A MAGNITUDE 5.4 earthquake which hit south of Trafalgar in June unleashed a barrage of aftershocks felt across Gippsland.
The strength of the aftershocks has varied, with the strongest being a magnitude 4.3 quake felt late in July; just over one month after the first earthquake.
The Warragul Citizen talked to Geoscience Australia seismologist Doctor Jonathan Bathgate to get the scientific background of what is happening below our feet.
Why are we getting so many earthquakes in Gippsland?
“The reason Australia in general gets earthquakes is because the tectonic plate is moving north by about seven centimetres per year, and that movement basically imparts stresses within the crustal rocks,” Dr Bathgate said.
“That stress at certain places around the country gets released… along local fault lines.”
“So you get areas like the Gippsland region, the Flinders Rangers and a very active region south west of Western Australia that are more susceptible to earthquakes than other areas of the country.”
“The Gippsland region is historically reasonably active.”
How do you define an aftershock and how long will they continue?
The recent magnitude 4.3 aftershock “is basically in the same spot as the main shock… and it is smaller so it generally qualifies as an aftershock,” Dr Bathgate said.
How long they will continue for “really does vary… the large one, the 4.6, has basically set off this siesmic activity that we have been seeing there ever since.”
“Generally what we see from a magnitude 5, 5.5 or so size earthquake is that you do get quite a lot of aftershock activity, and that can continue anywhere from just a few days to months so it really does vary.
“We can just go on a general pattern that they do tend to decrease in frequency and magnitude over time, but as we saw [with the 4.3 magnitude aftershock] you can still get some larger aftershocks with those smaller ones.”
“Before [the 4.3] it looked like it was settling down.”
“Aftershocks… are essentially just the faults coming back to an equilibrium point after having moved… Which will stop the earthquakes once it has found that equilibrium.”
“Once they die off the fault will start to build up stress again, but you may not see activity there for quite a long while.”
“It could be hundreds of years.”
Why do some houses shake when others do not?
“There’s a lot of factors at play there; building construction types can vary. A lot of it depends on the geology of the area,” Dr Bathgate said.
“Suburbs not too far apart may sit on different geology types, and sedimentary bases will amplify the effects of the quake so it will seem stronger in some places than others.”
“It’s dependant on what sort of activities people are doing as well, so if it is felt quite weakly then some people might not notice it based on what they’re doing at the time.”
How much of a factor is depth in an earthquake?
“They’re all generally quite shallow, all of the earthquakes that occur onshore of Australia are generally in the top 15 to 20 kilometres of depth… basically because of the rock,” Dr Bathgate said.
“Depth isn’t really a factor in what people feel because if it’s going to be very shallow to 10 kilometres, it’s not going to make much difference.”
“Definitely more shallow ones can be more destructive.”
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