Witnesses to Earthquakes

Standing left to right: Jean Toohey, Ralph Cutcliffe, Judy Smith (nee Archer), Keith and Clare White. Front: Irene Sharp and Pam Megaw (nee Archer)

Eugowra’s famous swarm of earthquakes occurred during August, September and October 1994. Most of the earthquake energy was released on the 19th, 20th and 21st August, with at least 50 tremors occurring each day.

The earthquakes ranged in magnitude up to 4.2 on the Richter scale. With the largest quake, which occurred on Sunday 21st August 1994 at 3.53 pm, items fell off walls and tables and a large rock crystal standing on the counter of Nangar Gems, toppled over and fell onto the floor. Cracks appeared in some homes. Tiles fell off roofs.

Normally with intensities as low as 3 or 4, earthquakes may not even be felt. However, the epicentre of the Eugowra earthquakes was about 2km northwest of the town, and only 2 km beneath the surface, making their effect more potent. Tremors were also made more intense where homes were built on sand (eg along Mandagery Creek).

 

Richter magnitudes

Events with magnitudes of about 4.5 or greater are strong enough to be recorded by seismographs all over the world.

The following describes the typical effects of earthquakes of various magnitudes near the epicenter. This table should be taken with extreme caution, since intensity and thus ground effects depend not only on the magnitude, but also on the distance to the epicenter, and geological conditions (certain terrains can amplify seismic signals).

Descriptor

Richter magnitudes

Earthquake Effects

Frequency of Occurrence

Micro Less than 2.0 Microearthquakes, not felt. About 8,000 per day
Very minor 2.0-2.9 Generally not felt, but recorded. About 1,000 per day
Minor 3.0-3.9 Often felt, but rarely causes damage. 49,000 per year (est.)
Light 4.0-4.9 Noticeable shaking of indoor items, rattling noises. Significant damage unlikely. 6,200 per year (est.)

 

 

Descriptor

Richter magnitudes

Earthquake Effects

Frequency of Occurrence

Moderate 5.0-5.9 Can cause major damage to poorly constructed buildings over small regions. At most slight damage to well-designed buildings. 800 per year
Strong 6.0-6.9 Can be destructive in areas up to about 100 miles across in populated areas. 120 per year
Major 7.0-7.9 Can cause serious damage over larger areas. 18 per year
Great 8.0-8.9 Can cause serious damage in areas several hundred miles across. The largest recorded earthquake was Great Chilean Earthquake of May 22, 1960 1 per year
Rare great 9.0 or greater Devastating in areas several thousand miles across. 1 per 20 years

(Adapted from U.S. Geological Survey documents.)

Eugowra’s earthquakes occurred due to the release of stress in the granite rock over which the town is built. The stress may have been due to Australia’s steady continental drift north, causing the tectonic plate on which Australia sits to collide with Papua New Guinea and Indonesia. The tough granite rocks which embrace the town were simply being squeezed to breaking point.

 

 

The Earthquake Cycle

Quiescence – energy builds up in the rocks

Precursory Activity – tremors occur in the high stress location over a period of months or years

Foreshocks – a swarm of tremors occurring at a fault minutes to days before the main shock

Main Shock – the largest tremor or earthquake in the swarm cycle

Aftershocks – occur over a period of days or weeks after the main shock

Adjustment Activity – tremors which occur in the area surrounding the main shock location

Quiescence again – lasting hundreds of years in an active area, and up to millions of years in non-active areas like Australia.

Earthquake is a shear movement along a fault plane, or break in the rock. The movement is caused by elastic energy seismic waves released as the rock breaks apart, then scrapes along the fault, creating friction heat energy.

In an active area like Indonesia, Japan or Papua New Guinea, it may take only ten years for enough stress to accumulate in the rocks to trigger earthquakes.

Australia however, is in an area of low activity, and it could take hundreds of years to build up enough energy for even a moderate earthquake, and even thousands of years to build up to a large earthquake like that of Newcastle…

On December 28 1989, a 5.6 Richter scale earthquake caused damage (ranging from small cracks to total collapse) to over 35,000 homes (10,000 significant – over $1000), 147 schools (42 structural), 3,000 commercial and other buildings/structures (and numerous vehicles were also damaged by falling debris) in the Newcastle – Hunter region.

Numerous buildings beyond the immediate region, as far away as Scone, Gladstone (near Kempsey) and Sydney also suffered minor damage bringing the estimated total to 50,000 buildings throughout central-eastern NSW.

The shaking, although only lasting about 5 to 6 seconds, was felt (particularly in tall buildings) over 500km away.

Very high uninsured infrastructure and commercial/industrial losses also resulted (eg to replace all damaged footpaths alone would have cost an estimated $27million.

The worst death toll and damage occurred at the Newcastle Workers Club, which was destroyed due to its location and building standards. Walls and multiple floors collapsed, sending 300 tonnes of concrete plummeting to the ground-floor car park, trapping several dozens under the rubble and killing 9 people. At the Kent Hotel in Beaument Street, Hamilton, another three died. Approximately 70,000 insurance claims (64,000 home/contents & 6,000 commercial) were paid in the months following the disaster.

If the earthquake had struck during the evening when 2ooo people were expected at a pop concert at the Workers Club, or if it had happened a month later when thousands of students had returned to school, a great many more deaths and injuries would have resulted.