Geoscience Reference
In-Depth Information
SISMALP NETWORK
10.02.2006 13:16
Conand aftershocks
CNDSZ
CNDSN
CNDSE
27
28
29
30
31
Fig. 3
Felt Conand aftershock (200-Hz sampling rate) used for computing seismic moment and
corresponding
M
L
-0.75 magnitude (4-s window). S - P = 120 ms; focal depth is about 900 m.
Amplitude window for each component is
±
300
ms
−
1
(same amplification as Fig. 2)
of 2,500 kg m
−
3
for Dogger limestone. We found that a 55
◦
-dipping, 40 m
50 m
source where a 2-mm slip propagated at 2,000 m s
−
1
with a rise time of 12 ms fitted
reasonably well the observed S-wave pulse. The seismic moment
M
0
, obtained by
multiplying the rigidy, the fault surface, and the slip, is 8.4 10
10
N m. To convert
it to local magnitude, we use the relation advocated by Bakun (1984) for
M
L
×
<
3
earthquakes:
log
10
M
0
=
1
.
2
M
L
+
10
.
Hence, under the assumed conditions,
M
L
is found equal to 0.75.
In February and March 2006, a total of 16 events were recorded by the Conand
station. On 28 Mar. 2006 at 07.34 in the morning, two late aftershocks were felt.
They were described as two explosions separated by 10 s, the first louder than the
second. This doublet was recorded by the local station (Fig. 4). The S - P intervals
(0.135 and 0.140 s) are slightly larger than for the 10-Feb. earthquake (0.120 s), but
we will assume that the difference in focal depth is not significant. By scaling the
maximum displacement amplitudes with that of the 10-Feb. shock, we find that the
corresponding magnitudes for these two felt events were
7.
The large discrepancy between the magnitude value computed by routine
Richter's technique (2.3) and that computed through the evaluation of the seismic
moment
M
0
(0.75) demonstrates—if ever it were necessary—that Richter's tech-
nique cannot be safely used for shallow (
z
−
0
.
2 and
−
0
.
<
∼
15 km) events observed at short
(
D
<
∼
15 km) epicentral distance.
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