Seismologists monitor the movements on and under the Earth’s crust on a regular basis as a rule of thumb; after all, once something big comes up to wreak havoc on the surface, they’re the first ones we run to for advice. And rightfully so, too, as earthquakes are among some of the most devastating natural disasters that can befall a community.
However, there’s an unorthodox upside to all this seismic movement. You see, part of some scientists’ jobs is to map out the Earth’s innards—which, to astute readers, would likely present a problem: the Earth has a diameter of 12,742 km. That’s an awful lot of Earth to map out, and we don’t have instruments that are even near the scale required to map out something that big.
Luckily for us, the Earth pings itself all the time with all the rumbles going on inside it, big or otherwise. Thus, scientists use some of Earth’s seismic movements as an opportunity to map out its inner contents, showing possible locations of dense mineral, water reserves, caverns, or features that are otherwise hidden too deep to be accessed normally.
In fact, one of this year’s earlier earthquake studies led to the digging of the deepest hole in the ocean: a bore hole 7,000-m (4.3-mi) under the Pacific Ocean that’s close to the Japan trench.
It was one of these earthquake-monitoring efforts that led to a peculiar case back in 2015. A team of researchers, led by University of Arizona seismologist Eric Kiser, tapped into Japan’s High-sensitivity Seismograph Network (Hi-net) array to obtain earthquake data. Japan, being one of the world’ most seismically-active areas on Earth, experiences around 200 earthquakes around its vicinity every year, hence the need for such powerful instruments.
There, they detected an earthquake off Japan’s Ogasawara Islands, or otherwise known as the Bonin Islands. These islands are located some 1,000 km (620 mi) directly south of Tokyo. There, an earthquake pinged off and registered as a magnitude 7.9. However, it wasn’t the main earthquake that piqued the interest of Kiser and team; instead, they paid attention to one of its aftershocks.
You see, Kiser and team found the aftershock 751 km (467 mi) below the surface of the Earth. Thing is, earthquakes commonly originate within the Earth’s crust and upper mantle, going only as far down as some 100 km (62 mi) on average. This makes this particular aftershock the deepest earthquake ever recorded, beating the previous record-holder, a 2004 magnitude 4.2 earthquake off Vanuatu, by around 15 km (9.3 mi). Their findings were published in the journal Geophysical Research Letters.
Part of the reason why scientists are so surprised about these findings is the fact that they don’t really expect the Earth to “tremor” at those depths. At or near the surface crust, the rocks are “brittle,” meaning we expect them to deform only slightly before breaking and releasing stress “like a coiled spring,” according to a news release by LiveScience. Meanwhile, rocks further down below are exposed to far hotter temperatures, and are compressed by the weight of the rock above it; this makes the rock deeper down in the mantle less prone to sudden breaking like those above it.
Some experts advise to take these results with a grain of salt, as more research is needed to confirm if the earthquake truly occurred so far down. Other experts, however, are far more positive about the study. Said Washington University in St. Louis seismologist Douglas Wiens, who was unrelated to the study, in a statement to National Geographic: “This is by far the best evidence for an earthquake in the lower mantle.”
At this detected depth, Kiser and team expect the tremors to have occurred near the bottom of a subducted piece of Pacific seafloor, which may have “[settled] very, very slightly” according to Kiser. Stresses were simultaneously released as it went deeper into the mantle, releasing the aftershocks.
Other experts recognize that quakes like it may be rare, but they do occur. According to University of Southern California geophysicist Heidi Houston, who’s also unrelated to the study: “”It can’t be ruled out. That’s one of the things that makes this interesting and exciting and important to look into.”
Despite this, much remains unanswered about the mysteries of the ground beneath our own feet. Ultimately, more research is needed for scientists to gather more details about the deeper Earth.
Finally, Houston added: “We can’t get down there. We only see what the earthquake waves show us.”
(Find similar deep-Earth studies in modernsciences.org, including the discovery of a brand-new volcano deep under the Indian Ocean.)
References
- Kiser, E., Kehoe, H., Chen, M., & Hughes, A. (2021). Lower mantle seismicity following the 2015 mw 7. 9 bonin islands deep-focus earthquake. Geophysical Research Letters, 48(13), e2021GL093111. https://doi.org/10.1029/2021GL093111
- Pappas, S. (2021, November 8). Deepest earthquake ever detected should have been impossible. LiveScience. https://www.livescience.com/deepest-earthquake-lower-mantle
- USGS Earthquake Hazards Program. (2004, April 8). M 4.2—Vanuatu region. USGS. https://earthquake.usgs.gov/earthquakes/eventpage/usp000cryq/executive
- Wei-Haas, M. (2021, October 25). Deepest earthquake ever detected struck 467 miles beneath Japan. National Geographic. https://www.nationalgeographic.com/science/article/deepest-earthquake-ever-detected-struck-467-miles-beneath-japan
