Cause of Earthquakes in South Australia- Alternative assignment PDF

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Why are here earthquakes in South Australia “Earthquakes are the vibrations caused by rocks breaking under stress” (Geoscience Australia n.d). This report aims to answer the question why are there earthquakes in South Australia? And this will be done by looking at the Tertiary and Quaternary geology and fault structures of the St. Vincent basin, discussing the past earthquake activity in SA and then explaining the cause of this earthquake activity.

“The St. Vincent Basin is an area of Cainozoic Sedimentation with dominantly paralic facies” (Geological Society of Australia 2007). The basin consists of areas around a gulf that existed during the tertiary period called St. Vincent Gulf. The sedimentation St Vincent basin has a maximum thickness of around 2200 feet and the rocks and strata consist of tertiary and quaternary sediment either surrounding or laying over more modern rocks (Geological Society of Australia 2007). Burnham limestone “is a pale grey, micritic limestone with poorly defined bedding and a fauna indicative of an estuarine or lagoonal environment” and is also the oldest quaternary sediment found in St. Vincent Basin (Belperio 1995). Burnham limestone was deposited in the St. Vincent basin by the constant movements of water across the land and into the St. Vincent Gulf during the early quaternary (Belperio 1995). There is an elevation difference present between the basin and upfaulted implants present within it, and this indicates 80 to 90 metes of relative subsidence in the St. Vincent basin during the quaternary (Belperio 1995) (see fig 1). There is also uplift or “vertical elevation of the Earth’s surface in response to natural causes” present in the St. Vincent basin, and this means that there are tertiary and quaternary faults present within the St Vincent basin as well. Because uplift only occurs due to the force created by movement of the Earths plates (The Editors of Encyclopaedia Britannica 2017). A fault is a break in the rocks that make up the Earths crust, along which rocks on either side have moved past each other (U.S. Geological Survey 2006) (see fig 2). There are multiple quaternary and tertiary faults present within the St Vincent Basin (see fig 3) (Verngust, Reid & Hill, 2015).

Figure 1 A cross section of Le Fevre peninsula and mount lofty that can show the relationship between Quaternary sediment and continental facies of the St Vincent Basin (Belperio 1995).

Figure 2: Example of a fault

Fig 2

Fig 3

Figure 3: A map displaying the major structures and fault lines of the Tertiary St. Vincent Basin (Verngust, Reid & Hill, 2015).

Summary of the Earthquake activity Over the past 150 years, South Australia has been one of the most earthquake-prone states in Australia. According to research conducted by Katherine Dix, there have been over 600 recorded earthquakes since 1837 in places such as Beachport, Belair, Mount Barker, Adelaide and many other areas within the state (Dix 2013). The Beachport earthquake occurred in 1897 and is the most powerful Earthquake to have happened in Australia since 1837. This earthquake registered a Magnitude of 6.5 on the Richter scale and caused enormous amounts of damage in the South East. The most affected areas were Robe and Beachport. There was visible slumping (liquefaction see fig 6) in Robe while there was some damaged property found in Beachport, but minor damage

from this earthquake reached Adelaide (see fig 5), while the tremors could be felt from as far away as Melbourne (Love 2014). Although the Beachport earthquake was the most potent to hit the region, the most infamous perhaps was the 1954 earthquake that hit Adelaide. This Earthquake measured 5.5 on the Richter scale its intensity (see fig 4) caused widespread damage including demolished buildings (see fig 7) and numerous surface cracks, the total losses for this earthquake equate to around $150 million in today’s currency (Love 2014) (Mcguire 2011).

Fig 4 Fig 5

Figures 4 and 5: Earthquake intensity maps of the Adelaide 1954 Earthquake (left) and 1897 Beachport Earthquake (right) (Love 2014).

Fig 6

Fig 7

Figure 6: liquefaction damage (slumping) caused by the Beachport earthquake. Fig 7: House demolished in 1954 Adelaide earthquake (Love 2014).

What causes this earthquake activity in SA? Earthquakes primarily occur at plate boundaries, (areas of the Earth’s crust where the tectonic plates are pushing against one another) (see fig 8) and approximately 80% of all earthquakes occur on the Pacific Plate, which affects Japan, the west coast of the Americas and New Zealand (Young 2011). However, as displayed in figure 8 South Australia does not lay on top of a tectonic plate boundary so how come there is a significant amount of seismic activity here? Well, the fact is that the sudden movement and collision of faults can also generate seismic waves, therefore also responsible for the occurrence of some earthquakes (GNS Science n.d). This information relates to South Australia because as was shown in figure 1, the stress put on the Earths crust by

the deposition and movement of rocks during the quaternary and tertiary has resulted in the formation of faults in the St. Vincent Basin as well as other areas in South Australia (e.g. Flinders mountain ranges, see figure 9). And more faults results in more fault movement which turns into more seismic activity.

References

Belperio, A.P 1995, ‘Quaternary’, The Geology of South Australia, vol. 2, pp 219-280, viewed 25th May 2018,

Dix, K 2013, ‘South Australian Historical Earthquakes in the Pre-Instrumental Period 18371963: A Comprehensive Chronicle and Analysis of Available Intensity Data’, MA thesis, University of Adelaide, Adelaide. Geological Society of Australia 2007, ‘IX. The St. Vincent Basin’, Journal of the Geological Society of Australia, vol. 5, no. 2, pp 115-126, DOI: 10.1080/00167615708728486 Geoscience Australia n.d, Earthquake, Geoscience Australia, viewed 25th May 2018, . Love, D 2014, Past Earthquakes, Minerals State Development, viewed 25th May 2018, < http://minerals.statedevelopment.sa.gov.au/geoscience/geoscientific_data/earthquakes/past_e arthquakes>. McGuire, M 2011, ‘Adelaide earthquake: where the faults lie’, The Advertiser, viewed 25th May 2018, . The Editors of Encyclopaedia Britannica 2017, Uplift, Encyclopaedia Britannica, viewed 27th May 2018, < https://www.britannica.com/science/uplift>. U.S Geological Survey 2006, What is a Fault, U.S Geological Survey, viewed 25th May 2018, . Vergunst, J, Reid, A.J, Hill, S.M 2015, ‘Origin of Eocene Sedimentary Cover Adjacent to the Hillside Cu-Au Deposit: Detrital Zircon Provenance of Quartoo Sand Member’, Geological Survey of South Australia, vol. 77, no. 2, pp 27-35, viewed 28th May 2018, ....