Page 428 - Laboratory Manual in Physical Geology

P D C
C
C
C
D
Fault
Up
Compression D C D
C
D
0 5 10 15
Scale in seconds D
D
P C
C D
D D
Down C
C
C
Dilation
Block X Block Y
FIGURE 16.6 How to read and plot earthquake first motions. Left —Sketch of typical seismograms for compressional first motion (first
P-wave motion is up) compared with dilational first motion (first P-wave motion is down). Right —Map of a hypothetical region showing a fault
along which an earthquake has occurred, and the P-wave first motions (C = compressional, D = dilational) observed for the earthquake at seismic
stations adjacent to the fault. Stress moves away from the field of dilation and toward the field of compression on each side of the fault, so the
relative motion of the fault is as indicated by the smaller half-arrows.

1 5 9

2 6 10

3 7 11

4 8 12

13

FIGURE 16.7 Activity 16.5 seismograms. The seismograms are from 13 numbered seismic stations in the Mississippi Embayment after an

earthquake that occurred in the New Madrid Fault System. Numbers in this figure correspond to the numbered sites on the map in Activity 16.5 .
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