Why earthquakes are more common in places such as Vanuatu

By Elissa Steedman for the ABC

A major earthquake has struck Vanuatu, causing major damage across its capital city.

While seismic activity is relatively frequent in the region, Tuesday's earthquake is considered to be one of the strongest in recent years.

Here's what you need to know.

What happened?

At 12.47pm AEDT, a magnitude-7.3 earthquake struck near Vanuatu's capital, Port Vila.

The quake hit 30 kilometres west of the city, at a depth of 57 kilometres, according to the United States Geological Survey (USGS).

Dan McGarry, a journalist in Port Vila, described the tremor as a "violent, high-frequency shake" that lasted for about 30 seconds.

A tsunami warning was issued by the US Tsunami Warning System soon after, but it was later cancelled.

Multiple buildings and streets have collapsed or been damaged, and videos on social media show vehicles being crushed by falling debris.

Witnesses have reported major landslides near Port Vila.

Where is Port Vila?

Vanuatu is an archipelago in the South Pacific Ocean, about 1,750 kilometres east of Australia.

Port Vila is located on the south coast of the island Efate.

What is the death toll?

McGarry said police on the ground had confirmed at least one person had died in the earthquake.

"It was a violent earthquake, more violent than I have ever seen in 21 years living in Vanuatu," he said.

"A policeman confirmed there is at least one fatality, and I saw a mass casualty triage being set up at the hospital."

There are fears there could be more deaths and injuries yet to be reported.

Is this considered a big earthquake?

Yes.

Magnitude refers to the size of an earthquake and those with a measure of more than seven are considered major.

The measurement is expressed in whole numbers and decimal fractions, and each whole number increase represents a tenfold increase in amplitude.

Fabio Capitanio, an associate professor at Monash University's School of Earth Atmosphere and Environment, said the magnitude of Tuesday's earthquake was "comparable to the explosion of Mt St Helens in 1980".

"Events of a similar magnitude are associated with the risk of tsunami, however the historical record shows low tsunami occurrence, as it seems in this very case," he said.

"The damage associated to such high-energy earthquakes is expectedly large, although this area has a record of low seismic damage."

How common are earthquakes in Vanuatu?

Earthquakes are relatively common in the island nation.

According to the USGS, 24 earthquakes with a magnitude of seven or greater have been recorded within a 250-kilometre radius of Tuesday's epicentre in the past century.

The largest-recorded event in the region was a magnitude-7.9 earthquake in December 1950.

Why do earthquakes occur more often in some areas?

It comes down to where Earth's tectonic plates meet.

Earth's surface is broken up into massive slabs of solid rock that make up seven major and eight minor plates.

Most of the boundaries between plates are hidden beneath the planet's oceans, the USGS explains.

"Tectonic plates probably developed very early in the Earth's 4.6-billion-year history and they have been drifting about on the surface ever since - like slow-moving bumper cars repeatedly clustering together and then separating."

As the plates move, they occasionally get stuck as they brush alongside their neighbours.

When the plates overcome the friction, a sudden slip occurs and produces an earthquake.

Because of this, areas along plates' boundaries are more likely to experience tremors than those that sit towards a plate's centre.

Vanuatu, for example, straddles the seismically active Ring of Fire - a belt where multiple tectonic plates butt onto the Pacific plate.

It's also known as the Circum-Pacific Belt.

The area where the Australia and Pacific plates meet - where Tuesday's earthquake occurred - is one of the world's most seismically active regions.

Why do earthquakes sometimes trigger tsunamis?

You might have noticed tsunami warnings are sometimes issued in the aftermath of earthquakes.

That was certainly the case on Tuesday and in California earlier this month following a magnitude-7.0 tremor there.

The National Oceanic and Atmospheric Administration says some big earthquakes can generate tsunamis.

"The energy from such an earthquake can cause the ocean floor to suddenly rise or fall," it says.

"This sudden vertical displacement of the ocean floor is what typically sets a tsunami in motion."

But Adam Pascale, the chief scientist at the Seismology Research Centre earthquake observatory, said not all large magnitude earthquakes triggered tsunamis.

"[That's] due to reasons such as earthquake depth and the orientation of the rupture of the fault," he said.

"In Tuesday's earthquake, it was more likely the depth of the event rather than the type of rupture that prevented the tsunami from being generated."

- ABC