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## Earthquakes produce two kinds of seismic waves: he longitudinal primary waves (called P waves) and the transverse secondary wave

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 8560 m/s while the S waves travel

at a slower speed of 5760 m/s.

If a seismic station detects a Pwave and

then 30.5 s later detects an S wave, how far

away is the earthquake center?

Answer in units of km.

Mi

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Physics
6 months
2021-09-01T16:18:04+00:00
2021-09-01T16:18:04+00:00 1 Answers
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## Answers ( )

Answer:d

=

s

t

Explanation:distance = speed * time

here, the distance is from the earthquake centre to the seismic station.

both the P and S waves travel from the earthquake centre before being detected by the seismic station, so the distance is the same for both.

the speeds are given as

8740

m

/

s

for the P wave and

4100

m

/

s

for the S wave.

we also know that the P wave arrives

47.4

seconds before the S wave.

we do not know the time that the P wave takes to travel, but we can denote it as

t

P

.

the time that the S wave takes to travel can be denoted as

t

P

+

47.4

, where time is in seconds.

for the S wave, speed * time is

4100

⋅

(

t

P

+

47.4

)

for the P wave, speed * time is

8740

⋅

t

P

.

since the distances that they travel are the same, the two expressions for speed * time are equal.

4100

⋅

(

t

P

+

47.4

)

=

8740

⋅

t

P

if you expand the brackets, you can find that

4100

t

P

+

194340

=

8740

t

P

then you can subtract

4100

t

P

:

4640

t

P

=

194340

and divide by

4640

to find

t

P

, which is the time that P takes to travel:

t

P

=

41.883

…

seconds

since distance = speed * time, the distance that the P wave travels is

t

P

⋅

the speed of P.

this is

41.883

s

⋅

8740

m

/

s

, which gives

366057.42

m

.

in kilometres, this is

366

k

m

to

3

significant figures.