Earthquakes –they’re all the same, right? Well, the answer to that is more like a politician’s reply than a scientist’s… it’s yes, and it’s no. Earthquakes are complicated things, caused by the earth’s tectonic plates moving in different ways and the shaking they produce is affected in different ways by local geology. We recently received a question, “Why do some earthquakes feel different? Some seem to shake side-to-side, and some seem to almost roll. What causes that?”
Let’s have a look at how these different types of quakes work.
Plate Movement Causes Earthquakes
It’s simplest to describe three types of movement which cause earthquakes. Some occur where plates move past one another and the movement is lateral. Sometimes the plates move apart and parts of the earth drop down: this is known as normal faulting. Where plates move together, and a plate or part of a plate is forced upwards, it’s called reverse faulting.
Body Waves and Surface Waves
Despite their varying origins, all earthquakes generate two main types of seismic waves – surface waves, which, as their name suggests, travel along the surface of the earth, and body waves, which travel through the earth itself. These waves generate different types of shaking and various factors affect how they are felt on the surface.
Let’s take body waves first. There are two types: ‘P’ waves, which have compressional motion (backwards and forwards) in the same direction as the wave motion: and ‘S’ waves in which there is shear motion (i.e. movement at right angles, vertically or horizontally or a combination, to the wave motion). Of these, ‘P’ waves are faster-moving.
Surface waves also come in two types – Love waves, which also involve shear movement though only in the horizontal plane; and Rayleigh waves, which are more like ocean waves and which generate a rolling motion. It is the surface waves which are responsible for most earthquake damage.
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