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!
!
!
!
(1)
×1010 cm/s = 30
18
/
1
4
30 / 4
1
7
7.5
(2)
km
200 km
vs
3.5
1.2 g/cm3
P24,25
1.0
「氷河」は流れるが「山」は数百年では崩れないのと同じ原理
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P
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y
P
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(2)
A B
(a)
1,000km
X
P56
30°
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X
x=
km
1000km
B
A
30°
40000km
40008km
X
(b)
X
1000 km
tan 30° =
x
x
30°
x
X
1700km
30°
1
(a)
5000km
(b)
1000km
(3)
1m
2m
2m / (3x1010 cm/s) = 200/(3 x1010 )
= 6.7 x 10-9 sec = 6.7 x 10-3 µsec
DVD
DVD
P22
P68
AB
cm/
100km
100km 2 x 106
= 5 cm/
= 100 x 105 cm
A
2Ma
2 x 106
B
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10cm/
A
B
A
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P
EW
S
3.1 Introduction to seismic sources and source parameters
Fig. 3.3 depicts schematically an idealized sub-surface explosion and tectonic earthquake (of
pure strike-slip type) in a homogeneous medium.
P
Fig. 3.3 Schematic sketches of an idealized underground explosion and of a strike-slip
earthquake along a vertically dipping fault. The fault motion is "left-lateral", i.e., counterclockwise. The arrows show the directions of compressional (outward, polarity +, red shaded)
and dilatational (inward, polarity -, green shaded) motions. The patterns shown on the surface,
termed amplitude or polarity patterns indicate the azimuthal variation of observed amplitudes
or of the direction of first motions in seismic records, respectively. While point-like
explosions in an isotropic medium should show no azimuth-dependent amplitudes and
compressional first motions only, amplitudes and polarities vary for a tectonic earthquake.
The dotted amplitude lobes in Fig. 3.3, right side, indicate qualitatively the different azimuth
dependence of shear (S) waves as compared to longitudinal (P) waves (rotated by 45°) but
their absolute values are much larger (about 5 times) than that of P waves.
P
S
It is obvious that the explosion produces a homogeneous outward directed compressional first

SNC
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1996
SNC
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