An earthquake is a sudden release of stored energy within the Earth’s crust, which radiates outward as seismic waves. These waves travel through the planet at distinctly different speeds, meaning they arrive at any given location at separate times. Understanding this arrival sequence is fundamental to seismology and earthquake preparedness.
The Initial Release: Primary (P) Waves
The seismic wave recorded first at any station is the Primary wave, or P-wave. This is the fastest seismic wave, traveling through the Earth’s crust at speeds ranging from about 5 to 8 kilometers per second. The P-wave is characterized by a compressional, or longitudinal, motion, pushing and pulling material in the same direction the wave travels, similar to sound waves. Because this motion involves volume changes, P-waves can travel through any medium: solid rock, liquid, and gas.
The first sensation of an earthquake is often a subtle, vertical jolt caused by the P-wave’s arrival. Though fastest, the P-wave generally carries the least amount of destructive energy compared to the waves that follow. Its ability to propagate through all materials, including the Earth’s liquid outer core, makes it a tool for studying the planet’s internal structure.
The Second Arrival: Secondary (S) Waves
The second type of seismic energy to arrive is the Secondary wave, or S-wave, which follows the P-wave. S-waves are also known as shear waves and travel more slowly, typically at 50 to 70 percent of the P-wave velocity, usually between 3 and 4.5 kilometers per second in the crust.
The motion of an S-wave is transverse, shaking the ground perpendicular to the direction the wave is traveling. This results in a side-to-side or up-and-down shearing motion, which requires the material to possess shear strength.
Because liquids and gases do not possess shear strength, S-waves cannot propagate through them, including the Earth’s molten outer core. When S-waves arrive, the ground shaking becomes noticeably stronger than the initial P-wave jolt.
The Slowest and Most Damaging: Surface Waves
Following the P and S body waves, surface waves are confined to the outer layers of the planet. These are the slowest seismic waves, but they are responsible for the most intense and prolonged ground shaking. Surface waves are generated when the P and S waves reach the surface and their energy is concentrated along this boundary.
There are two main types of surface waves: Love waves and Rayleigh waves. Love waves cause the ground to move horizontally from side to side, creating a shearing motion parallel to the surface. This horizontal displacement is particularly damaging to foundations.
Rayleigh waves exhibit a retrograde elliptical motion, rolling the ground both up-and-down and back-and-forth, similar to an ocean wave. The combined effects of these two waves are highly destructive because their energy remains focused near the surface. While they are the last to arrive, surface waves often possess the largest amplitude, making them the primary cause of structural failure.
The Practical Use of Arrival Timing
The predictable difference in arrival times between the fast P-wave and the slower S-wave is a powerful tool for seismologists. This time gap, known as the P-S interval, is directly proportional to the distance from the recording station to the earthquake’s epicenter. The longer the delay between the two arrivals, the farther away the earthquake occurred.
By measuring the P-S interval from at least three different seismic monitoring stations, scientists use triangulation to pinpoint the exact location of the earthquake’s origin. Travel-time curves, which plot the known speeds of these waves, allow the time difference to be converted accurately into a distance.
This arrival timing is the foundation of modern earthquake early warning systems. These systems detect the arrival of the P-wave and quickly issue an automated alert before the arrival of the slower, more destructive S and surface waves. This rapid warning can provide seconds to tens of seconds of notice, allowing for actions such as stopping trains or enabling people to drop, cover, and hold on.