How to see the earth rumbling.

The Earth starts to shake.
All the tension that was built up at the fault was released. An earthquake just happened somehow the ground starts moving. If you think about it, it’s fascinating how this energy can affect us several miles away from the actual earthquake. But how exactly does this work?

At the fault

A sudden jerk of two plates will create several mechanical waves. These move in different ways, which we call polarization. However, they all move away from the starting point of the earthquake, the epicentre. But the Earth is layered and everytime this wave will hit a significant layer, like the mantle or the core, some energy will be reflected. So when we’re sitting at the seismograph at the surface we record a complex image of different mechanical waves that were transmitted and reflected within the Earth.

How am I supposed to imagine this?!

The USGS has made this brilliant animation that shows the 2002 earthquake in Alaska that could be recorded all over the world since it had a magnitude of 7.9 on the Richter scale.

On the top part you can see the seismogramms of different seismic stations all over the world. On the bottom left you can see mechanical surface waves travelling along and und the bottom right, you see mechanical body waves.

What’s P PP PPP PS S SS SKiKP?

When you take a look at the labeling, this can get quite confusing, so let’s take a quick dive into seismic waves. P and S waves are the two major body waves, waves travelling through the body of an object, such as the Earth. The P wave always travels faster than the S wave and everytime a wave gets reflected at a boundary, another letter will be added, to note where it was reflected and what kind of wave emerges. So PcS is the P wave travelling down to the core-mantle boundary, and travelling back up as an S wave.

First P then S

So you might have notice, that I just told you that a P wave travels back up as an S wave. Now physics gets a bit of a nuisance. Instead of just staying the wave it is, a P wave can excite a S wave at a reflection point, which we call a converted wave. This doesn’t really make it easier for us to untangle the different waves in our seismograms.

The really interesting bit

This gif seems pretty complex, but someone created it with a smart program they coded (Thanks USGS!). However, not too far back in time, we did not have a clue about the Earth’s interior. The knowledge from the Earth’s interior actually comes from looking at seismograms and taking this information to figure out the different layers. Before that, there were even people believing that there were the seven circles of hell below the crust. Just from looking at those complex wiggles. Without any supercomputer, they were able to infer the inner and outer core, lower and upper mantle and many more astounding features.