EXPLANATOR: Why was the shallow earthquake in Indonesia so deadly?

While the magnitude is generally expected to cause light damage to buildings and other structures, experts say the proximity to fault lines, the shallow depth of the earthquake and the inadequate infrastructure that cannot not withstand earthquakes all contributed to the damage.

Here is an overview of the earthquake and why it caused so much devastation:

WAS MONDAY’S EARTHQUAKE CONSIDERED “STRONG”?

The US Geological Survey said Monday’s late afternoon quake measured a magnitude of 5.6 and struck at a depth of 10 kilometers (6.2 miles).

Earthquakes of this magnitude generally do not cause extensive damage to well-built infrastructure. But the agency stresses: “There is no magnitude above which damage will occur. It depends on other variables, such as the distance from the earthquake, the type of ground you are on, the construction of the building” 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.

SO WHY DID THE QUAKE CAUSE SO MUCH DAMAGE?

Experts said the proximity to fault lines, the depth of the shaking and buildings not constructed using earthquake-resistant methods were factors in the devastation.

“Even though the earthquake was medium in size, it (was) close to the surface…and located inland, close to where people live,” said Gayatri Marliyani, assistant professor in geology at Universitas Gadjah Mada, Yogyakarta, Indonesia. “The energy was still significant enough to cause significant shaking that resulted in damage.”

The most affected area is close to several known faults, 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 slips relative to the other.

“The region probably has the most interior faults compared to other parts of Java,” Marliyani said.

She added that although some well-known faults are found in the region, there are many other active faults that are not well studied.

Many buildings in the area are also not built with earthquake-resistant designs, which further contributed to the damage, said Danny Hilman Natawidjaja, an expert in seismic geology at the Geotechnology Research Center of the Indonesian Institute of Sciences.

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

DOES INDONESIA USUALLY HAVE EARTHQUAKES LIKE THIS?

The country of more than 270 million people is frequently 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 from “Ring of Fire”. The area stretches for some 40,000 kilometers (25,000 miles) and is where the majority of earthquakes in the world occur.

Many earthquakes in Indonesia are minor 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.

The Associated Press Health and Science Department is supported by the Howard Hughes Medical Institute Department of Science Education. The AP is solely responsible for all content.