2018 Palu earthquake was rare ultra-powerful supershear event

The Indonesian earthquake that impacted Sulawesi island in September 2018 was a rare ultra-powerful, superfast supershear event, according to new research out of NASA, UCLA, and more. The 7.5-magnitude earthquake caused an unexpected tsunami, the disaster ultimately claiming more than 1,500 lives and causing extreme damage. Using satellite images, researchers determined that the 7.5 Palu quake was one of fewer than 15 known supershear events.

The earthquake struck on September 28, 2018, with a steady rupture speed of 9,171MPH, according to NASA, which says the primary shock lasted for nearly one minute. This was an unusually fast speed compared to the typical 5600 to 6700MPH speeds most earthquakes present. In addition, and thanks to satellite images, the researchers found that the earthquake resulted in the fault — measuring 93 miles in length — had slipped by around 16ft.

The supershear event's rapid speed caused stronger shaking on the ground than would have resulted from a slower earthquake. Helping put this into perspective, the study's co-author Lingsen Meng explained via NASA, "The intense shaking is similar to the sonic boom associated with a supersonic jet."

In addition to the surprising nature of the earthquake was where it took place. According to NASA, this rare type of earthquake has been found to happen on very straight fault lines — that straight nature means there are less obstacles getting in the way. But the Palu quake's supershear event happened despite at least two "large bends" along the fault.

That surprising element may have been due to damaged rocks in the region caused by past earthquakes. "Real faults are surrounded by rocks that have been fractured and softened by previous earthquakes," explained the study's senior corresponding author Jean-Paul Ampuero.