The Science of Indonesia's Surprise Tsunami

Rescue workers look for victims at the site where a hotel collapsed after an earthquake hit near Palu, Central Sulawesi, Indonesia. As of Monday more than 800 people were confirmed dead after a tsunami triggered by a magnitude 7.5 earthquake slammed into Indonesia's coastline. PHOTOGRAPH BY ULET IFANSASTI, GETTY IMAGES

Rescue workers look for victims at the site where a hotel collapsed after an earthquake hit near Palu, Central Sulawesi, Indonesia. As of Monday more than 800 people were confirmed dead after a tsunami triggered by a magnitude 7.5 earthquake slammed into Indonesia's coastline. PHOTOGRAPH BY ULET IFANSASTI, GETTY IMAGES

On Friday, a powerful 7.5-magnitude earthquake shook the Indonesian island of Sulawesi, just after 5:02 PM Western Indonesian Time. Dramatic videos revealed the eerie growth of a mounting wave in a bay near Palu eventually crashing into the coast, sweeping away beach structures as onlookers scatter.

Scientists were surprised by the strength of the ensuing tsunami. And experts suggest that the area's unique geology could be to blame for the unexpected progression of the disaster.

The earthquake followed a series of temblors that began around 2:00 PM WIB with a 6.1-magnitude quake. The ground continued to roil with 27 aftershocksthat eventually gave way to the intense, shallow 7.5-magnitude temblor centered about 6 miles deep, according to the U.S. Geological Survey. Since then, dozens of aftershocks continue to rock the island.

As of Sunday evening, local time, the death toll had jumped to 832 people—and officials worry that number could still sharply rise since search-and-rescue teams have yet to reach many regions. This includes the regency of Donggala, home to some 300,000, where communication remains limited and landslides have severed rescue workers' access.

The total damage from the powerful quakes is still unclear—but devastation appears to be widespread. According to a statement from the Badan Nasional Penanggulangan Bencana (BNPB), the Indonesian natural disaster agency, the earthquakes have caused power outages, which have crippled communication, but local agencies are working to resolve the situation. Sutopo Purwo Nugroho, BNPB spokesperson, announced via Twitter that the Indonesian army has been deployed to assist with search and rescue.

Making Waves

Indonesian Agency for Meteorology, Climatology and Geophysics (BMKG) initially issued a tsunami warning but soon retracted it based on analyses at the time. Yet monstrous waves began crashing on shore, captured in videos that quickly began circulating on social media. According to BNPB, the destructive waves began before the warning was lifted. But the earthquake toppled telecommunications, which prevented officials from sending proper alerts, reports The New York Times.

Because of this, the towering waves took many by surprise. One particular video seems to be recorded from the upper floors of the circular parking structure at the Palu Grand Mall, showing waves rushing in below. It cuts off as the crowd starts to run, seeking safety from the wall of water.

Tsunamis typically are the result of the abrupt motion of large submarine earthquakes at tectonic plate boundaries. And earthquakes are not uncommonin Indonesia; the island chain sits within what's known as the Ring of Fire, a curving horseshoe-shaped chain of tectonic plate boundaries that hugs the Pacific basin. It is home to around 90 percent of the world's earthquakes.

But the monster waves were unexpected for this particular quake.

The 7.5-magnitude earthquake appears to be the result of what's known as a strike-slip fault, which takes place as two blocks of crust grind against one another, largely in a horizontal direction. Tsunamis more commonly followvertical movement in the crust, which disrupts the overlying water and can generate massive waves crashing onshore.

“It is definitely a surprise,” says Baptiste Gombert, a geophysicist at the University of Oxford. 

Read the full story on National Geographic’s website