The Speed of Seismic Waves
Steve Jones is an amateur seismologist in Huntsville, and he supplies us as well as teachers around North Alabama with supplemental information for Earth Science curriculum. If you are interested in getting updates on real-world seismic activity, please contact Steve from his website at Alabamaquake.com!
Steve also works closely with AMSTI schools in the Athens State University region, and since we are both interested in making science fun and enjoyable for students in this region, we will occasionally post some of his material here on Valleywx.com! He offers a classroom program called “Earthquake 101.”
The Speed of Seismic Waves vs Sound Waves
When an earthquake occurs, the mechanical properties of the rocks that seismic waves travel through quickly organize the waves into two wave types; and so two distinctly different types of waves begin to travel outward from the quake focus through the earth. The P-wave (a compressional or pressure wave) is the faster of the two, and is like a sound wave in the air. An S-wave (a side-ways or shearing type of wave) is also produced by the quake. A seismograph on the surface of the earth will record the P-waves of an earthquake first, and then the S-waves some short time later.
The speeds of the seismic waves in the earth, and sound waves in air, are much different, with seismic waves traveling through the earth with a much higher speed than sound waves in the air. Sound waves move through the air at about 1100 feet per second. The fastest seismic waves to leave the source of an earthquake (the P waves) travel (depending on the surrounding material and depth) at nearly twenty times that speed.
Typical P-wave speeds typically range between about 1 and 14 km/sec. The slower values correspond to a P-wave traveling in water, and the higher number represents the P-wave speed near the base of Earth’s mantle. The speed of the S-wave is about 60% of that of the P-wave. So while wave speeds may vary by a factor of ten or more in the Earth, the ratio between the average speeds of a P wave and its following S wave is quite constant.
A P-wave can generate a sound in the human-audible frequency range when it reaches the earth’s surface. A corresponding sound made by the usually-stronger S-wave, upon reaching the earth’s surface, will generally be below the human-audible frequency range.
Local soil or water conditions and the depth of the earthquake focus do cause some variations, but the number of seconds observed between the arrivals of the P-waves and S-waves, multiplied by 8 km/sec, is approximately the distance in kilometers to the earthquake location.