Question

when observing a seismograph, which waves would you see arriving at the recording station first? Describe the characteristics of p and s waves.

Answer 1

Wavelength and Period

To understand the difference between these seismic recording instruments, we need to discuss a little about the waves. First, we use the term wavelength the refer to the peak-to-peak distance on a wave measured at a single time - like in a snapshot. To measure the wavelength directly, we would need a group of instruments that measure the amplitude of the wave at the same time but at different locations. If we record the ground motion at a single location for a range of time, we can measure the time between peaks in the motion, which we call the wave period. Another important term is frequency, which is the inverse of the period, or one divided by the peak-to-peak time between wave crests.

Wave Defnition Cartoon

Waves are energy transmitting phenomena that have an amplitude and a wavelength. The upper panel shows a snapshot of the wave at a single time. The lower panel can be thought of as the motion of a single point for an interval of time.

The period and the wavelength are related by a simple expression connecting the two with the speed of the wave

wavelength = speed x period .

Note that the units of the quantities on both sides of the equation balance: wavelength is a measure of distance, such as kilometers, speed is usually specified in terms of kilometers/second, and period is measured in seconds. Checking the physical units of equations is an important way to make sure that you've got the mathematics correct.