Types of Earthquakes

The most familiar earthquakes are not random events — they are the result of rocks breaking and sliding along faults. But not every earthquake has the same cause, and the way an earthquake's energy travels through the Earth tells us a great deal about the planet's hidden interior. The classification of earthquakes and the behaviour of their waves are two sides of the same story.

Types of Earthquakes

The most common earthquakes are tectonic earthquakes. These are generated when rocks slide along a fault plane. The movement is not smooth; stress builds up until the rocks suddenly slip, releasing energy in the form of seismic waves.

A special class of tectonic earthquake is sometimes recognised as a volcanic earthquake. These are confined to areas of active volcanoes. They occur in association with volcanic activity, but they are still fundamentally tectonic in origin — hence the label "special class."

There is a third type mentioned in the text: reservoir induced earthquakes. These occur in the areas of large reservoirs. The weight of the impounded water and the changes in pressure it causes can trigger seismic activity in the surrounding rocks.

Earthquake Waves

When an earthquake occurs, it generates waves that travel through the Earth. These are recorded on a seismograph. There are two broad categories of waves, and within them, three main types to know.

Body waves travel through the interior of the Earth. There are two kinds:

• P-waves (primary waves) move faster and are the first to arrive at the surface. They are similar to sound waves. Crucially, they can travel through gaseous, liquid and solid materials.
• S-waves (secondary waves) arrive at the surface with some time lag. The important fact about S-waves is that they can travel only through solid materials.

This single characteristic of S-waves — that they cannot pass through liquids — is quite important. It has helped scientists understand the structure of the interior of the Earth. Because S-waves are blocked by liquid layers, their behaviour reveals where those liquid zones lie.

Surface waves are the last to report on the seismograph. These waves are more destructive than body waves. They cause displacement of rocks, and hence, the collapse of structures occurs. This is why the shaking felt during an earthquake — and the damage done — is so closely tied to surface waves.

Propagation of Earthquake Waves

Different types of earthquake waves travel in different manners. As they move or propagate, they cause vibration in the body of the rocks through which they pass.

• P-waves vibrate parallel to the direction of the wave. This exerts a push–pull effect on the rocks along the line of travel.
• S-waves, by contrast, vibrate perpendicular to the direction of travel, which produces a shearing motion. …