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ABC News
ABC News
Lucia Stein, Emily Clark and Lucy Sweeney

A shallow tremor in a densely populated town made this Indonesia’s deadliest earthquake this year

A magnitude-5.6 earthquake that struck Indonesia's island of West Java has killed at least 268 people, with authorities warning some people remained trapped in isolated places.

Residents appeared to be caught off-guard as the ground beneath them began to shake with tremors at 1:21pm (local time) on Monday.

Many school children were just finishing up their lessons and preparing to make their way to extra classes when the moderate tremor hit, damaging buildings and littering streets with debris.

Those who could were forced to flee to the road, some fearing another quake would follow.

Another did come, but not in Indonesia. Within 24 hours, Solomon Islands was struck by two earthquakes, the first measuring at a magnitude of 7.0 and the second with a magnitude of 6.0.

So far, no casualties have been recorded. 

Why was the comparatively smaller quake in Indonesia more deadly?

It's believed a combination of factors made this Indonesia's deadliest earthquake this year, including its shallow depth, the location and the region's population density.

'Shallow' quakes can be more destructive

Earthquakes occur all the time and can either strike at a shallow depth, or deep beneath the earth's surface.

They only become "destructive" when they occur near infrastructure or cities, according to Babek Hejrani, a visiting research fellow at the ANU.

"The closer we are to the earthquake epicentre, the more destructive it gets," says Dr Hejrani, adding that the 2011 Japanese earthquake did not have any effect on Australia, but devastated Japan.

The epicentre of Monday's quake was near the densely populated town of Cianjur in West Java, about 75 kilometres south-east of the capital.

It was also quite shallow, occurring at a depth of 10km, according to Indonesia's weather, climatology and geophysics council BMKG.

Shallow earthquakes will generate more material "waves" because of their close proximity to the earth's surface, making them powerful and more likely to be felt by people living nearby.

The waves can cause significant damage to the surrounding environment and its infrastructure, depending on the location of the epicentre and the quakes magnitude.

"Close to a fault with shallow earthquakes, the amplitude of seismic waves would be quite large, which causes more damage," Dr Hejrani told the ABC.

Surface waves are not observed in deeper earthquakes that strike at depths of hundreds of kilometres, according to Geoscience Australia senior seismologist Tanja Pejic.

But for shallow quakes, these waves "will be able to produce some really long and pronounced shaking".

"Commonly, it's exactly those waves that topple buildings," she told the ABC.

On the ground, reports are already emerging of the extent of the damage, with Metro TV footage showing some buildings in Cianjur reduced almost entirely to rubble.

Witnesses have also described seeing buildings "completely flattened," and watching debris fall onto the street as the ground shook.

The Indonesian province sits within the 'Ring of Fire'

The Indonesian archipelago lies on the "Ring of Fire," an arc of volcanoes and fault lines along the Pacific Basin.

It is known as the most active earthquake zone on the planet, with thousands of tremors detected along these fault lines each year, accounting for about 90 per cent of the world's seismic activity.

The Cianjur district sits in Indonesia's West Java province, and close to a tectonic plate boundary known as the Sunda trench.

The area is prone to flooding, landslides and droughts as well as earthquakes, tsunamis and volcanic eruptions.

"The Sunda trench is a very long plate tectonic boundary that goes from the Sukkah islands, past Java, past the south coast of Sumatra and onto the other islands. So it's really not uncommon that we observe earthquakes in that area of moderate to large magnitude," Dr Pejic said.

Indonesia's Meteorology, Climatology and Geophysics Agency (BMKG) has reported 23 earthquakes of at least magnitude 5 in the past month.

In 2018, more than 4,000 people died in the aftermath of a magnitude-7.5 earthquake and subsequent tsunami that struck the Sulawesi region.

The area was also heavily impacted by soil liquefaction, which is a major factor in building damage and can also trigger landslides.

Liquefaction is the phenomenon where solid ground suddenly behaves like a viscous liquid.

It usually occurs in areas with loose sediment or sandy soil, where the water table is high.

Tremors push the water up through the soil and destabilise the ground, often uprooting trees and disturbing building foundations.

Tsunamis following larger earthquakes are also often a significant factor in higher death tolls.

Since Java sits along the subduction zone, an area where the Indian Ocean-Australia plate encounters the highly active Eurasian plate, the area is more susceptible to earthquakes and some can trigger tsunamis.

On Boxing Day 2004, a magnitude-9.1 earthquake struck off the coast of Sumatra and triggered one of the deadliest tsunamis in history, killing 230,000 people across 14 countries, the vast majority in Indonesia.

Earthquake expert and director of the International Tsunami Information centre Dr Laura Kong said of the West Java quake that a magnitude-5.6 is a "fairly small earthquake to trigger their tsunami alarms".

"They may have just felt some strong shaking, or they may have felt some observations to cause them to err on the side of safety," she said.

"A shallow undersea earthquake or an earthquake near land that's large, and that could be anywhere from low magnitude 6s … and certainly if you get up to magnitudes of 8 and 9, there is a great likelihood that a tsunami would have been generated."

In Solomon Islands, which was struck by two larger magnitude earthquakes, a tsunami warning was cancelled after the quakes triggered an initial alert yesterday.

The region is known for its high density population

Though 75km from the Cianjur district, the people of Jakarta — one of the world's most populous cities — reported feeling Monday's earthquake.

Local media has reported some high rise buildings in the capital swayed and were evacuated.

Despite the Indonesian archipelago sitting on the Ring of Fire, it's unusual for the city of Jakarta and its 30 million inhabitants to feel an earthquake.

"I think a combination of it being shallow and of moderate magnitude of 5.6 is why it was felt in Jakarta," Dr Pejic said.

Over the years, Indonesia's cities have expanded and people have made their homes in more precarious areas.

Roughly 175,000 people live in the town of Cianjur, which is part of a mountainous district of the same name that is home to more than 2.5 million people.

Many residents of the area live in one and two-storey buildings and in smaller homes in the surrounding countryside.

As of Tuesday, Indonesia's disaster mitigation agency, BNPB, said more than 2,200 houses had been damaged and more than 5,300 people had been displaced.

Regional governor Ridwan Kamil put that number at 13,000 and said they would be spread out at various evacuation centres across Cianjur.

Dr Kong said there were a series of factors to the West Java earthquake that led to the high death toll.

"The first pictures that came out pretty much told the story, which is that the buildings there essentially collapsed, because they were probably not reinforced for earthquake shaking, so that the bricks just became rubble," she said.

"It wasn't necessarily what you would call a large earthquake, but the fact that it was where it was, it was shallow, and then it affected an area where buildings collapsed."

Indonesia is both one of the most populous nations in the world and one of the most vulnerable to earthquakes — a combination of factors that presents immense challenges to the agencies tasked with monitoring geological activity and keeping people safe.

"They can monitor the foreshocks … and we knew a big one was going to come … they can monitor the aftershocks, but they certainly weren't able to predict a [nearly] magnitude 6 there," Dr Kong said.

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