Why was the Indonesian shallow earthquake so deadly?

More than 260 people died and hundreds were injured in the magnitude 5.6 earthquake. Buildings collapsed and terrified residents fled for their lives.

Bodies continued to be pulled from the rubble on Tuesday morning in Cianjur, the worst-hit city in the country’s most densely populated province of West Java, about 217 kilometers (135 miles) south of the capital Jakarta. Some people are still missing.

This magnitude would normally be expected to cause minor damage to buildings and other structures, but experts say it was due to the proximity of the fault line, the shallowness of the quake, and inadequate infrastructure that could withstand an earthquake. All contributed to the damage.

Below are the details of the earthquake and why it caused so much damage.

Was Monday’s quake considered “strong”?

The quake, which struck late Monday afternoon, had a magnitude of 5.6 and an epicenter depth of 10 kilometers (6.2 miles), according to the U.S. Geological Survey.

Earthquakes of this magnitude typically do not cause extensive damage to well-built infrastructure. However, the agency points out that “the magnitude at which damage occurs is not a single magnitude and depends on other variables, such as distance from the earthquake, type of soil, and construction of buildings.”

Dozens of buildings were damaged in Indonesia, including Islamic boarding schools, hospitals and other public facilities. It also damaged roads and bridges and caused power outages in parts of the area.

So why did the earthquake cause so much damage?

Experts said the proximity to the fault line, the depth of the quake and buildings not constructed using seismic construction methods were factors in the devastation.

“The earthquake was of moderate magnitude, but close to the surface, inland and close to inhabited areas,” said Gayatri Marlyani, assistant professor of geology at Gadjah Mada University in Yogyakarta, Indonesia. said. “The energy was still great, and it caused a big tremor that led to damage.”

The worst-affected areas are near several known faults, Marliyani said.

A fault is a long break in the rock that forms the earth’s surface. When an earthquake occurs on one of these faults, the bedrock on one side of the fault slides against the other fault plane.

“This area probably has the most inland faults compared to the rest of Java,” Marlyani said.

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

Danny Hillman Natawijaya, an expert in seismic geology at the Indonesian Institute of Science’s Geotechnical Research Center, said many buildings in the area were not built with earthquake-resistant designs, which further contributed to the damage. said there is.

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

Do such earthquakes usually occur in Indonesia?

With a population of over 270 million people, the country is frequently hit by earthquakes, volcanic eruptions and tsunamis due to its location on the volcanic arc and fault lines of the Pacific Basin known as the “Ring of Fire”. This region spans about 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 were also deadly earthquakes.

In February, a 6.2-magnitude earthquake hit West Sumatra, killing at least 25 people and injuring more than 460. In January 2021, a magnitude 6.2 earthquake struck West Sulawesi province, killing more than 100 people and injuring nearly 6,500.

A powerful earthquake and tsunami in the Indian Ocean in 2004 killed 230,000 people in 12 countries, most of them in Indonesia.