60 Basic Seismological Theory
AMJ
KTJ
U
D
N
S
E
W
U
D
N
S
E
W
P
S
0
5
Sec
S
P
S
Earthquake
Fig. 2.4-8 Three-component seismograms
at two stations from an earthquake beneath
Japan. Because the stations are nearly above
the earthquake, the P wave has its largest
amplitude on the vertical (U–D, “Up”–
“Down”) components. (Ando et al., 1983.
J. Geophys. Res., 88, 5850–64, copyright
by the American Geophysical Union.)
0
5
10
15
20
25
Time (s)
E–W
N–S
U–D
MNV September 24, 1982 07:40:24
P
S
t s − t p
Fig. 2.4-9 Three-component seismogram of
a magnitude 4.9 shallow-focus earthquake
recorded 64 km away at Mina, Nevada. The
difference in the arrival times of the P and S
waves, t S − t P , can be used to estimate the
distance between the earthquake and the
seismometer.
These data also show an interesting effect. The S wave
on the north–south components arrives earlier than on the
east–west components. This observation has been interpreted
as indicating that material beneath the seismic stations is
~5% anisotropic, such that in this region shear waves with
displacements in the N–S direction propagate faster than those
with displacements in the E–W direction. The anisotropy
(Section 3.6) may reflect the presence of the mineral olivine,
in which seismic waves propagate at different speeds depending on their direction with respect to the crystal structure.
If enough olivine crystals are oriented in a consistent fashion,
significant anisotropy can result. A second effect that could
cause significant anisotropy is the presence of a region of
aligned cracks.
Figure 2.4-9 shows a different type of seismogram: a record
of a shallow earthquake in Nevada from a seismic station
within 100 km of the source. The times when the P and S waves
arrive can be measured from the seismograms. With a number
of such observations at different locations, we will see (Chapter
7) that the location and origin time of the earthquake can be
determined. Even with one seismic station, something about
the location of the earthquake can be learned. Although the
arrival times of the seismic waves cannot be converted to travel
times without knowing when the earthquake occurred, we can
learn something from the difference between the P and S arrival
times. For typical values of the compressional and shear velocities in the crust, α = 5.5 km/s and β = 3.2 km/s, the times
required for S and P waves to travel a distance of x km are
AMJ
KTJ
U
D
N
S
E
W
U
D
N
S
E
W
P
S
0
5
Sec
S
P
S
Earthquake
Fig. 2.4-8 Three-component seismograms
at two stations from an earthquake beneath
Japan. Because the stations are nearly above
the earthquake, the P wave has its largest
amplitude on the vertical (U–D, “Up”–
“Down”) components. (Ando et al., 1983.
J. Geophys. Res., 88, 5850–64, copyright
by the American Geophysical Union.)
0
5
10
15
20
25
Time (s)
E–W
N–S
U–D
MNV September 24, 1982 07:40:24
P
S
t s − t p
Fig. 2.4-9 Three-component seismogram of
a magnitude 4.9 shallow-focus earthquake
recorded 64 km away at Mina, Nevada. The
difference in the arrival times of the P and S
waves, t S − t P , can be used to estimate the
distance between the earthquake and the
seismometer.
These data also show an interesting effect. The S wave
on the north–south components arrives earlier than on the
east–west components. This observation has been interpreted
as indicating that material beneath the seismic stations is
~5% anisotropic, such that in this region shear waves with
displacements in the N–S direction propagate faster than those
with displacements in the E–W direction. The anisotropy
(Section 3.6) may reflect the presence of the mineral olivine,
in which seismic waves propagate at different speeds depending on their direction with respect to the crystal structure.
If enough olivine crystals are oriented in a consistent fashion,
significant anisotropy can result. A second effect that could
cause significant anisotropy is the presence of a region of
aligned cracks.
Figure 2.4-9 shows a different type of seismogram: a record
of a shallow earthquake in Nevada from a seismic station
within 100 km of the source. The times when the P and S waves
arrive can be measured from the seismograms. With a number
of such observations at different locations, we will see (Chapter
7) that the location and origin time of the earthquake can be
determined. Even with one seismic station, something about
the location of the earthquake can be learned. Although the
arrival times of the seismic waves cannot be converted to travel
times without knowing when the earthquake occurred, we can
learn something from the difference between the P and S arrival
times. For typical values of the compressional and shear velocities in the crust, α = 5.5 km/s and β = 3.2 km/s, the times
required for S and P waves to travel a distance of x km are