Businessconquerer wrote:
HELP HELP HELP
3. It can be inferred from the passage that if the S waves from an earthquake arrive at a given location long after the P waves, which of the following must be true?
(A) The earthquake was a deep event.
(B) The earthquake was a shallow event.
(C) The earthquake focus was distant.
(D) The earthquake focus was nearby.
(E) The earthquake had a low peak intensity.
No one could explain it properly
PLease could you?
GMATNinja GMATNinja2
NOTE: The OA for question #3 has been changed from (A) to (C).
From the second paragraph, we know that primary (P) waves and secondary (S) waves both move outward from the focus of an earthquake, traveling at different constant rates. Watadi studied the interval of time
between the two type of waves reaching a certain point.
Let's say an earthquake has an epicenter in Paris (sorry, Paris). If you measured the time interval between (P) and (S) waves at a point very
close to Paris -- maybe just outside the city -- you would expect the time gap to be very small, because the faster (P) wave hasn't had time to get too far ahead of the slower (S) wave.
What if you measured that time interval at a point far away from Paris -- maybe in Berlin? Now you would expect the time gap to be
larger, because the (P) wave has raced ahead of the (S) wave with each passing kilometer.
For most earthquakes, this is exactly what Watadi found!
In a few cases, however, there was a long interval between P and S waves even near the epicenter of the earthquake. What caused this larger gap? For these particular events, the earthquake
actually began several hundred kilometers below the surface of the earth (or, the "focus" of the earthquake was several hundred kilometers below the surface). This explains the larger time gap between P and S waves because they still had to cover long distances, giving the P wave time to get well out in front of the S wave. To continue with our example, instead of traveling from Paris to Berlin, the waves traveled UP from deep within the mantle to the surface of the earth.
So if S waves arrive long after P waves, we know that the focus of the earthquake is
far away from the point at which the measurement is taken -- whether that distance is horizontal (across the surface of the earth), or vertical (up from within the earth). This fits with answer choice (C), "the earthquake focus was distant."
I hope that helps!