On 2 June 1979 at 5:54 pm an earthquake measuring 6.1 on the Richter scale struck the small town of Cadoux and surrounding districts. It was the second most damaging earthquake in the history of Western Australia and one of only 12 earthquakes in Australia’s modern history to have caused the ground to rupture.
This earthcache science lesson is about surface ruptures associated with the Cadoux earthquake.
To log this earthcache…
To log this earthcache you will need to visit GZ and send me a message or email addressing each of the following earthscience tasks based on your observations and the information supplied:
1. What type of fault did the Cadoux earthquake produce and what is the primary type of movement associated with this fault type?
2. Examine the surface rupture you observe at GZ. Is it in rock, soil, sand, gravel or some combination of these? Can you see evidence of lateral displacement (side-to-side movement)? Measure or estimate the displacement along the surface rupture, including the width and visible depth.
3. Take a photo of the landscape looking east from GZ showing yourself or something with your team name on it and either post it with your log or send it to me with your answers. Feel free to include the sign showing ‘whose fault' it was here but please don’t include the actual surface rupture in your photo.
You may log your visit once you have submitted your answers to me. I will respond confirming permission to log, or seeking clarification if I feel that you have not properly answered the questions. Logs with no answers sent or photo included will be deleted.
Please do not post your answers in your log.
The Cadoux earthquake
The 1979 Cadoux earthquake occurred in the Southwest Seismic Zone (SWSZ) of Western Australia.
The SWSZ lies mostly within the Yilgarn Craton. The surface geology of the Cadoux area generally comprises Archean granite bedrock overlain by laterite, sandplains, and valley floors with loose, unconsolidated sediments 5-30 metres thick.
The earthquake is believed to have been caused by east-west compressive stress in the earth’s crust. It produced a shallow dipping fault along a more-or-less north-south axis.and caused a 15 Km long complex, multi-fault surface rupture consisting of six identifiable scarps. These were the 8 km long west-dipping Robb scarp, 3 km long south-dipping Cumming scarp, the Lone Tree, Carter and Tank scarps (which together define an east-dipping bow-shaped rupture) and the 2.5 km long southwest-dipping Kalajzic scarp.
It was one of the most structurally complex earthquakes globally for this moderate magnitude.
The epicentre is believed to have been within 6 Km of the south-eastern end of the surface rupture zone and about 4-6 Km below the surface.
Surface ruptures
Surface ruptures are cracks or displacements that occur along the earth's surface during an earthquake. They form as a result of the movement of tectonic plates and are often the most visible evidence of an earthquake’s impact.
There are two main types of surface ruptures:
1. Vertical surface ruptures: These occur when there is vertical displacement along the fault. This type of movement pushes the ground up or pulls it down.
2. Horizontal surface ruptures: These occur when there is horizontal displacement along the fault, where the ground moves side-to-side.
Surface ruptures can vary in size, from small cracks to large, dramatic displacements of several metres.
Surface ruptures associated with the Cadoux earthquake
The 1979 Cadoux earthquake was a strike-slip earthquake, which means the primary motion along the fault was horizontal, where the two sides of the fault slide past each other horizontally, rather than vertical.
The earthquake caused horizontal displacements of up to 2 metres in places. These displacements were concentrated along the fault line, resulting in a series of surface ruptures visible in the landscape. The ruptures can typically be observed as linear cracks that run along the direction of the fault or displaced ground where the land shifted horizontally due to the fault movement.
In addition to the main fault, there were also secondary ruptures created by the earthquake. These ruptures are smaller but still significant and typically show less displacement than the primary fault.
Although much of the evidence of the surface ruptures has been destroyed by farming, the ground in some parts of Cadoux still shows visible evidence of the seismic event and some sections of the rupture are still visible across ploughed fields in satellite imagery.
Nearby earthquakes
There have been a number of other significant earthquakes recorded in modern times in the Southwest Seismic Zone near Cadoux. These include earthquakes at Gabalong (1955), Meckering (1968), Calingiri (1970 & 1971) and Burakin (2000 - 2001).
The 1968 Meckering earthquake, approximately 100 Km south of Cadoux, was Western Australia’s largest recorded onshore earthquake and measured 6.9 on the Richter scale. To learn more about the Meckering earthquake, including how the intensity and effects of earthquakes are measured, visit ‘Meckering Moved’ (GC5Q4V3).
Additional information about the Cadoux earthquake can be found on an information board and monument in the town of Cadoux.