Indonesia’s deadly earthquake and tsunami

Map of the September 28, 2018 earthquake near Palu, Indonesia (Source: U.S. Geological Survey)

By Robert C. Jones Jr.

Map of the September 28, 2018 earthquake near Palu, Indonesia (Source: U.S. Geological Survey)

Indonesia’s deadly earthquake and tsunami

By Robert C. Jones Jr.
UM experts explain what happened, discuss if it could happen here, and talk about “strike-slip faults.” And then there is the pubic health risks from mosquitos.

The death toll from last Friday’s earthquake and tsunami that devastated the Indonesian island of Sulawesi has climbed to more than 1,200 and is likely to rise further, as survivors faced their fifth day with little food and clean drinking water.

In the city of Palu, which bore the brunt of the disaster, a mass grave has been dug as rescuers bury the dead and continue to search for survivors. 

Shocking satellite images show the impact of the devastation—entire villages, even a bridge, washed away. What has stumped scientists is the size and power of the tsunami, reported to have been as high as 20 feet in some places. 

“Large tsunamis are usually triggered by so-called megathrust faults associated with large, upward vertical movement,” said Guoqing Lin, associate professor of marine geosciences at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science, noting that the fault that ruptured on Friday along the coast of Sulawesi about 50 miles north of Palu was a strike-slip fault. 

“Strike-slip faults have more dominating horizontal motion,” said Lin. “This one was certainly unusual. At this stage, it’s not very clear why the earthquake triggered a tsunami of this scale. Some possible factors that might have influenced it are [that it could have been a] non-pure strike-slip fault with a vertical portion; the huge shaking due to the earthquake; or special characteristics of the area.” 

News@theU asked UM researchers with expertise in the area of geosciences, seismology, and public health to answer some of the most pertinent questions that have arisen in the wake of the disaster.

What spawns a tsunami?

It’s created by a sudden movement of a large chunk of the earth’s crust on the ocean floor. If there’s an abrupt movement such as an earthquake or a large landslide or large volcanic event, the shock of that movement creates a wave in the ocean. They’re not wind-driven waves. In the open ocean, they’re very long waves but not of big amplitude. A tsunami could pass by and you might not even notice it if you’re on a boat. But when that wave comes into shallow water, it slows down because of its interaction with the ocean floor. And then it builds up and creates a large wall of water.

—Brian Haus, professor of ocean sciences and director of the SUrge STructure Atmosphere INteraction Facility at the Rosenstiel School of Marine and Atmospheric Science

 

Why are tsunamis more common in the Pacific Ocean than in the Atlantic?

There are more tsunamis in the Pacific than the Atlantic because the Pacific Ocean crustal plate is colliding with nearly all of the continental plates surrounding it. Most of the boundaries are more or less straight on colliding with the ocean crust, subducting (going under the lighter continental crust). In some areas the plate boundaries are more or less sliding past each other (the San Andreas in California and the area around Palu are examples). This movement along plate boundaries is mostly a sticky business with the rocks on either side sticking together and gradually building up stress. When they suddenly release, that is an earthquake, and the rock can move up and down or laterally past each other at an amazing speed. The Atlantic Ocean crust is mostly locked in with the continental crust, and so there is not much in the way of earthquakes. The Caribbean has some areas with subduction and strike-slip faulting, and it is possible that we could be affected by a tsunami, but it would be a very rare event.

—Harold R. Wanless, professor in the Department of Geography and Regional Studies in the College of Arts and Sciences

It’s called the Ring of Fire (a major area in the basin of the Pacific Ocean where many earthquakes and volcanic eruptions occur). The Pacific is surrounded by subduction zones but not the Atlantic, which has passive margins.

—Falk Amelung, professor in the Department of Marine Geosciences at the Rosenstiel School of Marine and Atmospheric Science.

 

What type of fault ruptured on Friday?

It was a strike-slip earthquake with horizontal movement, which normally doesn’t create a significant tsunami. You need vertical seafloor movement or a submarine landslide. I suspect the latter happened here.

—Falk Amelung, professor in the Department of Marine Geosciences at the Rosenstiel School of Marine and Atmospheric Science.

 

Now that we know that the fault that ruptured on Friday was a so-called strike-slip fault, which does not ordinarily create a large tsunami, what happened in this case? Why was such a destructive tsunami triggered?

There are two possibilities as to why this strike-slip fault in Indonesia created a significant tsunami. First, it may not have been entirely strike-slip. In either strike-slip or subduction, the buildup of stress actually drags and deforms the rock. This is then violently released, causing a violent submarine push or pull of water. It is this violent movement that causes the tsunami (much like rapidly moving your hand down or sideways while underwater). The second probability is that Palu is at the end of a narrowing bay. The open end is facing north towards the line of the earthquake. Just as with a tide, even a small tsunami entering the bay will be amplified as the bay narrows and shallows.

—Harold R. Wanless, professor in the Department of Geography and Regional Studies in the College of Arts and Sciences

 

How effective are tsunami warning systems?

Very effective. There was a warning issued [after the earthquake]. But from what I’ve read in the news, there could have been problems with the dissemination of this information and the power network may have been downed by the quake. Local tsunamis are more difficult to detect, as they don’t travel over the open ocean where they could be readily detected by buoys.

—Falk Amelung, professor in the Department of Marine Geosciences at the Rosenstiel School of Marine and Atmospheric Science.

 

A tsunami warning system of seafloor sensors, data-laden sound waves, and fiber-optic cable that could very well have saved lives in the Indonesia tsunami hasn’t been installed due to lack of funding. How vital is it that effective early warning systems be in place to help minimize the aftermath of such disasters?

The economic value of warning systems far exceeds what they cost to install and to maintain. One of the case studies that I teach is the 2004 tsunami, and everybody reacts and recoils, asking why is it there was no warning system in place for the Indian Ocean as we have for the Pacific, and it’s a good question. But the cost of installing and maintaining these systems isn’t trivial. So we need to be vigilant about maintaining them.

—David Letson, professor in the Department of Marine Ecosystems and Society at the Rosenstiel School of Marine and Atmospheric Science

 

According to the World Health Organization, almost half of all Indonesians live in areas where malaria is endemic. How will the tsunami that struck the region worsen malaria outbreaks and other vector-borne diseases?

The extensive flooding caused by the tsunami will have an immediate impact on vector mosquitoes, first by washing out immature stages of mosquito larvae from most habitats. As things begin to dry out, pockets of water will be reinvaded by mosquitoes, and in such an environment they will thrive. So the real danger of malaria and other vector-borne diseases will be seen a month or so after flooding events.

—John C. Beier, professor and chief of the Division of Environment and Public Health in the Department of Public Health Sciences at the Miller School of Medicine. 

Exposure to malaria vectors will likely increase as homeless people become more exposed to mosquitoes. This happened in Haiti after the earthquake of 2010, when there was a large spike in malaria cases post-disaster. However, a potentially larger concern is increased risk and exposure to the vector of dengue fever, Zika, and Yellow Fever: Aedes aegypti. This vector breeds in refuse, trash, and anything that collects water for short periods. The amount of refuse, debris, and flotsam after the tsunami is likely to a) increase exponentially the potential breeding habitats of the mosquito and b) lead to more bites from Aedes aegypti, which feeds actively during the day outside. So, we can expect to see a spike in dengue cases, at a minimum, after the tsunami.

—Douglas O. Fuller, professor in the Department of Geography and Regional Studies in the College of Arts and Sciences.

 

Can rising sea levels make tsunamis worse?

Rising seas can make a hurricane storm surge worse. Brian Soden (a professor of atmospheric sciences at the Rosenstiel School) has modeled Hurricane Andrew coming with a 3-foot rise in sea level. The surge is much more than 3 feet higher than the real Andrew and penetrates much, much further inwards across Miami-Dade County. So, since we all—here and in Palu—live in low elevations near the sea, we should expect tsunamis to similarly be increasingly dangerous and damaging.

—Harold R. Wanless, professor in the Department of Geography and Regional Studies in the College of Arts and Sciences