It was the first time a tsunami had been recorded in three different oceans since the 2004 Indian Ocean earthquake, and scientists have only now discovered how the waves occurred.
The epicenter of the earthquake in August was measured 47 kilometers below the ocean floor, too deep to cause large tsunamis, even one with relatively small waves ranging in height from 15 to 75 cm (6 to 30 inches).
But as it turns out, this tsunami was not the product of a single 7.5 magnitude earthquake on the Richter scale. A fresh look at the seismic data suggests that it was actually a series of five sub-earthquakes, and in the center of it was hiding a much larger, less profound ratt that may have been the cause of the global tsunami.
This third “invisible” earthquake struck just 15 kilometers below the ground with a force of 8.2. But in the midst of earthquakes, surveillance systems were completely wrong.
“The third event is special because it was huge and was silent,” explains seismologist Zhi Jia, of the California Institute of Technology. In the data we usually look [to monitor earthquakes], it was almost invisible.”
After cutting seismic data into longer than 500 seconds, Jia and colleagues were able to detect an unprecedented shallow, slow earthquake.
Among other sets of regular tears, they found a 3-minute rattling that ruptured a 200-kilometer portion of the platefront. Altogether, this single event accounted for more than 70% of the total recorded seismic torque.
Thus, the researchers conclude: “The South Sandwich Island earthquake appears to be a combination of deep rupture and slow slide of tsunamis; this explains the somewhat unusual combination of relatively large depth and the observed tsunami globally.”
The results suggest that our earthquake and tsunami alarm systems need to be updated. If we want to warn coastal communities of similar events, our systems need to read between earthquake lines to see larger earthquakes.
Seismic monitoring systems tend to focus on short and medium periods of seismic waves, but longer periods also appear to contain important information.
“It’s hard to find the second earthquake because it’s buried in the first earthquake,” Jia says. Complex earthquakes like this are rarely observed, and if we don’t use the right dataset, we can’t really see what was hidden inside.”
Geologist Judith Hubbard, who works at the Earth Observatory in Singapore and who was not involved in the current research, says she is grateful that others are prospecting into unexpected tsunami data to better understand where it came from. With these complex earthquakes, the earthquake happens and we think, it wasn’t big, don’t worry. And then it hits a tsunami and causes a lot of damage.
This study is a great example of understanding how these events work, and how we can detect them faster so we can get more warning in the future.
The study was published in Geophysical Research Letters.