Report: Tsunami-inducing earthquakes caused by extinct undersea volcanoes

A rare type of earthquake that triggers powerful tsunami waves may be caused by extinct undersea volcanoes, said a report released Friday. Researchers behind the study hope that their findings will one day lead to early detection of such dangerous events and save lives.

The study, conducted by scientists at Imperial College London and GNS Science in New Zealand, was published in the online science journal Earth and Planetary Science Letters.

So-called tsunami earthquakes are characterized by their shallow ocean depths, often occurring at less than 6 miles deep. Though these types of quakes generally have smaller magnitudes than traditional earthquakes that lead to tsunamis — less than 7 on the Richter scale — they can result in very large tsunami waves.

Because such earthquakes are not accompanied by strong shaking, there is no natural warning for people to detect. The result can be catastrophic in populated coastal areas.

The report said at least nine earthquakes have been identified as tsunami earthquakes, including the ones in Nicaragua in 1992 and Java in 1994 and 2006. Hundreds of people perished in the resulting tsunamis, largely because of a lack of warning signs.

“Tsunami earthquakes don’t create massive tremors like more conventional earthquakes such as the one that hit Japan in 2011, so residents and authorities in the past haven’t had the same warning signals to evacuate,” Rebecca Bell of the Department of Earth Science and Engineering at Imperial College London said in a press release. “These types of earthquakes were only identified a few decades ago, so little information has been collected on them.”

Scientists know that when two sections of the earth’s crust, known as tectonic plates, slide under one another in a process called subduction, it can result in an earthquake. The risk of tsunami from a traditional earthquake depends on its depth and magnitude.

But tsunami earthquakes are different because large waves can be generated by relatively small-magnitude quakes. In these types of earthquakes, the process of subduction becomes stalled in areas of rough seafloor — creating a buildup of pressure that, when released, can lead to large tsunami waves.

In these cases, topographical features — such as extinct undersea volcanoes — can become crushed between two subducting plates, creating a “sticking point,” the report said. These points can create an elastic pressure like a rubber band that amasses, allowing a small earthquake that occurs at an abnormally shallow depth to create tsunami waves dozens of feet high.

The team of researchers studied two earthquakes that occurred in March and May 1947 off the coast of New Zealand — Offshore Poverty Bay and Tolaga Bay, respectively. The quakes shared many characteristics with other well-known tsunami earthquakes, including low magnitudes (the two were less than 6.0 on the Richter scale), long durations and unusually large tsunami waves.

Eyewitnesses from the March quake said at the time that they didn’t feel violent tremors, but felt the ground rolling for minutes in a manner that brought on a sense of seasickness. Approximately 30 minutes later the bay was hit by a 32-foot wave resulting from a mere magnitude-5.9 offshore earthquake. That May, a magnitude-5.6 earthquake off the coast of Tolaga Bay caused a 20-foot wave to smash into the coast.

For the report, scientists mapped the seafloor in the area where the two tsunami earthquakes occurred decades before and located two extinct volcanoes that had been squashed and had sunk beneath the crust off the coast of New Zealand.

Researchers suggested that the volcanoes provided a sticking point between a part of the Pacific plate, a section of the earth’s crust, and the New Zealand plate, which it was trying to slide under. The result was an elastic energy buildup that was released in 1947, “unsticking” the plates and causing the volcanoes to become submerged under New Zealand.

The energy release, though slow, caused large movements of the seafloor — resulting in very large tsunami waves.

Researchers said they hoped the information gathered by the study could provide insight into what geological factors cause large tsunamis, and help locate other areas of the world prone to tsunami earthquakes. They planned to survey sites with other sinking volcanoes to better understand their role in generating tsunami earthquakes.

“Thanks to oil exploration data and eyewitness accounts from two tsunami earthquakes that happened in New Zealand more than 70 years ago, we are beginning to understand for the first time the factors that cause these events,” Bell said. “This could ultimately save lives.”