Last fall, scientists were puzzled when an earthquake that hit the island of Sulawesi in Indonesia caused more shaking and destruction than would be expected given its magnitude. Two new studies may now offer an explanation.
The magnitude 7.5 earthquake struck Sulawesi near the city of Palu on September 28, 2018, triggering a devastating tsunami and ultimately killing more than 2,000 people. New evidence suggests that the extreme damage caused by this earthquake—the deadliest in 2018—could be attributed to its extraordinary speed. Two studies published earlier this month in Nature Geoscience suggest that this might have been a rare “supershear” earthquake: a temblor that travels so rapidly, it essentially creates a geologic sonic boom.
When earthquakes strike, the sudden burst of energy creates a rupture near the epicenter that cracks through the Earth’s crust like someone tearing a sheet of paper. While the rupture rips, waves of energy called shear waves cause most of the motion and destruction as they rattle the ground. Typically, these waves travel faster than the rupture itself.
But during supershear earthquakes, this process has an exceedingly rare quality: The rupture moves faster than the shear waves, causing the shear waves to become more powerful and damaging. The effect resembles what happens when a plane flies faster than the speed of sound. With supersonic planes, the buildup and quick release of pressure causes a sonic boom. During a supershear earthquake, the built-up energy is released in a burst of intense quaking, causing tsunamis, landslides, and other damage—much like that seen in Palu.
Jean-Paul Ampuero, a geophysicist and co-author on one of the new studies, told New Scientist that only a handful of supershear earthquakes have been studied extensively. Ampuero and his collaborators learned that the Palu earthquake likely belonged in this category after examining data from seismic stations that helped them calculate the temblor’s speed. Another team of researchers analyzed satellite imagery from before and after the quake and observed characteristics in the rupture and ground displacement that resemble previous supershear events.
The Palu earthquake struck along a fault that resembles others around the world, so this work could inform research into other potential areas where supershear earthquakes may occur.
“This is going to tell us something fundamental about the way the Earth works,” Wendy Bohon, an earthquake geologist who was not involved in the study, told National Geographic. “And it has the potential to actually save lives and help us inform people in a better way.”