Earthquakes are essentially sound waves—called seismic waves—traveling through the earth. Because the earth is solid, it can support both longitudinal and transverse seismic waves. The speed of longitudinal waves, called P waves, is 8000 m/s. Transverse waves, called S waves, travel at a slower 4500 m/s. A seismograph records the two waves from a distant earthquake. If the S wave arrives 2.0 min after the P wave, how far away was the earthquake? You can assume that the waves travel in straight lines, although actual seismic waves follow more complex routes.

Key Concepts

Seismic Waves

Seismic waves are energy waves generated by the sudden release of energy in the Earth's crust, typically during an earthquake. They are classified into two main types: longitudinal waves (P waves) that compress and expand the material they travel through, and transverse waves (S waves) that move perpendicular to the direction of wave propagation. Understanding these waves is crucial for analyzing how they travel through different materials and for interpreting data from seismographs.

Wave Speed

The speed of a wave is determined by the medium through which it travels. In the case of seismic waves, P waves travel faster at approximately 8000 m/s, while S waves travel slower at about 4500 m/s. This difference in speed is essential for calculating the distance to an earthquake's epicenter, as the time delay between the arrival of the two types of waves can be used to determine how far the waves have traveled.

Seismograph

A seismograph is an instrument that detects and records the motion of the ground caused by seismic waves. It measures the arrival times of P and S waves, allowing scientists to analyze the characteristics of an earthquake. By examining the time difference between the arrival of these waves, seismologists can calculate the distance to the earthquake's source, which is vital for understanding seismic events and assessing potential impacts.