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Researchers at UCLA; NASA's Jet Propulsion Laboratory in Pasadena, California; and other institutions analyzed high-spatial-resolution observations of the seismic waves caused by the devastating temblor, along with satellite radar and optical images,to characterize the speed, timing and extent of the magnitude 7.5 Palu earthquake that occurred on Sept. 28, 2018. They calculated that the quake ruptured at a steady speed of 9,171 mph (14,760 kph), with the main shock continuing for almost a minute. Earthquakes typically rupture at about 5,600 to 6,700 mph (9,000 to 10,800 kph). Processing the satellite images, the researchers found that the two sides of the 93-mile-long (150-kilometer-long) fault slipped by about 16 feet (5 meters) - a surprisingly large amount.


A rupturing fault creates several kinds of waves in the ground, including shear waves that spread out at 7,900 mph (12,700 kph). In a supershear earthquake, the fast-moving rupture overtakes the slower shear waves propagating in front of it and pushes them together into bigger, more powerful waves. "The intense shaking [that results] is similar to the sonic boom associated with a supersonic jet," said Lingsen Meng, a professor at UCLA and co-author of the report.