in version of seismicity of southeast Australia (Attanayake et al., 2019). The data (circles) are from the earthquake catalogue available on IRIS Wilber 3 for the period 1967-2018.
The 2012 Thorpdale Earthquake Sequences - A Case of Interacting Faults
A rare intraplate earthquake shook the residents of Thorpdale (~120 km southeast of Melbourne) in Southeast Australia on 19 June 2012 at 10:53:29 UTC (henceforth E1). A month later, on 20 July 2012, another significant earthquake struck approximately the same region at 09:11:31 UTC, which was initially thought to be an aftershock. We assembled a new seismic waveform dataset from regional and temporary networks to constrain source properties of these earthquakes.
Our primary finding is that the first (Mw 4.9) and the second (Mw 4.3) earthquakes did not occur on the same fault as was previously thought. In fact, they occurred on adjacent faults, suggesting that slip on the first fault activated the second one. To our knowledge, this is the first time seismic evidence have been presented in Australia to demonstrate that faults "communicate" with each other through geologic processes.
The spatial distribution of earthquakes (circles) located in this study color-coded by time. The blue earthquakes occurred closer to the first event in time and green earthquakes occurred in and around the time of the second event. The arrows roughly show how seismicity migrated during this time.
The first event (Mw 4.9, large triangle) and its aftershocks are clustered (triangles enclosed within the ellipse) along a fault plane dipping at 78˚to the northwest.
The second event (Mw 4.3) and its aftershocks are clustered (triangles enclosed within the ellipse) along a fault plane dipping at 27˚to the northeast.
Coulomb stress model for the source region. The stress is loaded in red regions, and that is relieved in blue regions. While stress increased surrounding the first earthquake location, it is slightly reduced (by 0.008 MPa) in the vicinity of the second event.
The co-seismic Coulomb stress transfer appears to have slightly relieved stress on the second fault, possibly delaying it from activating. It is also likely other geologic processes such as stress field rotation and/or fluid diffusion may have been responsible for the coupling of these two faults.
Attanayake, J., Sandiford, D., Schleicher, L. S., Jones, A., Gibson, G., & Sandiford, M. (2019). Interacting intraplate fault systems in Australia: The Thorpdale, Victoria, Seismic Sequences, J. Geophys. Res., 124. https://doi.org/10.1029/2018JB016945 (pdf)