Earthquakes: Are they becoming frequent?

Nature, despite providing all support to sustain life on Earth, has been displaying its ruthless force every now and then to living beings, ever since life evolved on Earth.

Among all natural calamities (disasters), an earthquake can be considered as one of the most dangerous since it is so spontaneous (instant), impossible to stop and there is very little you can do to get away from it.

You are sure to know about the earthquake that shattered southern China a few weeks back, causing more than 40,000 deaths and millions of displacements. Therefore, let’s have a brief look at this natural occurrence and whether there is a tendency for the number of earthquakes to increase in the future; there is already heated discussions about this.

Earthquakes in brief

An earthquake is basically a situation where the ground which generally is at rest starts to shake, making nearly everything on it unstable. But, technically speaking, this is a phenomenon which takes place as a result of a sudden release of energy in the Earth’s crust.

This energy is released in the form of seismic waves which cause all the havoc (destruction). Seismometers are used to measure the intensities of these seismic waves and the Richter Scale is used to check the severity of the earthquake.

In fact, the Richter Scale looks at the ‘moment magnitude’ of the seismic wave. Moment magnitude defines a physical quantity which depends on the slip on the fault and the area of the fault surface that slips.

In addition, another scale known as the ‘Mercalli Scale’ is used to measure the intensity of shaking of the ground. Where the Richter scale is concerned, quakes below the value 3 are almost imperceptible (too small to be felt) and quakes greater than 7 are considered catastrophic (devastating). The shaking in earthquakes can also trigger landslides and occasionally volcanic activity.

How and where they form

Generally, earthquakes can happen at plate boundaries or sometimes, at a certain distance from the boundary. First, let’s look at the earthquakes at plate boundaries. You might remember what we wrote about plate tectonics.

When the plates of the Earth move, there may be situations where they will move past each other smoothly only if there are no irregularities or asperities (rough features) along the boundary that would increase the frictional resistance.

But, most boundaries do have such irregularities and this leads to a form of stick and slip behaviour. This means, once the boundary has locked due to friction, continued relative motion between the plates leads to increasing stress and therefore, stores strain energy in the volume around the fault surface which relates to the ‘sticking’.

This continues until the stress has risen sufficiently to break through the irregularity, suddenly ‘slipping’ over the locked portion of the fault, releasing the stored energy. This energy is released as seismic waves, frictional heating of the fault surface, and causes an earthquake.

But, most importantly, it is estimated that only 10 per cent or less of an earthquake’s total energy is radiated as seismic energy. Most of the earthquake’s energy is used to power the growth of the earthquake fracture or is converted into heat generated by friction.

The earthquakes that happen away from plate boundaries are due to the deformations that are spread over a larger area than plate boundaries. Earthquakes happen when strains that get built within the faults increase and are released at once.

The special term ‘earthquake storms’ is used for earthquakes that occur in a cluster, where each earthquake, gets triggered by the shaking of the previous earthquake creating a series of earthquakes.

What’s the trend?

Right now there are two ideas coming up. One suggestion is that due to the varying pressure on the Earth caused by human constructions etc, the tendency for earthquakes to happen can increase.

Certain statistics support this by showing that the intensities of earthquakes in regions like China have increased due to large constructions like the Shanghai World Financial Centre and Taipei 101 which are among the tallest buildings in the world right now.

However, the other party argues that there is no real increase in tendency, but it just seems that way due to advancements made in reading earthquake data. Therefore, according to them, the improved data reception has implied that there have been too many earthquakes happening lately.

The fact that in 1931, there were only about 350 seismic stations operating in the world and today there are more than 8,000 stations, vouches for the second argument.

Anyway, right now it has been quite hard to predict the trends of future earthquake patterns due to the lack of accuracy in past data, and yet, advances in technology would surely provide much better monitoring and prediction mechanisms in the days to come by.

-Aravinda Dassanayake