Explainer: 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 traveling. And while some of these waves travel deep within the planet, still others move only along the surface. Studying where these various flavors 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 (Sighs-MAH-meh-turz) — 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 kilometers 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 kilometers 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 traveling.

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 percent as fast as p waves. So, along Earth’s surface they move at speeds of between 3 and 4.8 kilometers 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 traveling). 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 center, 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 epicenter.

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.

Here’s why: When a natural earthquake occurs, one side of a fault zone slides in one direction; the other side slides in the opposite way. (A fault zone is a fracture in Earth’s crust, or a boundary between two tectonic plates, where slippage can occur and seismic energy can be released.) Now, imagine that an earthquake occurs in an area that’s covered with a network of seismometers. For some of the instruments, the first P waves to arrive will be a “push” from the quake. But for others, the first P waves to arrive will be a “pull.”

For seismic vibrations generated by an unnatural explosion, the first P wave to arrive at every seismometer will provide a “push.” Not only that, the P waves generated by an unnatural explosion are typically sharp and sudden. So they die away pretty quickly. Vibrations produced by a natural earthquake instead tend to rumble for quite a while. That’s because the slippage along fault zones in a natural quake doesn’t happen all at once, like an explosion does.

Still more flavors of seismic waves

At first, all of a quake’s energy travels from its source deep within the planet as P waves and S waves. But when that energy reaches the surface, it now can spread as either of two different types of waves.

Think of a quake’s energy as a bubble rising from the bottom of a pond. The surface waves are much like the ripples in the pond’s surface. Here, the waves spread from the quake’s epicenter. These waves also are typically larger and cause much more damage than P waves and S waves.

The faster of these surface waves was named after British mathematician A.E.H. Love. More than 100 years ago, he worked out the math that explains how such waves move. The second type of surface waves were named for a British physicist who, in the 1880s, predicted their existence. This scientist was named John William Strutt. His father had been a British noble dubbed Lord Rayleigh. At his father’s death in the 1870s, Strutt inherited the title, becoming the next Lord Rayleigh. The waves he predicted are now known as Rayleigh waves.

Of these two surface waves, the Love type travels a bit faster.

Like S waves, Love waves shake the ground from side to side compared to the direction they’re headed. (In other words, for a Love wave traveling north, the ground shakes back and forth from east to west.) Rayleigh waves, on the other hand, cause ground movements in two directions at once. One of those motions is up and down, very much like waves on the ocean’s surface. The other is a push-pull movement along the same path that the wave is traveling. Together, those motions generate a rolling action that can cause extreme damage to buildings and other structures.

Other uses for seismic waves

Geoscientists often use seismic waves to map details of the inner structure of our planet. For instance, the time it takes P waves and S waves to travel down into Earth and then return to the surface helps scientists calculate how deep the boundaries of Earth’s major layers are. (Those calculations are made possible, in large part, because researchers have measured the speed of seismic waves through rocks under immense pressure in the lab.)

P waves and S waves tell scientists a lot more than the depth ranges of Earth’s major layers. In some cases, they also provide strong clues about the type and density of materials in those layers. For example, at distances of between 11,570 and 15,570 kilometers (7,190 to 9,670 miles) from a major earthquake, seismometers don’t record any S waves coming directly from that quake. That’s a big clue that Earth’s outer core is made of liquid, scientists say. (In areas more than 15,570 kilometers away from a quake’s epicenter, seismometers do detect S waves. Those waves develop when the energy of P waves that have traveled through Earth’s outer core once again enter the mostly solid mantle. That’s the very thick layer that lies between Earth’s outer core and its crust.)

At shallow depths in Earth’s crust, all types of seismic waves can be used to map out relatively small geological structures. These include things such as faults and sediment-filled basins. (Sediment-filled basins are broad bowls of solid rock where loose material accumulates. Such areas can be especially affected by earthquakes. That’s because seismic waves can get trapped and bounce around inside that basin, making the sediment shake like jelly in a bowl.) Again, the time it takes for a seismic wave to travel to a structure and then echo back helps scientists estimate how far away that structure is.

Even people setting off small explosions of dynamite can trigger seismic waves. That means these can be mapped from afar. It’s also possible to use data gathered by seismometers over a long period of time. Although such signals may be faint, they can be assembled into stronger signals (much in the same way that photographers can take photos in dim light by leaving their camera’s shutter open for minutes or even hours at a time).