The science of earthquakes: Examining what caused the devastating natural disaster in Fiji
In the past fortnight, three large-scale Earthquakes have hit Fuji, Mexico and Japan causing devastation to buildings and killing hundreds.
In general, any vibration sent through the Earth’s crust is an earthquake and they can be caused by volcanoes erupting, asteroids hitting the surface of the planet or even man-made causes such as fracking and nuclear explosions. The most common is the shifting in the planet’s tectonic plates.
The Earth is made up of a series of layers. The very core of the planet is molten iron, the source of the magnetic field which protects us from solar winds and creates light displays known as the Northern Lights.
Above this core lies a rocky shell called the mantle, which separates the core from the Earth’s crust. The mantle is always moving because hot material near the Earth’s core rises as colder mantle rock sinks.
The outer shell of our planet, the crust, is divided into several plates. Made of lighter rock, but much more hard and rigid, these plates glide over the mantle underneath, and the movement of these plates is described by plate tectonics.
In general, the most active parts of the world in terms of plate tectonics are at the boundaries where two or more plates meet, known as fault lines. For example, the San Andreas fault which extends for 750 miles (1,200km) through California lies at the boundary between the Pacific Plate and the North American Plate. This is why California experiences so many quakes.
Depending on the direction of movement of each of the plates, there are various kinds of faults that can develop.
When two plates move apart, allowing molten rock or magma to rise up to the surface, this is a divergent plate boundary. When plates move together and push against each other, either one plate will give way and sink below the other, or the two edges will rise up, forming a mountain. This is a convergent plate boundary.
Most earthquakes start when the force of a moving plate builds up enough to cause a sudden shift in the rock. This, in turn, might trigger another movement in a nearby fault, and is why earthquakes can trigger other earthquakes.
The severity of earthquakes are measured using shaking amplitudes – essentially the size of the waves given off from the centre. This is measured on the Richter scale – a logarithmic scale, meaning a magnitude 5 earthquake is ten times stronger than a magnitude 4.
Tsunamis and landslides
Depending on the nature of the earthquake and where it takes place, often other natural disasters can be triggered when an earthquake happens. For example, if the epicentre of an earthquake is under the sea, the seabed can be displaced, setting off a tsunami. On land, earthquakes give rise to landslides or volcanic activity.
The movement in the Earth’s crust causes vibrations to be sent rippling through the planet, called seismic waves. Just like water waves when you throw a stone in a pond, these waves radiate outward from the central point –the epicentre.
There are two main types of seismic waves; the fast-moving P waves and slower moving S-waves. Both bring with them the shakes in the ground that you may be familiar with if you have ever experienced an earthquake yourself. There are a variety of other kinds of seismic waves, but S and P are the main ones.
Seismic waves can be monitored using machines called seismographs, which measure the vibrations coming from each type of wave. The most important things to measure are the speed the waves are travelling and when they arrive at the seismograph. From this information, seismologists can work out the size of the earthquake and exactly where it started.
In order to minimise the damaged caused by earthquakes, scientists are constantly trying to improve methods to predict them. There are two main ways this is done; through studying the history of a particular place and through monitoring the amount of strain built up in the rock. However, we’re still a long way off successfully being able to predict earthquakes, and some scientists say it is a completely impossible task.