A new study claims that the deepest earthquake ever recorded occurred 6 years ago, at 467 miles (751 kilometers) below Japan.
Researchers at the University of Arizona analysed data collected by Japan’s network of seismometers in May 2015 during the Ogasawara earthquake.
The Richter scale measured magnitude 7.9, and the event’s largest jolt was located off the Bonin Islands (also known as the Ogasawara Islands).
But a series of less severe aftershocks registered deep in the Earth’s lower mantle, including one that reached a whopping 467 miles deep, where scientists had thought earthquakes ‘unlikely if not impossible’.
Most earthquakes strike within a few dozen miles of the planet’s surface, in the crust or upper mantle.
The 2015 event however, caused shocks to the lower mantle. This is where pressures and temperatures rise so intensely that rocks can bend.
One of the aftershocks from the 2015 Ogasawara earthquake registered a whopping 467 miles deep. Pictured is the epicentre of the event, just off the Ogasawara Islands, also known as the Bonin Islands
The lower mantle, which is the lower liquid portion within the Earth’s core, is the layer below which a crust and above which a core surrounds.
In news reports at the time, the quake was reported to reach a depth of 421 miles (678 km), but the new estimation extends this maximum by another 46 miles.
Douglas Wiens, a seismologist from Washington University in St. Louis, said that this is the best evidence for an earthquake occurring in the lower mantle. He wasn’t part the study.
However, scientists are arguing that further research may be necessary to confirm the quake actually struck the lower mantle. If so, it would be the largest earthquake in recorded history.
The lower mantle is the lower liquid portion of the Earth’s mantle – its layer bounded below by a core and above by a crust.
The exact depth at which the Mantle Transition Zone meets the lower mantle boundary varies around the globe, but it is approximately 410 miles (660 km).
Deep earthquakes such as Ogasawara 2015 can be felt farther away from tremors close to the surface but they are extremely rare.
Global Centroid Moment Tensor database (CMT), only 18% of the 56,832 large to moderate earthquakes that were recorded between 1976-2020 were greater than 70 kilometers (43 mi). This is because the upper mantle is not included in the CMT database.
According to news reports, the earthquake reached a depth of 421 miles (678km) in May 2015. This new study expands on this. Pictured, the intensity distribution across Japan on the Japanese seven-point scale from the Ogasawara earthquake
Even fewer, 4 per cent, struck below 300km (186 miles) – the depth commonly used as a cut-off for identifying ‘deep earthquakes’, National Geographic points out.
Deep earthquakes are not as destructive as earthquakes in crust and upper mantle, despite their incredible depth of origin.
However, the origins of 2015’s earthquake in the lower mantle remain a mystery.
It is well-known that earthquakes can be caused by subduction zones. This is where two tectonic plates are sliding in opposite directions and stick together, then suddenly slip.
Severe earthquakes normally occur over fault lines where tectonic plates meet, but minor tremors – which still register on the Richter sale – can happen in the middle of these plates.
It’s thought deep earthquakes strike near modern or ancient subduction zones that have reached extreme depths.
According to National Geographic, quakes in the Mantle Transition Zone could be triggered by changes to olivine, a rock-forming mineral.
In the Mantle Transition Zone, olivine’s crystal structure becomes no longer stable, which creates weak points in the rock where it can rapidly deform, causing deep quakes.
But even further down, in the lower mantle, another mineral – bridgmanite – starts to dominate the composition, meaning there must be some other explanation for earthquake triggers.
The Ogasawara Islands (pictured), also known as the Bonin Islands, were inscribed as UNESCO World Heritage in 2011. The islands were formed about 48 million years ago by volcanos that built an oceanic crust
The researchers suggest one possibility. The small aftershocks following the main magnitude 7.9 quake occurred near the base of a torn slab of subducted Pacific seafloor that pierced the top of the lower mantle.
The team suggests the large quake could have caused part of the mangled slab to settle ‘very slightly’, which in turn may have concentrated stresses at the base of the slab as it plunged into the lower mantle.
‘The causes of earthquakes below depths of approximately 60 km inside the Earth have been debated for nearly a century,’ the researchers say in their paper, published in the journal Geophysical Research Letters.
‘This study reports on the first detected earthquakes with source locations in the lower mantle.
‘These observations provide new insights into the mechanisms that can produce earthquakes deep inside the Earth.’
EARTHQUAKES ARE CAUSED WHEN TWO TECTONIC PLATES SLIDE IN OPPOSITE DIRECTIONS
Catastrophic earthquakes are caused when two tectonic plates that are sliding in opposite directions stick and then slip suddenly.
Tectonic plates are composed of Earth’s crust and the uppermost portion of the mantle.
Below is the asthenosphere: the warm, viscous conveyor belt of rock on which tectonic plates ride.
They do not all not move in the same direction and often clash. This builds up a huge amount of pressure between the two plates.
Eventually, this pressure causes one plate to jolt either under or over the other.
This releases a huge amount of energy, creating tremors and destruction to any property or infrastructure nearby.
Severe earthquakes usually occur along fault lines where tectonic plates meet. However minor tremors, which still register on Richter sales, can occur in the middle.
The Earth has 15 tectonic plate (pictured), that together have shaped the shape of the landscape around us today.
These are known as intraplate earthquakes.
These remain widely misunderstood but are believed to occur along minor faults on the plate itself or when ancient faults or rifts far below the surface reactivate.
These areas are weaker than the surrounding plate and can easily slip, causing an earthquake.
You can detect earthquakes by observing the size or magnitude of the shock waves that they produce, also known as seismic wave.
The magnitude and intensity of an earthquake are two different things.
The magnitude refers to how much energy was released at the site of the earthquake.
The hypocenter is the area below the earth’s surface where earthquakes occur.
One part of a seismicograph remains stationary during an earthquake while the other part moves with Earth’s surface.
The difference in the positions between the moving and still parts of the seismograph is then used to measure the earthquake.