Understanding P-Waves and S-Waves in Earthquakes

Why Seismic Waves Matter

When an earthquake occurs, the sudden release of energy sends shock waves radiating outward through Earth in all directions. These waves don't all behave the same way. Understanding the different types of seismic waves, particularly P-waves and S-waves, is crucial for earthquake science, from locating earthquakes to designing early warning systems that save lives.

As discussed in our guide to how earthquakes are measured, seismographs detect these waves and record their characteristics. But what exactly are these waves, and why do scientists care so much about the difference between them?

P-Waves: The Speed Demons

P-waves, or Primary waves, earn their name by being the first seismic waves to arrive at any location after an earthquake. They're the fastest seismic waves, traveling through Earth's crust at speeds ranging from 5 to 8 kilometers per second, depending on the rock type and depth.

How P-Waves Move

P-waves are compression waves, also called longitudinal waves. They move rock particles back and forth in the same direction the wave is traveling. Imagine pushing and pulling a spring or Slinky toy. As you compress one end, that compression travels along the spring's length. That's essentially how P-waves move through rock.

This push-pull motion alternately compresses and dilates the rock as the wave passes through. The rock temporarily becomes denser, then less dense, then denser again as successive wave compressions pass through it.

Where P-Waves Can Travel

Because P-waves work by compression, they can travel through any material that can be compressed: solids, liquids, and gases. This means P-waves can pass through Earth's entire interior, including the liquid outer core. This property has been invaluable for studying Earth's internal structure.

When you feel the initial jolt of an earthquake, that's often the P-wave arriving. P-waves typically produce a sharp, quick motion that people describe as a jolt or bump. The shaking is usually less intense than what follows with S-waves and surface waves.

S-Waves: The Powerful Followers

S-waves, or Secondary waves, arrive after P-waves at any given location. They're slower, typically traveling at 60-70% the speed of P-waves, or about 3 to 5 kilometers per second through the crust. Despite being slower, S-waves are generally more destructive than P-waves.

How S-Waves Move

S-waves are shear waves, also called transverse waves. Unlike P-waves that push and pull, S-waves move rock particles perpendicular to the direction the wave is traveling. Think of shaking a jump rope up and down. The rope moves vertically, but the wave travels horizontally along the rope's length.

This shearing motion requires the material to have rigidity, meaning it can resist being deformed sideways. When S-waves pass through rock, they twist and distort it, creating the characteristic side-to-side shaking people experience during the main shock of an earthquake.

The Liquid Limit

Because S-waves require a material that can resist shear forces, they cannot travel through liquids or gases. This limitation has profound implications for understanding Earth's interior. The fact that S-waves don't pass through Earth's outer core provided the first evidence that the outer core is liquid.

When you experience the main rolling or side-to-side motion during an earthquake, you're feeling S-waves. They produce stronger, more sustained shaking than P-waves and cause most of the earthquake damage to structures, especially when combined with surface waves that arrive later.

Side-by-Side Comparison

Using Wave Timing to Locate Earthquakes

The speed difference between P-waves and S-waves is one of the most powerful tools seismologists have for locating earthquakes. Here's how it works.

Both wave types leave the earthquake source at the same instant. However, because P-waves travel faster, they arrive at a seismograph station first. The farther the station is from the earthquake, the greater the time gap between P-wave and S-wave arrivals.

Scientists can calculate the distance from a seismograph station to an earthquake epicenter by measuring this time difference and applying the known speeds of P-waves and S-waves. This is called the S-P interval method.

By combining distance calculations from at least three different seismograph stations, scientists can triangulate the earthquake's exact location. This process, along with modern computing power, allows agencies like the USGS to determine earthquake locations within minutes of the event.

Early Warning Systems: A Race Against Time

The speed difference between P-waves and S-waves enables earthquake early warning systems. These systems detect P-waves, which cause less damage, to provide warning seconds before the more destructive S-waves and surface waves arrive.

Here's how it works: when an earthquake occurs, P-waves radiate outward from the source. Seismometers near the epicenter detect these P-waves almost immediately. Computer systems analyze the P-wave data to estimate the earthquake's location, depth, and magnitude in just seconds.

This information allows the system to predict when S-waves and surface waves will reach more distant locations. Because electronic signals travel much faster than seismic waves, alerts can reach people and automated systems before the destructive shaking arrives.

Real-World Warning Times

The warning time depends on your distance from the epicenter. Locations very close to the earthquake might receive only a few seconds of warning or none at all, since P-waves and S-waves arrive nearly simultaneously. However, locations farther away benefit from progressively more warning time.

For example, during the 2011 Tohoku earthquake in Japan, Tokyo received approximately 60 seconds of warning before strong shaking arrived, even though the city is about 370 kilometers from the epicenter. This was enough time for trains to slow down, elevators to stop at the nearest floor, and people to take cover.

The ShakeAlert system on the U.S. West Coast uses this principle to protect California, Oregon, and Washington. Even a few seconds of warning allows automated systems to shut off gas lines, halt surgeries, and open firehouse doors, potentially preventing injuries and saving lives.

What About Surface Waves?

While P-waves and S-waves are body waves that travel through Earth's interior, earthquakes also generate surface waves that travel along Earth's surface. These arrive last but often cause the most damage because they're typically the strongest and last the longest.

There are two main types of surface waves. Love waves move side to side horizontally, like an S-wave constrained to the surface. Rayleigh waves create a rolling motion similar to ocean waves, moving both vertically and horizontally in an elliptical pattern.

Surface waves travel more slowly than body waves but their energy is concentrated near the surface where all our buildings, roads, and infrastructure exist. This concentration makes them particularly destructive. The combination of S-waves and surface waves produces the intense shaking that causes most earthquake damage.

Understanding when these different waves will arrive helps explain why earthquakes can be felt at different intensities depending on distance from the epicenter.

Learning About Earth's Interior

P-waves and S-waves don't just help locate earthquakes—they're also our best tool for understanding what's inside our planet. Since we can't drill more than a few kilometers into Earth's crust, seismic waves act as natural probes that reveal the structure thousands of kilometers below our feet.

When seismic waves encounter boundaries between different materials inside Earth, they refract (bend) or reflect, similar to how light bends when it passes from air into water. By studying how waves from earthquakes all over the world travel through Earth, scientists have mapped out the major layers: the crust, mantle, outer core, and inner core.

The fact that S-waves cannot pass through the outer core revealed that it must be liquid. The way P-waves slow down dramatically in the outer core confirmed this liquid nature. Seismic wave studies also revealed that the inner core, despite extreme pressure, behaves as a solid because it's so compressed.

These wave studies continue to refine our understanding of Earth's interior, helping scientists understand not just the structure but also what causes earthquakes and how tectonic plates move.

Why This Matters for Earthquake Safety

Understanding P-waves and S-waves isn't just academic—it has real-world applications for earthquake safety and preparedness.

First, the ability to quickly locate earthquakes using wave arrival times means emergency responders know where to direct resources immediately. Instead of waiting hours to assess damage reports, they can predict which areas likely experienced the strongest shaking based on the earthquake's location and magnitude.

Second, early warning systems that detect P-waves can trigger automated protective actions. In Japan, bullet trains automatically brake when warnings are received. Gas utilities can shut off lines to prevent fires. Factories can shut down machinery to prevent damage. Hospitals can pause surgeries until shaking passes.

Third, understanding how S-waves and surface waves cause damage helps engineers design better buildings. Modern seismic design focuses on structures that can withstand the side-to-side motion of S-waves and the rolling motion of surface waves. Buildings that might survive P-wave shaking can fail catastrophically when S-waves arrive.

Finally, studying these waves helps scientists assess earthquake hazards. By understanding how waves travel through different geologic materials, they can predict which areas will experience stronger shaking, informing building codes and urban planning decisions.

The Bottom Line

P-waves and S-waves are fundamentally different types of seismic energy that travel at different speeds and cause different types of ground motion. P-waves arrive first with a sharp jolt, traveling through any material. S-waves follow with more destructive side-to-side shaking, able to travel only through solids.

This simple difference has profound implications. The time gap between wave arrivals allows scientists to locate earthquakes precisely. The P-wave's early arrival enables warning systems that provide precious seconds to take protective action. The inability of S-waves to pass through liquids revealed Earth's liquid outer core. The destructive nature of S-waves informs building design and safety measures.

Next time you experience an earthquake or see seismograph data, you'll understand that those wiggly lines represent different types of waves racing through Earth at different speeds, each telling scientists something important about the earthquake's location, size, and the planet's interior structure. From pinpointing epicenters to enabling early warnings that save lives, the difference between P-waves and S-waves matters immensely for understanding and preparing for earthquakes.