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2018 Indonesia Earthquake That Killed 2,000 Traveled At 'Forbidden' Speed

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The earthquake that struck near Palu City in Indonesia's Sulawesi Island on Sept. 28, 2018, devastated the region. A tsunami followed, destroying houses and buildings near the shores.

Deadliest Earthquake In 2018

The 7.5-magnitude earthquake was the deadliest earthquake in 2018. It left more than 2,000 people dead and thousands missing.

A pair of new studies now revealed that the quake was a rare and an incredibly fast breed of tremor scientists call supershear. Less than 15 of these superfast-moving and extremely powerful earthquakes have so far been identified.

Earthquakes happen when built up stress in the Earth's crust hits a breaking point that causes rocks on either side of the tectonic fault to shift directions.

The S waves that shear rocks normally propagate at about 3.5 kilometers per second, and the P waves, which compress rocks, propagate faster at about 5 kilometers per second.

Geophysical observations show that the speed at which earthquake ruptures along the fault tend to be slower than the S waves or nearly as fast as the P waves.

In supershear events, the rupture moves along a fault extremely fast, causing the side-to-side and up-and-down waves that shake the ground to pile up and intensify. This results in a much stronger shaking than in slower quakes.

Sonic Boom

Analysis of seismological data revealed that the Palu earthquake ruptured at a much faster speed of 4.1 kilometers per second. It moved along the 180-kilometer-long fault line faster than the shock waves it generated.

Earthquake geologists described the event as like a sonic boom in an earthquake.

Jean-Paul Ampuero, from the Université Côte d'Azur in France and coauthor of one of the studies, said the earthquake ran in the forbidden speed range.

His team found that the path of the fault rupture was not straight but rather had two major bends. Nonetheless, these obstacles did not reduce the speed of the quake.

"The rupture propagated at a sustained velocity of 4.1 km s-1 from its initiation to its end, despite large fault bends," Ampuero and colleagues wrote in their study published in Nature Geoscience.

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