A seismograph at a monitoring station records ground vibrations from a distant earthquake. Two types of seismic wave arrive at the station: - Primary (P) waves, in which rock particles vibrate back and forth along the same direction that the wave travels. - Secondary (S) waves,…

Answer: ((a)) P-waves are longitudinal, because the rock particles vibrate parallel to (along) the direction of travel. S-waves are transverse, because the rock particles vibrate perpendicular to the direction of travel. ((b)) Wavelength = v ÷ f = 6.0 ÷ 2.0 = 3.0 km. ((c)) Frequency = v ÷ = 3.6 ÷ 1.2 = 3.0 Hz.. Part (a): Classifying P-waves and S-waves To classify a wave, compare the direction the particles vibrate with the direction the wave travels. - P-waves: particles vibrate back and forth along the same direction as the wave travels, so P-w…

IGCSE Physics 0625: Classifying and Calculating Seismic P-waves and S-waves

A seismograph at a monitoring station records ground vibrations from a distant earthquake. Two types of seismic wave arrive at the station:

Primary (P) waves, in which rock particles vibrate back and forth along the same direction that the wave travels.
Secondary (S) waves, in which rock particles vibrate from side to side, perpendicular to the direction the wave travels.

(a) State, with a reason based on the descriptions above, whether P-waves are transverse or longitudinal. State, with a reason, whether S-waves are transverse or longitudinal. [2]

(b) The P-wave arriving at the station has a frequency of $2.0\text{ Hz}$ and travels through the rock at a speed of $6.0\text{ km/s}$ . Calculate the wavelength of this P-wave, giving your answer in km. [3]

(c) The S-wave from the same earthquake has a wavelength of $1.2\text{ km}$ and travels through the rock at a speed of $3.6\text{ km/s}$ . Calculate the frequency of this S-wave. [3]

Part (a): Classifying P-waves and S-waves

To classify a wave, compare the direction the particles vibrate with the direction the wave travels.

P-waves: particles vibrate back and forth along the same direction as the wave travels, so P-waves are longitudinal.
S-waves: particles vibrate from side to side, perpendicular to the direction of travel, so S-waves are transverse.

Part (b): Wavelength of the P-wave

The wave equation links speed, frequency and wavelength:

$v = f\lambda$

Rearranging for wavelength:

$\lambda = \frac{v}{f}$

Substituting $v = 6.0\text{ km/s}$ and $f = 2.0\text{ Hz}$ :

$\lambda = \frac{6.0\text{ km/s}}{2.0\text{ Hz}}$

$\lambda = 3.0\text{ km}$

Part (c): Frequency of the S-wave

Rearranging the wave equation for frequency:

$f = \frac{v}{\lambda}$

Substituting $v = 3.6\text{ km/s}$ and $\lambda = 1.2\text{ km}$ :

$f = \frac{3.6\text{ km/s}}{1.2\text{ km}}$

$f = 3.0\text{ Hz}$

Final answers

(a) P-waves are longitudinal; S-waves are transverse
(b) Wavelength of the P-wave $= \boxed{3.0\text{ km}}$
(c) Frequency of the S-wave $= \boxed{3.0\text{ Hz}}$