Earthquakes and Seismology

 Macquarie Island, year 2000. I am in the science lab doing routine stuff. Suddenly I hear a frantic scratching sound coming from the back room. About half a second later the ground shifts a little bit under my feet and I hear this powerful rumbling sound that seems to come from the centre of the Earth. A minor earthquake. The scratching sound came from the pens of my trusted old seismograph that went crazy about the thing a little bit before I could feel it with my body and my ears.

The cause of earthquakes is the friction of tectonic plates. When two plates rub together, they do not glide smoothly. Sometimes they get a little bit stuck and energy builds up. When the motion suddenly resumes the energy is released, creating shock waves in all directions. These shockwaves travel from the quake epicentre all around the Earth and can be picked up by seismographs.

A seismograph consists of three sensors: X, Y and Z. X and Y detect horizontal ground motion (East-West and North-South) and Z does vertical motion. Comparing X and Y gives the direction a shockwave comes from. So with data from two seismographs located a fair distance from each other, it is possible to pin-point the epicentre.

Shockwaves bounce off the boundaries of the Earth core and mantle. So analysing a seismograph network data is not unlike doing an ultrasonic scan on a human body, but on a bigger scale. This is an important part of geophysics.

Reference:

https://en.wikipedia.org/wiki/Seismology

The pics below are from this site.

Basic Seismographs

Shockwaves propagation