Land of earthquakes and volcanoes?
ON WEDNESDAY 24TH FEBRUARY 2010 at 9:27am a small earthquake shook the Lithgow area of New South Wales. This is the sixth earthquake in NSW to be widely recorded on seismographs throughout southeastern Australia during February.
At a time when the Australian Government is planning to build a radioactive waste repository and being urged to join the nuclear power club by eminent lobbyists such as Bob Hawke amongst others, these earthquakes are a reminder to all that the Australian crust is not a geriatric, stable, immobile basement.
The continent as a whole is being carried along northward at 70mm/yr by deep-seated tectonic forces, so it is perhaps no surprise that stress has built up to breaking point in the 25-km-thick brittle upper crust in many parts of Australia.
NSW regularly experiences small earthquakes and occasional moderate ones; like the December 1989 Newcastle earthquake, with a magnitude of 5.6 on the Richter scale. Similar sized earthquakes have occurred in NSW before; near Dalton in 1886, 1934 and 1949; at Bowral in 1961; and Picton in 1973.
By sheer luck no one was killed in these earlier earthquakes although there was substantial damage and landslides.
Australia’s earthquake history is mild but present
BUILDINGS IN THE SOUTH Australian capital Adelaide were damaged by a magnitude 5.6 earthquake in March 1954, just prior to the young Queen Elizabeth’s first visit. Amazingly no one was killed and the damage was cleaned up quickly prior to the royal visit.
In1902 two people died in Adelaide as a result of an earthquake southwest of Adelaide, that was magnitude 6.0 and there was an even larger earthquake in the southeast of South Australia in 1897, which caused damage locally and was felt as far as Melbourne and Tasmania.
The outstanding feature of this southeast region of South Australia is the landscape of young volcanoes there, Mt Gambier and Mt Schank, whose eruptions were suffered by local Aborigines, their campfires buried in volcanic ash.
These volcanoes are just the western end of a chain of hundreds of volcanoes that stretch across Victoria to the vicinity of Melbourne where many buildings and walls feature the local scoria or welded ash. Young volcanoes with extensive lava flows exist too in Queensland, so much for the stable continent!
Large earthquakes have occurred in the Northern Territory, one near Tennant Creek as recently as February 1988 was almost as big as the recent Haiti earthquake. It cracked the Earth’s surface leaving behind as evidence a 35 km long, 2 m high fault scarp.
The largest recorded earthquakes in Australia
THE FAULTING CRUMPLED and shortened the buried natural gas pipeline providing energy to Darwin and caused damage in the Tennant Creek hospital. Even larger earthquakes have occurred in Western Australia cracking the whole thickness of the 40 km thick crust.
In 1906 the largest recorded Australian earthquake occurred offshore from Exmouth, its magnitude at least 7.3. It was felt throughout the state of WA, about one-third of the land area of Australia.
The ancient Archaean shield, supposedly one of the most stable areas on Earth, was fractured in 1968 by a magnitude 6.8 earthquake that turned the standard gauge railway into a giant pretzel (pictured), ruptured the Kalgoorlie-Mundaring water pipeline and raised a 1 m high step in the main east-west highway.
Prior to the first useful seismographs being established in Australia in 1901, an incredible sequence of earthquakes struck north-eastern Tasmania, some of them up to magnitude 6.5 – yet Tasmanians have experienced few strong earthquakes since then.
Like Haiti the time between destructive earthquakes in Australia may be four hundred years or more and we have not yet lived long enough in this land to understand the nature of our earthquake problem.
The next big earthquake could occur at anytime
EVEN SO WE KNOW that a large magnitude 6 or greater earthquake occurs on average about every five years somewhere in this ancient land, the last one in north-west WA was in 1997, nearly 13 years ago.
We expect another large earthquake at any time and there is no way of knowing where or when it will occur. The only way to achieve some peace of mind is to design and construct buildings to resist earthquakes so they will not collapse, putting lives at risk.
To this end a relative earthquake hazard map has been drawn up based on the data from 110-odd years of earthquake monitoring and another 50 to 100 years of anecdotal information on earthquakes that people noticed in the main population centres. The basic premise of the map is that earthquakes in the next 100 years are more likely to occur where they did in the last 100 years.
This hazard map is one plank of the Australian Earthquake Loading Code which has been called up as part of the Australian Building Code. Engineers and architects have a legal responsibility to consider earthquakes when they are designing and building structures, from houses to major apartment buildings.
Adequate monitoring should be in place to record the amplitude and frequency of strong earthquake shaking so that future structures can be better prepared, properly designed for Australian earthquakes. These data are not available at present so we rely on information collected overseas.
Are Australia’s buildings prepared for earthquakes?
SOME STRUCTURES DESERVE special treatment, a higher level of resilience, such as schools and hospitals, dams and power stations, nuclear containment repositories. Let’s hope someone in government is aware of the risk and ensuring that earthquakes are a factor in the design and construction of such buildings as few if any building certifiers are qualified to make that call.
The actual situation is that the law courts are left to judge whether the designer and engineer are culpable, but only after a structure has collapsed in an earthquake. There are no building regulators qualified to ensure that the earthquake code is followed and the public are generally unaware of the risk of earthquakes.
The reasons for the lack of public knowledge about Australian earthquakes include; that the media get bored with news stories when there is no damage and there are no casualties, government agencies monitoring earthquakes are not required to advise the media or the public and are reluctant to do so for fear of litigation. All of these deficiencies could be remedied but will they?
Kevin McCue is the Director of the Australian Seismological Centre, the President Australian Earthquake Engineering Society and an adjunct professor at Queensland University.