A deep dive into Portugal’s 1930 deadly tsunami reveals how ocean depth affects tsunamis triggered by landslides.
By Lauren A. Koenig, Ph.D., Science Writer (@Lauren_A_Koenig)
Citation: Koenig, L., 2022, Shallow waters trap “silent” tsunamis from volcanic landslides, Temblor, http://doi.org/10.32858/temblor.241
In the aftermath of January’s massive eruption from the submarine volcano Hunga Tonga-Hunga Ha’apai, recovery efforts across the tropical islands of Tonga, now blanketed in monochromatic ash, are underway. As scientists sift through weather station data to figure out what happened, rising evidence suggests that the tsunami that followed the eruption was caused by rapid atmospheric pressure changes that violently displaced the Pacific Ocean.
In the case of the Tonga volcano, the blast created an atmospheric shock wave that circled the earth three times and hurled tsunami waves as far as California and the Caribbean over 7,800 miles (12,500 km) away. Overall, it was an incredibly rare event. Only 5% of tsunamis are caused by volcanos, whereas more than 80% are triggered by earthquakes.
A small percentage of tsunamis can also be created through a lesser-known mechanism known as tsunamigenic – meaning tsunami-generating – landslides. These types of waves form without any warning when steep, unstable cliffs give way after an earthquake, or due to erosion, drought, or heavy rain. Because of this, Amir Salaree, a seismologist at the University of Michigan, calls these landslide tsunamis “silent killers.” They’re usually smaller than earthquake- or volcano-generated tsunamis and more localized, making them harder to detect.
They’re also incredibly challenging to study. But new research has provided insight on how the danger to those living nearby can shift depending on the depth of water in which tsunami waves form. By investigating the 1930 Cabo Girão “Deadly Wave” tsunami that flooded Portugal’s island of Madeira, the researchers show how shallow waters can trap a tsunami near the shoreline, resulting in devastating effects to the island it originated from. Flatter stretches of coastline, where people more often work and live, end up bearing the brunt of the tsunami’s force, and their proximity to the landslide gives them little time to run to higher ground.
The island of Madeira, known for its namesake wine, is surrounded by plunging cliffs that reach up to 1900 feet (580 meters) above the azure waters of the North Atlantic Ocean. It also has a rich historical record documenting landslides along its coast. On the morning of March 4, 1930, a rock wall near Cabo Girão, the second highest sea cliff in the world, suddenly split apart. Rock and debris capable of filling 287,000 dump trucks hit the relatively shallow water, forming a shelf that extended the coastline beneath the newly scarred cliff face.
Immediately, an enormous 26-foot (8-meter) wave arose from the ocean. Eyewitnesses said it looked “like a cloud” as it moved towards the village. When the wave reached the village only a few minutes later, it had fallen to about 15 feet (4 meters) in height. Nevertheless, 19 people were killed as it swept across the beach.
Based on these descriptions, the research team worked backwards; by matching the island’s current geography with historical records of the event, they developed a numerical model that captures what happens when tsunamis like these occur. They found that shallow water dissipates the energy of the landslide, making it less likely to send waves that impact more distant islands. If the landslide falls in deeper water instead, the wave’s energy is free to build and travel towards the open ocean. The difference is similar to doing a cannonball into the deep end of the pool, which drenches everyone near and far, versus jumping into the shallow end and creating turbulent, choppy waves that mainly stay within reach.
In the case of Madeira, the island’s shelf also guided the wave back towards the nearest coasts and stopped the wave from propagating into deeper water. Even though the tsunami’s energy dissipated as it traveled along the island, it wasn’t quick enough to prevent the tsunami from having an intense local impact. Other steep volcanic islands similarly rimmed by shallow waters face the same hazard risk.
Landslides make waves
Tsunamigenic landslides along the sheer cliff faces of Madeira and other islands in the Northeast Atlantic are relatively common, but they can impact many places on the planet. Described by geologists as a type of “mass wasting” event, defined as the down-slope movement of soil and rock, tsunamigenic landslides were also responsible for the tallest-recorded wave on earth when a magnitude-7.7 earthquake hit Alaska in 1958. The shaking caused 90 million tons of rock to drop into Lituya Bay and sent a whopping 1,720-foot (524 meter) wall of water, taller than the Empire State building, up the opposite side of the fjord.
Landslides triggered by causes other than earthquakes, however, are more sporadic and have fewer direct or instrumental observations that could help scientists understand what sets them off. They have also been understudied. According to Ricardo Ramalho, a geologist at Cardiff University and one of the co-authors of the study, tsunami researchers have historically focused on rare, megatsunamis or regions of high risk such as the precariously unstable Canary Islands.
“If you add the lack of adequate monitoring tools, if any, for these more scattered, smaller events to the mix, you realize that we need to learn and do a lot to prepare ourselves,” said Salaree, who was not involved with the study.
Small but frequent tsunamis gain attention
In 2018, the southwestern flank of Indonesia’s Anak Krakatau volcano collapsed, generating a tsunami that killed hundreds of people. After this event, scientists started shifting their attention towards these “smaller,” but more frequent tsunamis. Among those interested were Ramalho and his colleagues, who identified Madeira as an ideal case study to investigate tsunamigenic landslides and model their hazard. “We decided to study the [Cabo Girão] event of 1930 because we had a good description of the event and submarine data (good bathymetry) to map the underwater development of the failed material and run the propagation of the tsunami with more accuracy,” said Rui Quartau, a marine geologist at Portugal’s Instituto Hidrográfico and another co-author of the study.
Although, the reasons for why the Madeira Island cliff split in 1930 will likely remain a mystery, understanding what happened next is the first step towards preparing for future events. “Crucially, the [methods used here] … guide us to propose a conceptual model for tsunamis triggered by ocean islands landslides that can be used to develop tsunami forecast capabilities for such unpredictable events,” added Quartau.
Although the supercharged volcanic eruption in Tonga had an impact that was heard around the world, it is most likely a once-in-a-lifetime event. It does, however, highlight the vulnerability of island nations to tsunamis, especially since climate change may make these landslides more likely to happen.
“Climate change has started to drastically change the water content (and what we call the pore pressure) of coastal rocks. We can expect that this will change the potency of landslides and their tsunamis,” said Salaree. Unstable rocks and sediment that are already overstressed by water pressure are more likely to move or collapse when even more water floods in during a tsunami.
Volcanic islands, isolated in large swaths of ocean, sit on the frontline of the climate crisis. Low-lying island nations are likely to experience worsening effects from tsunamis as they have little to no buffer from rising sea-levels. Tsunamis can also destroy reefs, which are under attack by the warming oceans, leaving these coastlines unprotected from more frequent storm surges and erosion.
“There is no doubt that with rising sea-levels and increased storminess, with global warming, more attention should be focused on smaller but much more frequent events,” said Ramalho.
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