Underground noises
Experts from Kerala and Centre will study land area in a village in Kottayam where mysterious underground sounds have been heard I Representational pic

Melbourne earthquake 2023: Are they becoming more common?

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Last night at 11:41 pm local time, the greater Melbourne region was shaken by a magnitude 4.0 earthquake as calculated by the Seismology Research Centre centred near Sunbury, approximately 30km north of the CBD.

Geoscience Australia have so far received more than 25,000 reports from people who felt this earthquake, some as far as Hobart, which is 620 km away from the epicentre. In the Melbourne region, the earthquake reportedly produced shaking which lasted roughly 1020 seconds, according to witness reports on social media. It was followed two minutes later by a magnitude 2.8 aftershock, which was reported by some people in the epicentral region between Sunbury and Cragieburn.

Also read: 7.7 magnitude earthquake in far Pacific creates small tsunami off Vanuatu

Tremors felt far away

In September 2021, a magnitude 5.8 earthquake rattled Melbourne, its epicentre being at Woods Point to the east of the city. This earthquake was felt as far away as Brisbane and Adelaide. Last night’s earthquake follows a magnitude 2.5 earthquake near Ferntree Gully, to the east of Melbourne, two weeks ago on May 16. Another one with a magnitude of 2.0 was felt by about 1,300 people in roughly the same region on May 22, according to Geoscience Australia.

Although this means some Melbournians have experienced two or even three earthquakes in the last two weeks, earthquakes are not becoming more common in Melbourne. It is not unexpected for there to be 1012 felt earthquakes a year somewhere in the greater Melbourne region these need not occur at regular intervals.

Earthquakes in Australia occur as a result of stresses at our surrounding tectonic plate boundaries where different plates collide, grind past one another, or are being forced apart.
These stresses make their way towards the middle of the plate, too.

In southeast Australia, the forces at the Pacific-Australian plate boundary to the east of us the same plate boundary which passes through Aotearoa New Zealand produce a buildup of strain. This is eventually released in the form of earthquake ruptures at weak zones or faults in the crust. As a result of all this, earthquakes occur in the greater Melbourne region about once a month. Many of these typically more than three quarters are too small to be felt.

Also read: Strong earthquake hits Japan; no tsunami threat

What determines if you feel an earthquake?

Generally speaking, a larger magnitude earthquake is more likely to be felt than those of smaller magnitudes. But other factors play a part, too.

Earthquake depth affects how strong the associated ground shaking is the shallower the earthquake, the stronger the shaking. Last night’s magnitude 4.0 near Sunbury was a relatively shallow earthquake at just 3 km depth. Because shallow earthquakes produce stronger ground-shaking, they’re also more likely to cause damage.
Minor damage, such as cracked plaster and fallen pictures, were reported as a result of last night’s earthquake.

The closer you are to the earthquake epicentre, the more likely you are to feel it. You’re also more likely to experience an earthquake if you’re stationary, rather than jogging or riding a bike or driving. Some people reportedly slept through last night’s earthquake.

The earthquakes reported as felt by those near the epicentre are mostly ones above a magnitude 2.02.5, although smaller events can be felt especially at shallow depths, and in populated areas. If an earthquake happens in a remote region, there are often no people to report having felt it.

Also read: Earthquake measuring 4.5 on Richter scale hits Western Nepal

Booms and aftershocks

Very small, shallow earthquakes sometimes do not produce any shaking closest to the epicentre, but instead produce a sound akin to an explosion – a short, sharp, loud boom. This occurs when the seismic waves reach the surface and transform into sound waves.

This is different to the rumbling sound which is more commonly heard, and is often described as a train approaching. It is the result of the shaking of the built environment as the seismic wave passes through. In addition to the magnitude 2.8 aftershock from last night’s earthquake, there have been additional aftershocks less than magnitude 1.0.

These are still being examined by seismologists. There may still be aftershocks large enough to be felt in the coming days, weeks, and months, though the likelihood of these diminishes with time. Occasionally, a larger earthquake may occur, in which case the magnitude 4.0 will be considered a foreshock.

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