Why was Indonesia’s shallow earthquake so deadly?

JAKARTA, Indonesia (AP) — A magnitude 5.6 earthquake killed more than 160 and injured hundreds as buildings collapsed and terrified residents ran for their lives on Indonesia’s main island, Java.

In the worst-hit city of Cianjur, located in the most populous province of West Java and about 217 kilometers south of the capital Jakarta, bodies were still being pulled from the rubble on Tuesday morning. A number of people are still missing.

While the magnitude is normally expected to cause minor damage to buildings and other structures, experts say the proximity of fault lines, the quake’s shallowness and inadequate infrastructure that cannot withstand earthquakes all contributed to the damage.

Here’s a closer look at the earthquake and some of the reasons why it caused so much devastation:

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WAS MONDAY’S EARTHQUAKE CONSIDERED “STRONG”?

The US Geological Survey said the quake measured 5.6 late Monday afternoon and struck at a depth of 10 kilometers (6.2 mi).

Earthquakes of this size usually do not cause widespread damage to well-built infrastructure. But the agency points out: “There is not one size above which damage will occur. It depends on other variables such as distance from the earthquake, what kind of ground you are on, building construction and other factors.

Dozens of buildings were damaged in Indonesia, including Islamic boarding schools, a hospital and other public facilities. Roads and bridges were also damaged, and parts of the region experienced power outages.

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WHY HAS THE POPULATION CAUSED SO MUCH DAMAGE?

Experts said proximity to fault lines, depth of the quake and buildings that were not built earthquake-resistant were factors in the devastation.

“Although the earthquake was moderate in magnitude, it was close to the surface … and inland, close to where people live,” said Gayatri Marliyani, an assistant professor of geology at Universitas Gadjah Mada in Yogyakarta, Indonesia. “The energy was still high enough to cause significant tremors that resulted in damage.”

The hardest hit area is close to several known fault lines, Marliyani said.

A fault is a place with a long break in the rock that forms the Earth’s surface. When an earthquake occurs on one of these faults, the rock on one side of the fault slides relative to the other.

“The area probably has the most inland fractures compared to the other parts of Java,” Marliyani said.

She added that while there are some known faults in the area, there are many other active faults that have not been well studied.

Many buildings in the region were also not built with earthquake-resistant designs, further contributing to the damage, said Danny Hilman Natawidjaja, an earthquake geology expert at Indonesia’s Institute of Science Geotechnology Research Center.

“This makes an earthquake of this size and depth even more destructive,” he said.

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DOES INDONESIA USUALLY HAVE EARTHQUAKES LIKE THIS?

The country of more than 270 million people is often hit by earthquakes, volcanic eruptions and tsunamis due to its location on the arc of volcanoes and fault lines in the Pacific basin known as the ‘Ring of Fire’. The area spans some 40,000 kilometers (25,000 miles) and is where most of the world’s earthquakes occur.

Many of Indonesia’s earthquakes are small and cause little or no damage. But there have also been deadly earthquakes.

In February, a magnitude 6.2 earthquake killed at least 25 people and injured more than 460 in West Sumatra province. In January 2021, a magnitude 6.2 earthquake killed more than 100 people and injured nearly 6,500 in West Sulawesi province.

A powerful earthquake and tsunami in the Indian Ocean in 2004 killed 230,000 people in a dozen countries, most of them in Indonesia.

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The Associated Press Department of Health and Science receives support from the Department of Science Education at the Howard Hughes Medical Institute. The AP is solely responsible for all content.

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