The Physics behind Earthquakes

Article by, Adrian Vatchinsky

To amend for the previous seasonally inappropriate snowflakes post, I figured I provide a more appropriate post this time around dealing with the physics behind earthquakes since basically the entire New York City shook along with the east coast today!

For starters, those who want to tackle the subject head on, I found this neat paper from CalTech which does a thorough job in giving a more rigorous background on earthquakes than the informal description which is about to follow.

For those who do not have the time (or patience) to read the 30+ paper allow me to take a stab at this fascinating event.

The reason behind earthquakes was not well understood until the tectonic plate revolution of the 1960s. It was actually in 1915 when Alfred Wegener first proposed the idea of continental drift as a way to explain the findings of similar fossils across lands separated by entire oceans. It took a while for this idea to become accepted and it was not until the 1960s that the concept of tectonic plates fully caught on.

But what exactly are tectonic plates? Well a really basic way of understanding what these "plates" are is to imagine the Earth as an egg drifting through space on its merry way, a space faring Humpty Dumpty if you will. And having the string of poor luck which the Dumpty family is notorious for, our poor space egg finds itself one day on a collision course with a rock floating in space. There is nothing that can be done to avoid this impact and the two bodies collide. The result is Humpty Dumpty's space cousin with a cracked shell!

The earth is sort of like that, the crust which we all live on top of is not one continuous solid. Instead it is subdivided among numerous "plates" which actively move around and interact with one another at their borders called fault lines.

Fig 1. A map of the Tectonic Plates which make up the Earth's crust.

But how does this all play into earthquakes you may wonder? Well as mentioned previously, these plates are in constant motion and interact with each other. For example the Pacific Plate and North American Plate are moving apart from one another, evident at the San Andreas Fault Line in California. Well, in the simplest terms, these plate interactions cause earthquakes!

Take a look at the following figure which shows areas of earthquake activity.

Fig 2. A map of earthquake activities

If you look at Fig 1 and Fig 2 you can kind of see that the earthquakes align with the plate boundaries in a sense. These interactions which occur (slipping, sliding, pushing) are not immediately noticed but instead build up elastic strain energy within the system. Eventually this form of potential energy becomes too great to bear and the rocks "slip" in a sudden fast motion. The location of the fault where this slip occurs is called the hypocenter from which seismic waves propagate throughout the surrounding area.

These waves, much like any other wave travel along their median, the earth, and cause displacements to occur along that medium. These are the waves people tend to feel during an earthquake.

Fig 3. Simple representation of an earthquake

As these waves travel they lose their energy with distance. Depending on the strength of the earthquake the waves propagate near or far from the hypocenter.

The reason why so many people were surprised today as they felt the residual waves of the Virginia earthquake was because there really are no major fault lines on the east coast to cause major earthquakes. This just goes along the point raised in the paper I initially cited which states that even today earthquake causes are not completely understood and the explanation is not as simple as two plates colliding.

In any case for those who are interested in a more detailed explanation I urge them to look it up.

In conclusion, I will leave with this video showing a simulation of an earthquake hitting a major city area.