Figure
1
Background: The focus of an earthquake is the place inside the Earth where the energy is release as movement occurs on a fault. The deepest earthquakes that have been recorded occur at depths of 600 km. The epicenter is the point on the Earth’s surface directly above the focus. Seismologists plot the epicenter.
When an earthquake occurs, seismic waves immediately radiate away from the focus in all directions. P-waves are compressional waves, material expands and compresses in the direction of wave motion. P-waves are the fastest moving seismic waves. P-waves moving along the surface can cause the ground to buckle and fracture.
S
Figure
2
Use the earthquake seismogram in figure 3 to answer the following questions:
Figure
3
How long did the entire earthquake recording last? ___________________
How much time elapsed between the P- and S-waves? ___________________
How much time elapsed between the arrival of the P-wave and the first surface wave? ____________
How does the maximum amplitude of the surface waves compare with the maximum amplitude of the P-waves? _________________________________________________________
What was the approximate average period of the surface waves? ___________________
Figure
4
P - and S-waves leave the focus at the same instant in time. As they get further from the focus, P- and S-waves get further and further apart. The time difference between the P- and S-waves allows seismologists to determine how far away an earthquake occurred from a seismic station. The time-travel graph at right is used to determine distance to an earthquake epicenter.
How do the arrival times of the first P-waves and S-waves change with distance from the epicenter?
What is the difference in arrival times between the P- and S-waves for stations at the following distances from an epicenter:
1000 km
2000 km
3000 km
Using figures 3 and 4, answer the following questions:
What is the distance from the epicenter to the seismic station? ___________________
How long after the earthquake did the P-wave arrive at the station? _________________
If the P-wave was recorded at 4:30 am local time, when did the earthquake occur? ___________________
E
Figure
5
When the seismogram stations are closer to the earthquake epicenter, we need to use a time-travel graph that is scaled appropriately. Use Figure 6 to determine the distances the stations are from the epicenter then draw circles centered on the stations with a radius of that distance to determine the location of the earthquake epicenter.
Figure
6
Seismic Station—Fresno, CA
Seismic Station—Phoenix, AZ
Seismic Station—Las Vegas, NV
Station |
Tp |
Ts |
ΔT |
Distance |
Fresno, CA |
0 Seconds |
|
|
|
Phoenix, AZ |
0 Seconds |
|
|
|
Las Vegas, NV |
0 Seconds |
|
|
|
Seismic station---Akita, Japan
Seismic station—Pusan, South Korea
Seismic station—Tokyo, Japan
Station |
Tp |
Ts |
ΔT |
Distance |
Akita, Japan |
0 Seconds |
|
|
|
Pusan, S. Korea |
0 Seconds |
|
|
|
Tokyo, Japan |
0 Seconds |
|
|
|
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Tags: figure, background, focus, earthquake