After a tsunami struck the island of Sulawesi, Indonesia last week, leaving over 1,400 people dead, experts have been trying to piece together what caused the deadly event in order to better prepare for future disasters.
USC researchers weighed in to explain how tsunamis work, and what might have caused this one.
Most tsunamis are a result of plate tectonics, said Steve Lund, a USC professor of earth sciences.
"The earth's surface is composed of tectonic plates," Lund said. "These plates are moving relative to one another. Most of the nasty things that happen to us occur at plate boundaries. That's where big earthquakes occur, that's where big volcanic eruptions occur."
Earthquakes occur at plate boundaries, like the San Andreas fault, which runs along the coast of California. The fault lies right between two major plates, the Pacific Plate and the North American Plate, which are continuously sliding past each other. But the process is not smooth. The two plates might spend hundreds of years locked, building energy. When they come unstuck, a huge amount of energy is released all at once, causing a powerful earthquake.
Although Hollywood would have us believe otherwise, a big earthquake does not always produce a big tsunami. Dangerous tsunamis are only caused when an earthquake occurs underwater, pushing a huge quantity of water up toward the surface. The tsunami thus begins as a fast-moving ripple, racing from the epicenter of the earthquake until it reaches the shore.
Evidence from the Sulawesi tsunami, however, suggests an alternate cause. According to USC graduate student Alexander Lusk, the Palu-Koro fault, where last week's earthquake occurred, should not have produced such a deadly tsunami.
"Usually you see more up and down motion [from a fault] to cause a major tsunami," Lusk said.
The Palu-Koro fault, like the San Andreas, is a strike-slip fault. In simple terms, this means that the Palu-Koro fault involves plates sliding horizontally past each other. This side-to-side motion does not push up any water.
One suggestion, according to Lund, is that the Palu-Koro fault is not entirely strike-slip. "The fault could have some vertical motion [to it] because it created a tsunami," Lund said.
Another explanation, Lusk said, is that the earthquake could have caused an underwater avalanche. This would have displaced massive amounts of water, resulting in a destructive tsunami like the one that wiped out the city of Palu last week.
In fact, the tallest recorded tsunami in history is the Lituya Bay tsunami of 1958, which was caused by a similar earthquake that unleashed an underwater avalanche. The resulting tsunami pushed water up to 1,720 feet above sea level onto the land around Lituya Bay.
With a better understanding of what causes these events comes a better chance at preventing them. Lund cites the Southern California Earthquake Center, a consortium of research institutions which is headquartered at USC, and has worked with national, state and local communities to develop a comprehensive disaster preparedness system.
Indonesia, in fact, did have a warning system, complete with tsunami sirens and text messages; both capabilities were knocked out by the earthquake.
Despite promising systems like SCEC, Lund said that a certain amount of damage from earthquakes and tsunamis is inevitable.
"We will never come even close to completely preventing these disasters," Lund said. "The amount of energy stored in the earth's crust is too enormous."