Ancient coral structures called microatolls preserve geological history, and now some in Indonesia have told the tale of a record-breaking 32-year earthquake

Most earthquakes last seconds to minutes, but others will rumble along slowly for days, weeks or even months, at low frequencies that may not be felt at the surface. Now, researchers in Singapore have discovered the slowest earthquake ever found, which lasted 32 years.

Earthquakes are caused by stress between two tectonic plates pushing against each other, which eventually builds to the point where the rocks can’t handle it and shift. Most of the time that shift is sudden, producing vibrations that can be felt at the surface and potentially cause destruction. But other times, the plates will slide past each other more slowly, producing vibrations that may only be detectable by extremely sensitive equipment.

These slow slip events (SSEs) typically last a few weeks, although some have been recorded to go for as long as three years. But that’s nothing compared to the marathon earthquake that researchers at Nanyang Technological University have now discovered.

The team was investigating some unusual ancient coral structures known as “microatolls” off the coast of Sumatra, Indonesia. These structures grow slowly, providing a good record of changes in sea level and land elevation in the process.

The team found signs of a normal baseline between 1738 and 1829, when the microatolls were sinking at a steady rate of 1 to 2 mm per year. But around 1829, the rate suddenly increased, sinking at up to 10 mm per year – a gear shift that has been associated with tectonics in previous studies. The faster sinking continued until about 1861, indicating a record-breaking 32-year slow slip earthquake.

Unfortunately, the drawn-out rumbling ended with the 1861 Sumatra earthquake, a devastating 8.5-magnitude event that claimed the lives of thousands of people. The researchers say that better monitoring of these long-lasting SSEs could one day provide an early warning system for these larger quakes.

The research was published in the journal Nature Geoscience.

https://newatlas.com/science/worlds-slowest-earthquake-32-years/

> The team was investigating some unusual ancient coral structures known as “microatolls” off the coast of Sumatra, Indonesia.

This wouldn’t happen to be off the north coast would it?

If so, I have an explanation.

If not, it could be more complicated.

mollwollfumble said:

> The team was investigating some unusual ancient coral structures known as “microatolls” off the coast of Sumatra, Indonesia.

This wouldn’t happen to be off the north coast would it?

If so, I have an explanation.

If not, it could be more complicated.

No idea other than there is a North and South Sumatra.

What is your explanation?

PermeateFree said:

mollwollfumble said:

> The team was investigating some unusual ancient coral structures known as “microatolls” off the coast of Sumatra, Indonesia.

This wouldn’t happen to be off the north coast would it?

If so, I have an explanation.

If not, it could be more complicated.

No idea other than there is a North and South Sumatra.

What is your explanation?

A possibility is this. During the infamous 2004 tsunami in the Indian Ocean that wrecked from Sri Lanka to Bangkok, the earthquake observed by seismometers just off the west of Sumatra completely failed (on its own) to account for the spread pattern of the tidal wave.

It was quickly shown that there was a length of fault NW off Sumatra more than 200 km long that had slipped completely silently, not triggering seismometers. The explanation given was that this section of fault is so well lubricated that it doesn’t exhibit the slip-jam-slip-jam operation of almost all other faults. Instead it slipped smoothly almost without friction.

So, with the slow sub-crustal tectonic movement of the descending plate, this section of fault off Sumatra would slip continuously, smoothly, over a long period of time. Which correlates nicely with the slowness of this earthquke.

If further south, though, some different mechanism would have to be invoked, such as sea level change or crustal tilt due to buckling.

mollwollfumble said:

PermeateFree said:

mollwollfumble said:

> The team was investigating some unusual ancient coral structures known as “microatolls” off the coast of Sumatra, Indonesia.

This wouldn’t happen to be off the north coast would it?

If so, I have an explanation.

If not, it could be more complicated.

No idea other than there is a North and South Sumatra.

What is your explanation?

A possibility is this. During the infamous 2004 tsunami in the Indian Ocean that wrecked from Sri Lanka to Bangkok, the earthquake observed by seismometers just off the west of Sumatra completely failed (on its own) to account for the spread pattern of the tidal wave.

It was quickly shown that there was a length of fault NW off Sumatra more than 200 km long that had slipped completely silently, not triggering seismometers. The explanation given was that this section of fault is so well lubricated that it doesn’t exhibit the slip-jam-slip-jam operation of almost all other faults. Instead it slipped smoothly almost without friction.

So, with the slow sub-crustal tectonic movement of the descending plate, this section of fault off Sumatra would slip continuously, smoothly, over a long period of time. Which correlates nicely with the slowness of this earthquke.

If further south, though, some different mechanism would have to be invoked, such as sea level change or crustal tilt due to buckling.

The slip would have been a result of the earthquake, so not sure where that fits in the subsidence of the coral that exists in much shallower waters, presumably well away from the slips.

PermeateFree said:

mollwollfumble said:

PermeateFree said:

No idea other than there is a North and South Sumatra.

What is your explanation?

A possibility is this. During the infamous 2004 tsunami in the Indian Ocean that wrecked from Sri Lanka to Bangkok, the earthquake observed by seismometers just off the west of Sumatra completely failed (on its own) to account for the spread pattern of the tidal wave.

It was quickly shown that there was a length of fault NW off Sumatra more than 200 km long that had slipped completely silently, not triggering seismometers. The explanation given was that this section of fault is so well lubricated that it doesn’t exhibit the slip-jam-slip-jam operation of almost all other faults. Instead it slipped smoothly almost without friction.

So, with the slow sub-crustal tectonic movement of the descending plate, this section of fault off Sumatra would slip continuously, smoothly, over a long period of time. Which correlates nicely with the slowness of this earthquke.

If further south, though, some different mechanism would have to be invoked, such as sea level change or crustal tilt due to buckling.

The slip would have been a result of the earthquake, so not sure where that fits in the subsidence of the coral that exists in much shallower waters, presumably well away from the slips.

Apart from the pattern of tsunami waves, the main way we know about the free slip along the fault north of Sumatra in the 2004 tsunami is by is by coral reef island rises and falls along the fault line.

mollwollfumble said:

PermeateFree said:

mollwollfumble said:

A possibility is this. During the infamous 2004 tsunami in the Indian Ocean that wrecked from Sri Lanka to Bangkok, the earthquake observed by seismometers just off the west of Sumatra completely failed (on its own) to account for the spread pattern of the tidal wave.

It was quickly shown that there was a length of fault NW off Sumatra more than 200 km long that had slipped completely silently, not triggering seismometers. The explanation given was that this section of fault is so well lubricated that it doesn’t exhibit the slip-jam-slip-jam operation of almost all other faults. Instead it slipped smoothly almost without friction.

So, with the slow sub-crustal tectonic movement of the descending plate, this section of fault off Sumatra would slip continuously, smoothly, over a long period of time. Which correlates nicely with the slowness of this earthquke.

If further south, though, some different mechanism would have to be invoked, such as sea level change or crustal tilt due to buckling.

The slip would have been a result of the earthquake, so not sure where that fits in the subsidence of the coral that exists in much shallower waters, presumably well away from the slips.

Apart from the pattern of tsunami waves, the main way we know about the free slip along the fault north of Sumatra in the 2004 tsunami is by is by coral reef island rises and falls along the fault line.

But what caused the reef islands to rise and fall, the earthquake or the slip. I would think they are both the result of the earthquake.