Seismic Waves Come In Different ‘Flavors’

Earthquakes and underground explosions can release a lot of energy. That energy ripples away from its source in a variety of ways. Some of those vibrations will move forward and back through the material they travel through. Other waves travel just like ocean waves, where they make the material they pass through move up and down compared to the direction the wave is travelling. And while some of these waves travel deep within the planet, still others move only along the surface. Studying where these various flavours of waves are and how they move not only can help scientists pinpoint where an earthquake or explosion occurred, but also can shed light on the structure of our inner planet.

Seismic waves are vibrations in the ground. These can be generated by a number of phenomena, including earthquakes, underground explosions, landslides or collapsing tunnels inside a mine. There are four major types of seismic waves, and each typically travels at different rates of speed. That’s one big reason why scientists are able to tell them apart. If the waves arrived at vibration-detecting instruments — seismometers — all at the same time, it would be difficult to tell them apart.

Another major difference between these types of waves is how a material will move as the wave passes through it. With these differences in mind, let’s review the major types of seismic waves.

P versus S waves

Seismologists are scientists who study earthquakes. They also study how a quake’s energy spreads through Earth’s crust, as well as the deeper layers of our planet. The fastest seismic waves are known as P waves. That “p” stands for primary. And early seismologists called them that because these waves were the first to arrive at seismometers from some distant quake.

At Earth’s surface, P waves travel somewhere between 5 and 8 kilometres per second (3.1 and 5 miles per second). Deeper within the planet, where pressures are higher and material is typically more dense, these waves can travel up to 13 kilometres per second (8.1 miles per second).

P waves travel through rock the same way that sound waves do through air. That is, they move as pressure waves. When a pressure wave passes a certain point, the material it is passing through moves forward, then back, along the same path that the wave is travelling.

P waves can travel through solids, liquids and gases. That’s one big difference between them and the other types of seismic waves, which typically travel only through solids (such as rock).

The next-fastest type of seismic waves are “secondary.” They earned that name because they were typically the second set to reach seismometers from a distant quake. Not surprisingly, they’re known as S waves.

In general, S waves are only 60 per cent as fast as p waves. So, along Earth’s surface, they move at speeds of between 3 and 4.8 kilometres per second (1.9 and 3 miles per second).

As an S wave passes through a material, the site of its passing moves from side to side or up and down (as compared to the direction the wave is travelling). This is why S waves are also known as transverse waves. “Transverse” comes from the Latin words for “turned across.”)

S waves cannot travel through liquids or gases. That’s because the types of stresses set up by those waves can only be transmitted through solid materials.

Distinguishing earthquakes from nuclear shakes

Because P waves and S waves travel through Earth — not just along its surface — they are also known as “body waves.” This trait makes them useful in a number of ways. For one, scientists can use P waves and S waves to identify where an earthquake began. To do that, they need to have data gathered by seismic instruments at three or more different locations. That lets them triangulate to find the source of Earth’s shimmying.

Triangulation is only possible when there are accurate measurements of the times at which P waves and S waves show up at each seismometer. Some techniques use only the P waves. Others also consider the time difference between the arrival of the first P waves and S waves. (The farther the distance between the seismometer and the source of the quake, the more exaggerated that time difference will be.)

Whatever method is used, it gives scientists only an estimate of how far from a seismometer the earthquake’s source happens to be. So with a seismometer as a centre, scientists draw a circle of the proper size on a map. But using only one seismometer, there is no way to tell in which direction the source was. It could be anywhere along the outer edge of that circle. By plotting the circles for at least three instruments on the same map, however, there will be a single point where those circles overlap. That marks the point on Earth’s surface above the quake site.

Most quakes occur deep within Earth’s crust. The point where a quake originates is called its hypocenter. The point on Earth’s surface directly above the hypocenter is the quake’s epicentre.

But scientists don’t just use these waves to map earthquakes. Those same seismic waves also can be generated by underground explosions. These might arise from a small blast inside an underground coal mine, for example. Or, they might signal the test detonation of a nuclear weapon (such as several that recently took place in North Korea). And P waves, in particular, can strongly point to whether the seismic waves come from a natural quake or an unnatural blast.

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