Seismic tomography is similar to a CAT scan. When earthquakes occur, they sent out seismic waves, and by recording the patterns of the wave arrivals from many earthquakes on many seismometers, seismologists can calculate the wave speeds through the crust. Fast and slow wave speeds can then be interpreted to geologically understand the tectonic and/or volcanic structure of the earth.

A
B
C
D
E

F
G
H
I
K
L
M
O
P

Q
R
S
T
U
V