Laboratory Manual for Introductory Geology 4e

EXERCISE 11.2
Name:
Course:
The Logic of Locating Earthquake Epicenters (continued)
Section:
Date:
The logic of locating earthquake epicenters based on the arrival times of different types of seismic waves.
Car 1
travel time 60
Car 1
(1 mile in 60 seconds)
Car 2
travel time
61
Car 2
(1 mile in 61 seconds)
Delay time
1 mile 2 miles 3 miles 4 miles 5 miles 6 miles 7 miles
1
Of course, geologists don’t know at first how far seismic waves have traveled or when they were generated by an
earthquake. But as with the cars in this exercise, geologists do know (1) the velocities of the different types of seismic
waves; (2) that the waves started at the same time (when the fault moved); and (3) the precise times when the different
wave types were recorded by seismometers. To figure out how far away an epicenter was and when the earthquake
happened, we work backward to derive the information.
You can see how this works by continuing to use the two imaginary cars instead of seismic waves and working
backward like a seismologist. Imagine you are waiting for Car 1 and Car 2 to drive by a streetlight outside your window.
You don’t know where they came from or what time they left, but you do know (1) their velocities (you have been told
that Car 1 will arrive traveling at 1 mile in 60 seconds and Car 2 at 1 mile in 61 seconds) and (2) that they started at
exactly the same time. As they drive by, you (3) record the precise delay between the times they pass the streetlight
outside your window.
(c) Using that information, how far did the two cars travel if Car 2 passed the streetlight
(i) 25 seconds after Car 1?
miles
(ii) 45 seconds after Car 1?
miles
(d) Now determine the precise time that the cars started their trip (hour:minute:second) if Car 1 passed the streetlight at
12:25:00 and was
(i) 25 seconds before Car 2: : :
(ii) 45 seconds before Car 2: : :
(Hint: Determine how far the cars traveled and then use the known velocity of Car 1 to estimate how much travel
time it needed to go that distance.)
Now put the two pieces of reasoning together.
(e) If Car 1 arrives at precisely 1:45:22, followed by Car 2 at 1:46:37, how far did they travel?
(f) At what precise time did they leave? : :
miles
11.3 LOCATING EARTHQUAKES
281