An interesting question came up in a discussion about earthquakes recently – namely, could one ever be powerful enough to fracture the planet, or even completely break it apart? Well, short answer, no, but this is a surprisingly complex question to answer, but let’s take a look at the science to see how possible this type of apocalypse really is.
First off, let’s just remind ourselves what causes an earthquake and just how ludicrously energetic they are at their worst. There are so many different ways to get earthquakes, but for the sake of simplicity, we’ll stick to the most common for now.
You’ve got fault networks like the San Andreas complex, which has formed at the point where the Pacific Plate is moving northwards with respect to the North American Plate. This type of tectonic margin is called a “transform” boundary, and it tends to produce very shallow and damaging earthquakes.
Then you’ve got two plates coming together to collide. On these “convergent” boundaries, one of two things happens: either the denser plate sinks beneath the other one and gets destroyed in the mantle (see: Japan Trench) or they both smash into each other and rise up to form a mountain range (see: Himalayas).
In the case of the former, you get deep-seated earthquakes, and in terms of the latter, they are at middling-to-shallow depths. With some exceptions, these are how earthquakes are generated on Earth, so how powerful can they get?
Here are the top five most powerful earthquakes on record, in ascending order:
5 – Kamchatka, former Soviet Union: 9.0M (November 4, 1952) on a convergent boundary along the Kuril-Kamchatka trench. It created a devastating tsunami, and over 2,300 people perished.
4 – Tohoku region, off the eastern coast of Japan: 9.1M (March 11, 2011) on a convergent boundary along the Japan Trench between the Okhotsk and Pacific Plates. The tsunami was one of the deadliest in human history, killing up to 20,000 people.
3 – Sumatra, Indonesia: 9.2M (December 26, 2004) on a convergent boundary where the Indian Plate slides beneath the Burma Plate. Scary fact: At one point, the rupture was moving 2.8 kilometers (1.7 miles) per second, which equates to speeds of 10,000 kilometers (6,200 miles) per hour. The resulting tsunami caused up to a quarter of a million deaths.
2 – Prince William Sound, Alaska: 9.2M (March 27, 1964) on a convergent boundary along the Aleutian Trench. Despite a powerful tsunami, just 30 or so people died.
1 – Valdivia, Chile: 9.5M (May 22, 1960) on a convergent boundary between the descending Nazca Plate and the South American Plate. It also produced a colossal, Pacific Ocean-wide tsunami, but “only” 1,000-6,000 people died.
So how powerful are these beasts?
Most of us are familiar with the Richter scale, which measures earthquakes in terms of the amplitude, or length of one wave cycle. The higher the amplitude, the more powerful the quake. This scale, now-defunct, was replaced by another in the 1970s called the Moment Magnitude Scale, whose values align similarly with the original.
It’s a logarithmic scale, meaning that a 2.0M quake is 32 times as energetic as a 1.0M event. Similarly, a 3.0M quake is 1,000 times more powerful than a 1.0M event. Technically, the scaling factor is 31.6, but I'm cheekily rounding up here.
Seismologists can use the seismic waves unleashed by these quakes to work out how many joules of energy they release. For a point of comparison, an apple falling a meter to the ground from a tree involves one single joule of energy.
The Valdivia quake, using the somewhat basic Richter formula, released 4.5 quintillion joules of energy in mere seconds. This was around 23 times more powerful than the most explosive nuclear weapon ever detonated, the Tsar Bomba. Make no mistake, these quakes are insanely powerful.
Some of you may have spotted a pattern here – all of these quakes took place on convergent plate boundaries. They’re what are known as “megathrust” quakes, which describes one fault sliding upwards relative to another on a gargantuan scale. Forget the fabled “Big One” due to rock San Andreas sometime soon – megathrusts are where the planet is at its most destructive.
To say these five quakes were devastating is a massive understatement. The release of so much energy triggered enormous landslides, turned the soil into a fast-flowing fluid, and cities were literally washed away. They were even powerful enough to ever-so-slightly change the length of a day by causing the planet to wobble on its axis.
It’s safe to say, however, that even these megathrust monsters could not literally crack open the crust though, and as far as geologists can tell, it’s never happened in the entire history of the world. But why?
Well, first off, the crust is already cracked open.
The average thickness of the continental crust is about 35 kilometers (22 miles), compared to the oceanic crust thickness of around 9 kilometers (5.6 miles). Fault networks are often shallower than 80 kilometers (50 miles) deep, but they can go as deep as 600 kilometers (375 miles). The crust, the upper mantle beneath it, and the tectonic slabs subducting into it are all cracked in some way.
The majority of the mantle, however, cannot be cracked open. It's solid, sure, but it’s so hot and squashed together that any attempt to split it would be like putting a dent in a highly pressurized tube of toothpaste. Instantly, mantle material would rush in to fill the gap and the shock would be absorbed.
But that’s no fun, is it? Surely we can generate an artificial earthquake and tear the planet apart, you may wonder. Why yes, yes we could. Let’s take a look at the energy that would be required to do so.
Earthquakes create frictional heat, particularly megathrusts. Assuming the crust is generally made of granite, which melts at 1,260°C (2,300°F), we can use the magic of physics to work out how much frictional heat would be required to obliterate the crust during a megaquake: 4.4 x 1023 joules, in fact.
That would require an earthquake far more powerful that the 1960 Valdivia earthquake. A 12.8M earthquake, roughly speaking.
If a supervillain made this happen, then it would cause Earth’s orbit around the Sun to wobble dramatically, perhaps messing up our seasonal cycles forever more. The molten crust would explode into the ocean, causing a gigantic steam blast that would obliterate whatever country it was near at the time.
(Un)fortunately, this type of megathrust quake could never happen in real life. The forces required to generate it are far beyond the mechanical strength of any rock we know of, which means that the stress of two plates would cause them to jut forwards long before they could build up enough energy.
If you really want to crack open a planet, we’d suggest hitting it with an asteroid. The famous dino-killing monster was enough to crack open the crust as far north as Colorado.