GY305 Geophysics Lab 4: Seismology
Problem 1.
Seismic P-waves travelling through the lower oceanic crust of density 3.0 encounters upper mantle of density 4.0 at the crust/mantle interface. If
the incident angle is 25 degrees, what is the incident angle when the ray path is refracted? What angular change in ray path does this produce?
Use Figure 5 to estimate seismic velocities.
1. Incident angle in lower layer = ______________
2. Angular change = __________________
Problem 2.
Given the following seismic reflection data solve for the depth to the reflector at the ends of a 24-geophone seismic line. Also calculate the dip
angle of the reflector assuming the seismic reflection line is perpendicular to strike:
Shot point offset: 50 feet
Geophone spacing: 25 feet
Layer velocity: 8000 ft./sec.
2-way travel time recorded by geophone #1: 62.8 msec
2-way travel time recorded by geophone #24: 285.9 msec
Ray path distance to geophone #1=: ____________________
Depth to reflector @ geophone #1: _________________________
Ray path distance to geophone #24: ____________________
Depth to reflector @ geophone #24: ________________________
Dip of reflector: ____________________
Problem 3.
Figure 1 is a plot of 1st motion data from (a) Iran and (b) the Eastern Atlantic. For both of the diagrams filled circles are compressional and open
circles are dilational. For both stereographic plots:
1. Plot the 2 nodal planes and list the strike and dip
a. Strike and dip = ________________
b. Strike and Dip = ________________
2. After investigating tectonic maps of Iran and the eastern Atlantic make a “best guess” on which nodal plane is the true fault plane.
Classify the fault motion:
a. Fault strike and dip = _______________
b. Type of Fault = _________________
Problem 4.
Using the diagrams in Figure 2 :
1. Plot the nodal planes and list the strike and dip of the nodal planes:
a. Strike and dip = ________________
b. Strike and Dip = ________________
2. Using the tectonic map in Figure 2 list the strike and dip of the true fault plane and classify the fault:
a. Fault strike and dip = _______________
b. Type of Fault = _________________
Problem 5.
Using the map in Figure 3, the graph in Figure 4 (P-wave velocity =4.5km/sec ; S-wave velocity =2.3km/sec), and the following table:
Seismic Station
A
B
C
P-wave arrival time
05:33:45
05:33:53
05:34:28
S-wave arrival time
05:34:12
05:34:28
05:35:37
t (time differential)
Epicentral distance
Find the following:
Seismic Station
A
B
C
Plot the position of the epicenter on the map in Figure3. Interpolate the position from the latitude-longitude grid:
Epicenter Longitude: ____________________ (decimal degrees)
Epicenter latitude: _____________________ (decimal degrees)
Figure 1: 1st motion data from Iran and eastern Atlantic.
Figure 2: 1st motion data from the mid-Atlantic Ridge.
Figure 3: Map of Seismic Stations A, B, and C.
-91°
-90°
-89°
-88°
37°
-92°
-87°
-86°
-85°
-84°
-83°
-82°
-81°
36°
KENTUCKY
MISSOURI
C
!>
NORTH
CAROLINA
35°
35°
36°
TENNESSEE
34°
SOUTH
CAROLINA
ARKANSAS
B
33°
34°
!>
GEORGIA
MISSISSIPPI
32°
33°
ALABAMA
A
!>
31°
32°
LOUISIANA
FLORIDA
-92°
-91°
-90°
-89°
-88°
-87°
-85°
-84°
0
Legend
!>
-86°
25
50
-83°
100
Rivers
Lakes
State Boundaries
150
200
Kilometers
Seismic Stations
Interstate Highways
-82°
Southeast U.S.
Coordinate System: Albers
Central Meridian: 96°0'0"W
1st Std Parallel: 20°0'0"N
2nd Std Parallel: 60°0'0"N
Latitude of Origin: 40°0'0"N
Epicentral Distance
250.00
200.00
150.00
Travel Time
(seconds)
P-wave
100.00
S-wave
50.00
0.00
0.00
100.00
200.00
300.00
Distance (km)
Figure 4: P- and S-wave travel time curves for southeastern U.S.
400.00
500.00
600.00
Figure 5: Seismic Velocities (filled circles = igneous/metamorphic rocks; open circles = sedimentary rocks).