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Why the 5.6-magnitude earthquake in Indonesia was so deadly

Scientists weigh in on the geologic drivers of the tragedy

Rescuers transport an injured victim of the Nov. 21 earthquake at a hospital in Cianjur, West Java, Indonesia. (Adi Weda/EPA-EFE/Shutterstock)

The earthquake that shook Indonesia’s West Java province on Monday might sound deceptively mild — the 5.6-magnitude quake struck at 1:21 p.m. local time in a seismic hot zone that frequently sees much larger temblors.

But this technically moderate quake has so far killed at least 268 people and injured hundreds more.

Scientists who study earthquakes named several factors that could have contributed to its tragic death toll. The epicenter of the quake was shallow, just about six miles beneath the surface of the earth, so the seismic energy didn’t have to travel far before it hit people and buildings. It also occurred on the densely populated island of Java, in a region of the world where many structures are not built to withstand earthquakes.

“A 5.6 [earthquake], in the scheme of things, is just not a huge earthquake. There’s lots of faults that can produce an earthquake that big,” said Susan Hough, a seismologist with the U.S. Geological Survey (USGS) in Pasadena, Calif. She noted that it was similar to a 2008 earthquake in California — one that most people probably don’t remember.

“Unfortunately, you put an earthquake like this in the wrong place, you can cause damage,” Hough said. “It’s kind of a perfect storm, in terms of damage relative to the magnitude.”

Indonesia sits in a notoriously active part of the world called the Ring of Fire, where the Pacific plate collides with several other tectonic plates in a roughly ring-shaped zone encircling the Pacific Ocean. As these plates grind past or dive under each other, seismic stresses build up until the energy gets released as earthquakes, volcanic eruptions and other activity along the plate boundaries.

In 2004, a 9.1-magnitude megathrust earthquake occurred under the Indian Ocean off the coast of Sumatra, causing a devastating tsunami. That kind of quake occurs at a subduction zone, where one tectonic plate dives under another.

By contrast, Monday’s temblor was a strike-slip earthquake, the same type of quake that occurs along California’s San Andreas fault. In a strike-slip quake, energy builds up as two tectonic plates grind against each other.

USGS scientists pinpointed the epicenter of the latest quake underneath Java’s landmass, not far from the capital of Jakarta. That’s part of why it did not generate a tsunami. The scientists also say that it struck about six miles below the surface. That may sound deep, but earthquakes can originate from hundreds of miles underground, and depth can be a factor in what’s felt at the surface.

Think of it like throwing a rock into the center of a pond, said Don Blakeman, an earthquake analyst for the USGS. The ripples radiate from the point where the rock plopped down, getting weaker until they dissipate.

“If you have an earthquake like this close to the surface, it’s close to people and buildings,” Blakeman said. “If it was 500 miles deep, they’d be 500 miles away from where it happened.”

Blakeman added that much of the destruction caused by an earthquake depends not only on how much shaking occurs, but also on how strict the building codes are in forcing people to make earthquake-resistant structures.

Hough said that while the destruction caused is sobering, from a scientific perspective this was in many ways a garden-variety earthquake. But she added that seismologists will be closely analyzing the data from the event, because similar-magnitude earthquakes can sometimes generate different levels of shaking — something that seems possibly to have happened in this case, considering the scale of destruction.

“The level of shaking seems high,” Hough said. “It was on land. It was close to people. There’s a chance it actually generated more than average shaking.”