Earthquakes: How big can they get?

A subduction zone is where a plate goes underneath another, into the mantle where it gets melted down.

If I remember correctly theres a much lower chance of there being a quake in the centre of a plate, it depends upon what the local rock structures are like. The UK has had some smal localised shallow quakes in the past couple of centuries, but I think their mechanisms are different from those at plate boundaries.

 

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There are quakes in the center of plates. The New Madrid quake is a good example. I believe that the New Madrid fault zone represents the third arm of a failed rift dating back to the break up of Pangea. These quakes are rare in comparison to quakes at plate boundaries. There was a good sized quake in Charleston, SC back in the 1800's. I haven't heard a good explanation of that one.

 

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How do you mean, shatter? You mean break the planet apart completely? That certainly couldn't be true - the Earth's gravitational binding energy is stronger than all the energy released by the Sun in a second.

 

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Xyth, the magnitude of a seismograph is exponential? A 12 is a 10^1,000th increase of that of 9? (I don't bother with seismology, just was passing on what I remember being told by some seismologist way back, heh). So to me, the saying that a 12 would just "rock the world apart" doesn't seem far fetched, considering that it'd be emmensly more energetic.

Consequently, it'd be impossible, because I doubt the Earth's system contains that much energy to be released...I don't know where the cut-off point by energy restrictions in the system must be...10 something I should imagine?

Anything else would be requiring of something similar to a bolide impact.

Speaking of which, I'd suggest (from further memory) that the 12 magnitude figure is a bit off as well, because I was playing around with some "figures" of bolide impacts and achieving a 12 was similar to creating a geologic anomally about the size of mount everest, and a crater about 6,000 miles in diameter, so that isn't the "destruction" of the world per-say? lol

 

Quakes happen where ever there is a fault, regardless of the location on the plate. All an Earthquake is is the motion of the ground, the ones we care about are the ones we can actually feel, which tend to happend when a fault block wants to move but doesn't, until it finally just "snaps". Uhmm...well I suppose this is best caused by the fact that kinetic friction is less than static friction. So the potential energy is building-up until it is suddenly released in motion. Same as what happens when you try to push something heavy, at first it doesn't move, but when it does it moves more easily...

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They occur most often at those regions. But there are exceptions, like the New Madrid earthquake and numerous other ones in the East Coast of the U.S. There are also earthquakes in other parts of the world that develop stress within plates, i.e., the recent earthquake in Xinjiang from the same tension that created lake Baikal due to the collision of the Indian plate w/ the Eurasian plate. Also in China, the Tangshan earthquake in the 70s was around an 8, but the city itself is pretty far from the nearest plate boundary, which if I recall correctly is the Phillipine plate.

 

So the bottom line is earthquake can happen if the region it hapens in is rather near tectonic cracks or something (faults etc)? But if something else other than these tectonic cracks were to cause a quake, what could it be?

Oh, and I just remembered to ask this specific question:
Have the tectonic plates been moving in one direction (I mean, one set direction for one plate)? If so, some questions:
1) What stops the Indian plate from stopping or even going backwards after hitting, er, Eurasian (or else?) plate, the movement that makes Himalayan mountain range goes higher and higher (albeit some minor collapses IIRC) as time passes by?
2) Still speaking of Indian plate, relative to its movement, is there any plate which move at the same direction towards Indian plate? If not, what happens to the 'gap' Indian plate leaves as it goes norther and norther each time?

Oh, and please answer my curiosity on whether earth's rotaion can affect earthquakes, in whatever way (frequency or power)...

 

Good questions.
1. The plates are driven by convection currents in the mantle. The Earth still contains a lot of heat generated by the decay of radioactive elements. The mantle is sufficiently hot that, over long time periods, it behaves like a liquid and moves. So there are places on the Earth's surface where currents are rising, then spreading out. It is this spreading motion of the mantle, that carries the plates (which consist of the crust and the very upper part of the mantle) forward.
Along the line where the currents come to the surface the Earth literally splits apart and, because the constraining pressure is now low, becomes molten, giving us an eruption. This is what is taking place along the mid-ocean ridges. It also accounts for the East African rift valley and the Red Sea. In this way new ocean floor is generated. As the plates separate small shallow earthquales are generated.
At the other 'end' of the plate it will either plunge below its neighbour, or ride over it. That movement will generate the larger earthquakes, such as this disasterous one.
Google ['plate tectonics' introdcution'] for more information.

Edit: Because this has been going on for hundreds of millions of years, there are lots of old plate boundaries 'buried' in the middle of continents. With the uneven stresses applied to each plate movement may sometinmes occur along these old fault lines, giving us the New Madrid type example.

 

Actually, the magnitude of an earthquake is measured using a logarithmic scale ... in fact, for every unit increase in magnitude is an increase in energy of approximately 30 times. In other words, A 12 would result in (30)^3 times more energy released than a 9.

To put that in better perspective, a Mag 9 earthquake releases an equivalent amount of energy of 32 billion tons of TNT whereas a Mag 12 earthquake is equivalent to 1000 trillion tons of TNT. And according to this website I found, that's enough energy to "fault Earth in half through the center" ... though that's hard to imagine but I'm only writing down what they're saying.

 

Actually if the exponent you gave is accurate, the 12th magnitude earthquake would be 27000 times as powerful as a 9th magnitude: this would equate to [32 billion x 27000 = ] 864 trillion tons of TNT...

Over 80 times the energy probably released by the K-T mass extinction impact. Globally catastrophic to say the least, but not sufficient to physically split the entire planet.

 

The exponential I gave is a rough estimate to simplify the calcualtion. It's not exactly 30 times ... it varies between 27 and 33 times for each unit increase of magnitude, depending where you look ... it's more in the neighbourhood of 27.3 from what I found.

I know that doesn't give the 1000 trillion ton value I stated before but that number is something I just grabbed off a particular website for a magnitude 12 earthquake.

Regardless, I don't even believe myself that something in that neighbourhood is enough to split the planet ... it's just something this particular website said, and I can't find another source to back that claim. The 1000 trillion ton of TNT would only represent 1/1000th the Earth's gravitational energy anyways.

 

Thanks Singularity, by the way to save-face, I did not mean 10^1000, I meant 10^3 lol...but it was late for me...

Anyway, I wasn't so much saying it as "gospel" it was just off-the-cuff as certainly a 12 has not been recorded. I'd have to say that 1/1000th of the Earth's gravitational energy is a huge portion.

 

The Richter scale, it seems, involves 2 different exponents: a 31-fold increase in energy equates to only a 10-fold strengthening of the actual shockwaves.

http://www.pgc.nrcan.gc.ca/seismo/eqinfo/richter.htm

A scale increasing in multiples of 10 seems most rational.

Then again, there is also the Mercalli Scale, which rates the effect of an earthquake rather than its raw power.

http://www.pgc.nrcan.gc.ca/seismo/eqinfo/richter.htm

 

Someone in the net just said that some scientists believed that the recent quake had caused the earth to 'shrink' and rotate faster by a few microseconds.... Not only that, they also said the quake causes the earth to tilt (from its pre-recent statistics) as well...

Makes me wonder.... Can earth actually cause a quake which can shatter itself? Okay, you are discussing it, so, those things I found would be just another news or something anyway ^_^

 

About 9 is the strongest one we can ever expect. The reason is that rocks are just not strong enough to accumulate more energy.

 

I suppose that could be the crucial factor: too much stress and rocks would simply crumble, rather than transmitting the power as earthquake waves.

Mantle rocks are denser and toughter, obviously - but earthquakes aren't generated in the mantle. Although magma plumes do sometimes force their way up through it, so stresses must accumulate.

 

Of course, the earthquake will be the least of the worries if we are hit (again) by such an object.

I wonder what the magnitude of the shockwaves were when we were struck by that Mars-size object back in the early Pre-Cambrian?

 

The idea of magnitude is meaningless for an event which was literally world-shattering. There's no scale to classify the force of planetary collisions.

All one could do would be estimate the energy released by such a cosmic smash. Probably amounted to pentillions of megatons.

 

How do you mean exactly? I know that a magnetar is a neutron star with a particularly intense magnetosphere... are you saying that it can somehow break up, or undergo a complete restructuing, due to internal mechanical instabilities? :bugeye:

Tiny variations in the periods of pulsars have been attributed to "starquakes;" that too I know.