I would like to respond to the letter written by Ily Ystra in the June 20, Edition of the Armidale Independent.
The recent earthquake that occurred north of Lake Keepit, between Tamworth and Gunnedah was triggered by rupturing along an ancient weakness in the Earth’s crust. This weakness, a thrust fault, marks the separation between two major geological provinces, the Sydney — Gunnedah Basin to the west and the old New England Orogen to the east. The hypocentre (point of rupture of the earthquake) was 17km deep, and can be projected at depth to coincide with the moderately east dipping thrust fault.
Mining activities in this area are minimal and consist of shallow coal operations that are significant distances away from the epicentre (the point of rupture projected to surface). These operations could not have been the cause of the earthquake (as they have barely removed 100m of soil and rock). There are no underground, or deep open cut mining operations in the Tamworth district.
Tamworth is not situated on and around a dormant volcano, as stated. The geology around Tamworth comprises ancient sediments that were deposited in a shallow to deep marine environment. These sediments are derived from an ancient volcanic environment, similar to what we see today in Indonesia. These volcanoes were over 400 million years old and have long since eroded away (the heavily eroded Warrumbungles Volcano is only 13 to 18 million years old — a twentieth the age!).
The prominent hills that form around Tamworth are not volcanic in origin. A very large NNW striking fault, called the Peel Fault, is located immediately east of Tamworth and runs in a line on the Earth’s surface from Warialda down to Tamworth (near Bingara, Barraba and Manilla) and then east to Port Macquarie. This again is an old structure that separates geological provinces within our area.
Similar to the thrust fault near Lake Keepit, this fault has been the site of many earthquakes over the past 300 million years. Some of these movements have resulting in significant vertical displacement on one side of the fault, creating linear mountain chains (a modern analogy is the Alpine Fault in the south Island of New Zealand, creating the Southern Alps). Other significant hills in the Tamworth district, particularly to the north, are of granite, a hard, crystalline rock that forms from cooling magma within the Earth’s crust. This rock is more resistant to weathering than the surrounding sediments.
Australia is a seismically quiet zone as our continent is located in the middle of a large tectonic plate (the Australian plate). Our near neighbours, Indonesia and New Zealand, lie on the boundary of this plate and here forces pushing plates together and grinding them create frequent and large earthquakes. Although we are lucky in our tectonic location, Australia is not immune to earthquake activity, as recently experienced in Melbourne as well as New England. The Australian Plate is the fastest moving plate on earth and is racing northward at about 2cm per year. This creates internal stresses and strains as some parts of the continent move faster than others, and in slightly different directions. The most active places in Australia are the Flinders Ranges in SA, the Wheat Belt of WA and south eastern Victoria.
I would finally like to point out that mining, along with other land uses, must be undertaken responsibly, within the current laws, and with respect to the local residents and communities. There are strict laws in place to protect the environment and to protect the rights of all interested parties.
Mining in Australia affects 0.26 per cent of the total landmass, much of which is undertaken over a small period of time (in the order of years, not decades) and in most cases, land can be successfully rehabilitated. Given the geological nature of the Tamworth area, it is very unlikely that large corporations and mining magnates will ever take an interest.
Mining contributes a significant amount to the NSW economy as well as providing Australians with a standard of living most other countries dream of. It would be ludicrous to halt such activities for baseless and ignorant fears.
Dr Nancy Vickery
