AAMC Test 7 PS #46 Doppler question

[wareagle87]

I know this question has been asked, but I still don't understand the reasoning behind the answer.

"The primary rupture in the Landers quake moved from south to north as it progressed to its full 74-km length. How would the Doppler effect influence this?"

A) By decreasing the wavelength of seismic waves propagating eastward and westward

B) By increasing the wavelength of seismic waves propagating eastward and westward

C) By decreasing the wavelength of seismic waves moving northward and increasing the wavelength of waves moving southward

D) By increasing the wavelength of seismic waves moving northward and decreasing the wavelength of waves moving southward

The answer is C. The reason from AAMC is the Doppler effect will cause a bunching or squeezing of the waves with the rupture and an elongation of the waves opposing the rupture.

It seems to me that it depends on the reference point that one is looking at to arrive at the correct answer. I picked the answer D because standing from the south the waves would lengthen and frequency would decrease when the wave travels from south to north. However, it seems as though the reference point is from the north. What am I missing? Thanks

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[sciencebooks]

I know this question has been asked, but I still don't understand the reasoning behind the answer.

"The primary rupture in the Landers quake moved from south to north as it progressed to its full 74-km length. How would the Doppler effect influence this?"

C) By decreasing the wavelength of seismic waves moving northward and increasing the wavelength of waves moving southward

D) By increasing the wavelength of seismic waves moving northward and decreasing the wavelength of waves moving southward

I'm going to try to explain this how I reasoned it. Hopefully it doesn't conflict with anything AAMC has said.

I thought of it two ways.

First, the Landers quake moved south to north. Movement of waves in the same direction (also south to north) would experience an increased frequency by the Doppler effect. Frequency is inversely proportional to wavelength. Thus, seismic waves moving northward have higher frequency = lower wavelength.

Then, I thought about it rationally. Because you want the wave to move northward, you want the energy propagated northward. Energy is proportional to frequency so you want to increase frequency northward and decrease frequency southward = decrease wavelength northward and increase wavelength southward.

Hope that helped.

 

[MD013]

If it helps, whenever I think about doppler effects, I picture a water droplet in my head. The water droplet (the source) as it drops down to a puddle, you see the waves correct? As it moves outward, the wavelength increases. The wavelengths are smaller when it is closest to the source so the frequency is highest since they are inversely proportional. I always remember, wavelength is largest at its tail and smallest at the beginning. Hope my weird explanation did not confuse you.

 

[wareagle87]

That is similar to how I think about Doppler effect. However, for this problem that is why I got it wrong. If south is where the quake originates wouldn't that have the shortest wavelength and highest frequency, and then the wave spreads out to the north with a lower frequency and longer wavelength.

The one thing that maybe be the problems is that when I did the problem I was thinking of the wave as a stationary wave. For example, a stationary ambulance with its siren blasting and not coming toward the observer, but should I be thinking of the wave as a moving wave like an ambulance moving towards me with the siren blasting? That would give me the right answer if I thought of it like that. Thanks

 

[MD013]

Yea since it is not stationary, the south should then have the longest wavelength since it is moving towards the north. It is leaving a trail of longer wavelengths as it goes further away from its original location (south).

 

[circulus vitios]

Doppler effect is really all about the compression/expansion of waves.

The seismic waves are moving from south to north. The northern tip of the wave is compressed because it's moving north. The southern tip of the wave will be decompressed because it's at the tail end. Compression at the northern tip = shorter wavelength. Decompression/expansion at the southern tip = long wavelength.