While Melbourne escaped major damage in last month’s magnitude 5.9 earthquake, experts say thorough building inspections are needed to check for any undetected problems.
Buildings in Melbourne – particularly structures with masonry and reinforced concrete – may be vulnerable to “significant damage” even with low seismic activity, according to scientists who have assessed the impacts of the 22 September tremor.
Mark Quigley, an associate professor of earthquake science at the University of Melbourne, says: “Parts of Melbourne have extreme vulnerability to relatively low intensities of ground shaking.”
After the magnitude 5.9 earthquake struck near Mansfield in September, scientists “raced to the scene” to monitor the seismic activity around the epicentre.
The team observed shallow landslides, toppled trees, cracks in roads and several toppled chimneys in Woods Point, 15km from the epicentre.
Quigley said the depth of the earthquake – an estimated 12.7km – and the way the fault ruptured meant it was not felt as strongly near the epicentre as might be expected for a tremor of its magnitude.
“At the surface in the epicentral region you’re still several kilometres away from the seismic source fault,” he said. “The top of the fault that ruptured is probably 4km deep.”
Mansfield avoided major damage likely because the fault rupture sent more seismic energy outwards rather than upwards. Tremors were felt hundreds of kilometres away, as far afield as Sydney and Launceston.
“The strength of the Australian crust meant the earthquake was felt over much greater areas than would be the case in New Zealand [for example],” Quigley said.
Much of Melbourne’s southeast and the southern part of its central business district is built on soft sediments, which is potentially concerning for the structural integrity of buildings post-earthquake. “The soft sediments in Melbourne amplify the seismic waves and therefore increase the potential for damage,” Quigley said.
“It highlights the vulnerability of some of the buildings in those areas to … significant damage under relatively low ground motions.”
Construction codes introduced in 1979 require all buildings to be designed for earthquake resistance.
But Quigley said much of Melbourne city has legacy builds that have not been assessed for their ability to withstand earthquakes.
“Older buildings that have not brought up to code are particularly vulnerable,” he said, adding that although less likely, damage was still possible in structures that have been built to comply with requirements.
A spokesperson for a major construction and engineering company said the firm had implemented an earthquake inspection procedure, visually checking projects currently underway for fatigue and cracking in their structures.
More detailed inspections of ground conditions would not be able to take place until construction activities resumed in Victoria next week, he said.
Quigley said there was a misconception that a large earthquake “reset the clock” before another significant tremor would occur. “The statistically most likely time to experience future earthquakes is immediately following prior earthquakes.”
As of last Saturday, more than 150 aftershocks with a magnitude greater than 1.0 had been recorded, the largest of which was a magnitude 4.2 aftershock that occurred 15 minutes after the initial earthquake.
