french institute of science and technology for transport ... - Ifsttar

2_SCIENTIFIC AND TECHNICAL ACTIVITIES
Acceleration (gals)
700
600
400
200
0
a
CHB024 (Vs30 = 248.5 m/s)
liquefaction
The major
earthquake
in Japan
Analyzing ground
movements
-200
10.0
ESD in horizontal plane (S-transform)
Ew
NS
On 11 March 2011, an Mw 9 earthquake struck the
East coast of Japan. This was the strongest tremor that
has been felt in the country and one of the largest earthquakes
to have occurred since seismological measurements
began.
50
Frequency (Hz)
Acceleration (gals)
Normalized Acceleration
1.0
0.1
4 000
2 000
1 000
-1 000
50 100 150 200 250
b
Time (s)
MyGH10 (Vs30 = 347 m/s)
3 000 NS2
0
1.0
0.5
0.0
-0.5
-1.0
0
500 1 000 1 500 2 000 2 500
Evolution of predominant frequency
500 1000 1500
2000 2500
Times (s)
Ew2
NS1
Ew1
15
14
13
12
11
10
9
8
7
6
5
Frequency (Hz)
Japan’s exceptional instrumentation system, consisting
of roughly 1000 accelerometric stations on the surface (the
K-NET network) and 700 seismograph pairs located on the
surface and in boreholes (KiK network) meant that this large
earthquake was the best recorded in the world, from distances
that were close to the fault plane (40 – 50 km).
The recordings show that the strong movements lasted approximately
3 minutes, with a maximum ground acceleration
value at sites near the fault plane that often exceeded 1g. This
combination of factors led to the mobilization of interstitial pressure
which was responsible for cyclic mobility and liquefaction
at the regional level (Figure a). The ANR ONAMAZU project,
which got under way in October 2011, brings together partners
from France and Japan in order to conduct a thorough analysis
of the ground movements that were recorded during the earthquake.
This research involves IFSTTAR, BRGM, IRSN and
CETE Méditerranée in France and DPRI, NIED and Shimizu
Corp in Japan. The project began with the construction of
a database containing accelerometer measurements from
the mainshock and the aftershocks. This was used to compute,
amongst other things, the response of the soil at station
MYGH10 (Figure b), which shows that the medium did not
return its initial state after the principal event. The intention is
for the project now to conduct a fuller study of this phenomenon
in order to calibrate constitutive models for soil under
seismic loading.
@ CONTACT
fabian.bonilla@ifsttar.fr
Figure a. Time-frequency analysis of the recordings of station CHB024
(Chiba Prefecture). The signals show the effect of cyclic mobility (high frequency
peaks between 120 and 130 s) and liquefaction (after 130 s).
Figure b. Change in response over time in the case of the soil under station
MYGH10 (predominant frequency). Note the sharp drop in frequency during the
mainshock and the slow recovery during the first 45 minutes of the recording.
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