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In-Depth Information
Seismic snapshots
Electric potential snapshots
×10 -8
×10 -5
0
0
4
3
t
= 305 ms
3
t
= 305 ms
200
2
200
CS
2
1
SE
1
400
400
0
0
RE
600
600
CS
−1
−1
−2
−2
800
800
−3
CS
−3
1000
−4
1000
0
200
400
600
800
1000
0
200
400
600
800
1000
×10 -5
Position x (m)
Position
x
(m)
×10 -8
0
2
0
4
3
200
200
CS
1
2
400
1
400
0
0
600
−1
600
−2
−1
800
800
−3
t
= 480 ms
−4
t
= 480 ms
−2
1000
1000
0
200
400
600
800
1000
0
200
400
600
800
1000
Position
x
(m)
Position x (m)
Figure 5.4 Same as Figure 5.3 for later times. Note that electrical field present around the seismic source and associated with the
diffusion of the pore pressure around the source (type I electrical signal, which is dipolar in nature). CS, coseismic P- and S-wave signals
(type III electrical signal); SE, seismoelectric conversions at interfaces (type II electrical signal); RE, reflected wave.
distribution associated with groundwater flow at the
position of the seismic source diffuses slowly over time,
depending on the value of the hydraulic diffusivity of
the medium. The left-hand sides of Figures 5.3 and 5.4
show the direct seismic waves and the reflected waves.
The right-hand sides of Figures 5.3 and 5.4 show the three
types of electrical disturbances. The type I electrical distur-
bance corresponds to a dipolar field associated with the
diffusion of the pore fluid pressure with time. After the
occurrence of the seismic source, this field decreases
exponentially with time. The type II electrical distur-
bances correspond to the electrical field generated by
the arrival of the direct seismic wave at the interface
between the layers L1 and L2. This field is also dipolar
in nature. The type III electrical disturbances are seen to
accompany the seismic waves.
The horizontal and vertical components of the seismic-
solid displacements and two components of the electric
field are shown in Figures 5.5 and 5.6, respectively, for
each of the 8 stations located in the vicinity of the ground
surface. The seismic displacements are created from the
mechanical conversion mechanism at the interface
between L1 and L2 such as the converted P
-
S-wave as
well as the direct P
-
P- and S
-
S-waves. The converted
S
P-wave is not visible on these time series because
of the minimum radiation pattern of S
-
-
P-waves in the
( x
z ) plane for the horizontal and vertical components
of the seismic displacement (Aki & Richard, 2002; Gao
& Hu, 2010).
The electrical field time series, due to the seismic distur-
bance, is shown in Figure 5.6. Note that the vertical com-
ponent of the electrical field is computed at a depth of 5m.
-
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