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Hidden Magnitude-8.2 Mysterious 2021 Global Tsunami Source

Geophysical Research Letters According to a new study by, the magnitude 8.2 earthquake was “hidden” in the 2021 magnitude 7.5 earthquake, sending a mysterious tsunami around the world. Credits: ZheJia and AGU “width =” 800 “height =” 530 “/>

The magnitude 8.2 earthquake was “hidden” in the 2021 magnitude 7.5 earthquake, sending a mysterious tsunami around the world. Geophysics Research Letter.. Credits: ZheJia and AGU

Scientists have discovered the source of the mysterious 2021 tsunami that has sent waves around the world.


August 2021, magnitude 7.5 earthquake Hit near the South Sandwich Islands, tsunami It has spread all over the world. The epicenter was 47 kilometers below the surface and was too deep to cause a tsunami. The rupture was nearly 400 kilometers long and should have caused a much larger earthquake.

Seismologists were confused and tried to understand what really happened in the faraway South Atlantic that day.

A new study reveals that the quake was not a single event, but a series of five sub-earthquakes spread over several minutes. The third sub-earthquake was a shallower, magnitude 8.2 earthquake, just 15 km below the surface of the water. The rare “hidden” earthquake was probably the trigger for a global tsunami.

This study was published in the AGU journal Geophysics Research LetterHas published a short, influential paper with implications that spans earth sciences and space sciences.

The South Sandwich Islands earthquake was complex, with multiple sub-earthquakes. Seismic signal Interpretation was difficult, according to Zhe Jia, a seismologist and principal research author at the California Institute of Technology. The magnitude 8.2 earthquake was hidden in the entanglement of seismic waves. The seismic waves interfered with each other in the course of the event. The hidden seismic signal wasn’t clear until Jia filtered the waves using a much longer period of up to 500 seconds. Only then was the 200-second-long quake revealed, which Jia said accounted for more than 70% of the energy released during the quake.

“The third event is special because it was huge and quiet,” Jia said. “In the data we usually see [for earthquake monitoring]I could hardly see it. “

As the South Sandwich Islands earthquake shows, the danger of complex earthquakes can be difficult to predict. The USGS initially reported a magnitude 7.5 earthquake and added only 8.2 events the next day. This is because the tsunami occurred on the coast up to 10,000 km away from the point of occurrence.

“We need to rethink how we can mitigate the risk of an earthquake and tsunami, which requires us to quickly and accurately characterize the actual size of a major earthquake and its physical process,” says Jia.

It is important to improve the predictions as this type of earthquake can cause unexpected earthquakes. “We think these complex quakes caused quakes,’Oh, it wasn’t that big, so don’t worry,’ and the tsunami struck and caused a lot of damage.” Told. “This study is a great example of how to understand how these events work and how to detect them more quickly so that we can issue more warnings in the future.”

Sneaky seismic signal

When an earthquake occurs, a wave of vibration is sent to the entire earth. The global network of seismic monitors uses these seismic waves to identify the time, location, depth, and magnitude of an earthquake. General monitoring often focuses on short- and medium-term waves, and longer periods can be omitted. However, incorporating long periods into surveillance is not sufficient to capture complex earthquakes with messy seismic signals.

“It’s hard to find a second quake because it’s buried in the first quake,” Zia said. “We rarely see such complex earthquakes … and without the proper datasets, we can’t really see what’s hidden inside.”

Simple earthquakes can be easily identified and explained, Jia said. However, nasty earthquakes need to be carefully broken down into their components in order to find a unique combination of simple earthquakes that make up a complex earthquake.

Jia and his colleagues have developed an algorithm that separates seismic signals between these nasty earthquakes. The algorithm can identify the location and characteristics of different earthquakes by “decomposing” complex seismic signals into a simpler form using waves of different periods (20-500 seconds long). .. This is similar to someone who has perfect pitch, hearing five discords being struck at once, but can identify individual sounds.

“I think a lot of people will be disappointed when trying to tackle an event like this,” says Hubbard. “It’s really helpful that someone was willing to actually dig into the data and understand it.”

Both Jia and Hubbard have stated that the long-term goal is to automate the detection of such complex earthquakes, as in the case of simple earthquakes. In the 2021 earthquake, the tsunami was small by the time it reached the coast, and most of the permanent residents of remote volcanic islands were penguins. However, complex earthquakes can pose a significant danger if they cause larger tsunamis and strikes in densely populated areas.


A 7.7 seafloor earthquake in the South Pacific causes a small tsunami


For more information:
Zhe Jia et al, 2021 South Sandwich Island M w 8.2 Earthquake: A slow event sandwiched between regular bursts, Geophysics Research Letter (2022). DOI: 10.1029 / 2021GL097104

Quote: Hidden magnitude-8.2 mysterious 2021 global tsunami source (February 8, 2022) https://phys.org/news/2022-02-hidden-magnitude-earthquake- Obtained February 8, 2022 from source-mysterious.html

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Hidden Magnitude-8.2 Mysterious 2021 Global Tsunami Source

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