Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology
SIMULATED GROUND MOTIONS OF THE MAY 12 2008, WENCHUAN (CHINA)
EARTHQUAKE – COMPARISON WITH DAMAGE DISTRIBUTION
L. W. Bjerrum (1), M.B. Sørensen (2) and K. Atakan (1)
(1) Department of Earth Science, University of Bergen, Allegaten 41A, NO‐5007 Bergen, NORWAY. louise.bjerrum@geo.uib.no,
kuvvet.atakan@geo.uib.no
(2) Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences, Public Law Foundation State of Brandenburg, Telegrafenberg,
D‐14473 Potsdam, GERMANY. sorensen@gfz‐potsdam.de
Abstract: The May 12, 2008 Wenchuan earthquake had its epicenter along the Longmenshan Fold and Thrust Belt (LFTB) and ruptured along an
approximately 300 km long thrust fault. There is applied a broadband frequency (0.1‐10 Hz) hybrid strong ground motion simulation technique,
combining deterministic low‐frequency and semi‐stochastic high‐frequency simulation, in order to assess the contributions of the strong ground
shaking in the resulting destruction. The simulated ground motion distributions have been calibrated by the actual strong motion records from
the Wenchuan earthquake at short distances. Comparisons with the damage distribution from reconnaissance field observations confirms the
fault rupture complexity in the resulting ground motion distributions which also control to a large extent the damage distribution. The applied
simulation methodology provides a promising platform for predictive studies.
Key words: Ground motion simulation, Wenchuan, earthquake damage.
INTRODUCTION
The large Wenchuan earthquake of 12 May 2008 (M=8.0)
in central China had disastrous consequences and caused
at least 69,195 casualties, injured 374,177 and 18,392 are
still missing (presumed dead) as of May 2009 (USGS). The
earthquake occurred along the Longmenshan Fold and
Thrust Belt (LFTB). Earthquake of this size has previously
not been associated with the LFTB, and the area was in
the seismic zonation map of China classified as intensity
VII zone (Zhao et al., 2009). The LFTB results from the
deformation of the Tibetan Plateau, which is situated
between the northward moving Indian plate (4 cm/yr)
and the stable Eurasian plate. The collision of the Indian
plate results in eastward escape of the Tibetan Plateau.
This motion is accommodated along large east‐west
oriented strike‐slip systems as the Kunlun fault in the
north and the Xianshuihe fault in the south. The LFTB
marks the eastern termination of the actively deforming
Tibetan Plateau and represents the boundary from the 65
km thick weak crust in the Tibetan Plateau to the thinner,
35 km, strong crust under the Sichuan Basin. Across the
LFTB the eastward motion of the Tibetan Plateau
decreases from 14 mm/yr on the western side of the fault
12
to 10 mm/yr on the eastern side in the Sichuan Basin.
This differential motion results in compressional stresses
of a rate of 4 mm/yr across the fault belt, and it is this
motion which reactivates the faults, as observed during
the May 12, 2008 earthquake.
In this study we have simulated the ground motion during
the Wenchuan earthquake, following the approach of
Pulido and Kubo, (2004), Pulido et al., (2004), Sørensen et
al., (2007). The low frequency ground motion simulations
are based on deterministic wave propagation from an
asperity model in a flat layered velocity structure
(Bouchon, 1981), while the high frequency ground
motions are calculated from a semi‐stochastic
simulations, also based on an asperity model. For the
high‐frequencies the stochastic methodology of Boore
(1983) is combined with the empirical Green’s function
method of Irikura (1986). In order to get a smooth
transition from the low to the high frequency domain, a
frequency‐dependent radiation pattern is applied (Pulido
and Kubo, 2004). Finally, the different contributions are
summed. All calculations are done for bedrock conditions,
and site effects are therefore not considered in this study.
Fig. 1: Input source model, modified from Ji and Hayes (2008). The black box outlines the rupturing fault
for which the ground motion simulations are computed and the red boxes outline the asperities. Asperity
1 and 2 are high‐slip asperities, while the asperities 3‐6 are intermediate‐slip asperities.