Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

The attenuation of the recorded seismic data is shown in
Fig. 3. It is clear that the recorded ground motions gives
much higher peak ground acceleration values, than what
is predicted by the empirical attenuation relationships.
The largest ground motions were recorded by the three
near fault stations Wolong, Qingping and Baijiao, which
recorded maximum values of 957.7, 824.1 and 633.1
cm/s 2 , respectively (Li et al., 2008b). The attenuation of
the simulated ground motions found in the present study
is shown as the lower graph in Fig. 3. From this it can be
seen, that the simulated peak acceleration values are
comparable to those recorded during the earthquake.
Fig.4: Top: Simulated Pseudoacceleration response spectra
for the three stations Qingping, Baijiao and Wolong. The
two horizontal components are shown (see legend for
specifications). Bottom: Acceleration response spectra
obtained from Wolong station in Wenchuan during the
mainshock of 12 May 2008, form Li et al., (2008a).
Fig. 4 shows the pseudoacceleration response spectra,
from the ground motion simulation, for the stations in
Wolong, Qingping and Baijiao (see Fig. 2 for locations).
Tthe predominant periods of the simulated ground
motions are between 0.1s and 1s. The simulation results
are well in agreement with Li et al. (2008a), which
estimate the predominant spectral acceleration to be
distributed between periods of 0.1s to 0.6s (see the lower
graph in Fig. 4) and the ground motion simulation
therefore reproduces the frequency content of the
seismic waves to a very large extent.
STRONG GROUND SHAKING AND DAMAGE ON
BUILDINGS
The affected area suffered severe destruction during the
earthquake. The economic losses due to the event are
estimated to be 86 billion USD (USGS). The city of
Hanwang is located along the fault, approximately 90km
NE of the epicentre and had 70,000 inhabitants before
1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology
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the earthquake. During the earthquake 4,500 have lost
their lives. In an area of approximately 3km 2 in the central
part of Hanwang, 90‐95% of the buildings were totally or
partially collapsed, resulting in a large uninhabitable area.
The picture in Fig. 5 is taken in the area of almost total
collapse. The typical buildings in this area are residential
buildings of 4‐5 stories. In the foreground, rubble from
totally collapsed buildings is visible, whereas the buildings
in the background are still standing. In this area the adobe
structures have almost completely collapsed, whereas the
more recent reinforced concrete structures seem to have
been able to withstand the strong ground shaking.
Fig.5: Picture taken in Hanwang, inside the 3km 2 area of
almost total collapse.
Using a simple empirical relation for fundamental periods
of structures (Kramer, 1996), for the typical structures
dominating the area in Hanwang, the predominant
periods are estimated to be 0.16‐0.21s for two story
buildings. The first floor is usually used as a shop and the
second floor as living area. For five‐story apartment
buildings on the other hand, the fundamental frequency
is estimated to be 0.47‐0.56s. Comparing the fundamental
periods of the structures in the area to the predominant
periods of the seismic signals coincide to a large extent,
which is the reason why these buildings have not been
able to withstand the ground motion experienced in the
area. Furthermore, the older buildings suffered from an
insufficient design code, since there have been many
revisions of the seismic design code for buildings in China
during time, latest in 2001 (Zhao et al., in press). The
buildings which are still standing in the area (e.g. like the
buildings to the left in the picture) were under
construction at the time of the earthquake, and they are
assumed to be designed after the latest adequate seismic
design code.
CONCLUSIONS
In the present study we have used a hybrid broadband
technique for modeling ground motions caused by the
May 12, 2008 Wenchuan earthquake. The following
conclusions can be drawn from the simulation results:
- The ground motions experienced during the
May 12 Wenchuan earthquake were
reproduced, and values similar to those
recorded on the near field stations in the area,
with maximum bedrock ground motions in the
order of 850 cm/s2 (PGA) and 200 cm/s (PGV),
were obtained.
- The values of the simulated PGA are higher than
predicted commonly by empirical attenuation