Earthquakes produce two kinds of seismic waves: he longitudinal primary waves (c

Question

Earthquakes produce two kinds of seismic
waves: he longitudinal primary waves (called
P waves) and the transverse secondary waves
(called S waves). Both S waves and P waves
travel through Earth’s crust and mantle, but
at different speeds; the P waves are always
faster than the S waves, but their exact speeds
depend on depth and location. For the pur-
pose of this exercise, we assume the P wave’s
speed to be 9090 m/s while the S waves travel
at a slower speed of 4070 m/s.
If a seismic station detects a P wave and
then 54.5 s later detects an S wave, how far
away is the earthquake center?
Answer in units of km.

Explanation / Answer

   Let the distance of earthquake centre be 'x'm.    Time   t   =   distance/ speed    for Pwave   tp   =   x/ 9090      s    for Swave   ts   =   x/ 4070      s    given   ts   -   tp   =   54.5   s    =>   x /4070   -   x /9090   =   54.5       x ( 9090 - 4070 / 9090 *4070)   =   54.5    distance   x   =   54.5* 9090 * 4070 / 5020   =   4.0165 *105   m   =   401.65  km

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