Distribuição de deslizamentos, deformação cosismica e

ENCONTRO DE PÓS-GRADUAÇÃO EM CIÊNCIAS FÍSICAS E DA TERRA DA
UNIVERSIDADE DE ÉVORA
Distribuição de deslizamentos, deformação cosismica e transferência
de tensão do sismo de Wenchuan (China, Mw7.9) de 12 de Maio de
2008
Slip distribution, coseismic deformation and Coulomb stress change
for the 12 May 2008 Wenchuan (China, Mw7.9) earthquake
Zadonina E.(1), Caldeira B.(1,2), Bezzeghoud M.(1,2), Borges J.F.(1,2)
(1) Centro de Geofísica de Évora e (2) Departamento de Física, Universidade de Évora;
[email protected]
SUMMARY
This work will present the coseismic deformation and Coulomb stress associated with the Wenchuan
(China, Mw7.9) earthquake. The results were determined from a slip distribution model obtained by
inversion of 40 well-distributed broadband waveforms. The aftershock distribution and coseismic
deformation will be interpreted on the basis of these results.
Introduction
The concept of earthquake source is not very well understood yet and remains one of the main issues for today’s
seismologists. When a large earthquake happens, its effects are recorded by many instruments such as
seismographs, GPS stations, SAR and optical satellites and strain meters. However, each of those datasets is only
sensitive to part of the phenomenon and usually yield very different results in terms of rupture history and slip
distribution.
The SISMOD/LISMOT project mainly aims at developing a computational algorithm for studying extended
seismic sources independently from their dimension and geometry. To account for the partial representativeness
of each type of data and achieve robust results, we choose to develop a joint algorithm capable of inverting GPS,
InSAR, teleseismic and strong motion data simultaneously.
The first step of the project was to collect a full dataset related to a well-studied large earthquake and test our
approach on it by computing slip distribution and co-seismic surface deformation and compare to existing data
and other authors’work. In that regard, we selected the May 12, 2008 Wenchuan earthquake (Mw7.9) to serve as
a test dataset. The Wenchuan earthquake took place at the transition between the mountainous chain of Shan and
the basin of Sichuan along the Longmen Shan Fault zone (31.1ºN, 103.3ºE; USGS). With a magnitude of 7.9 and
a depth of ~19 km the earthquake produced a 300-km-long fault rupture. It was the largest earthquake recorded
in the region during the last hundred years. It claimed more than 69,000 lives, induced widespread destruction
over the region and raised concern about seismic hazard and source characterization for the Sichuan province. As
a consequence, the event has been thoroughly surveyed and studied and is the focus of numerous projects. In the
frame of our study, we selected 40 broadband waveforms (IRIS Consortium, USA) with satisfactory quality and
azimuthal distribution.
Objectives
Test the use of various methods that joint contribute to the characterization of the finite seismic source.
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ENCONTRO DE PÓS-GRADUAÇÃO EM CIÊNCIAS FÍSICAS E DA TERRA DA
UNIVERSIDADE DE ÉVORA
Methodology
After processing the body waveforms data set, we used a finite fault model (length=300 km, strike=229º,
dip=33º, width=60 km) and the Kikuchi and Kanamori's method [1] to obtain the spatiotemporal slip distribution
model. From this slip distribution model and the coseismic deformation data,, the stress changes maps was
determineted using the Coulomb 3.0 software [2].
Results and Discussion
Our coseismic deformation model was compared with data from GPS stations located near the fault area and
results shows that directions of coseismic deformations are consistent with GPS-observations from near-fault
area but amplitudes are significantly different (Fig. 1). This could mean that inversion parameters should be
refined or that our simple fault geometry does not account for complexities.
Finally, to confirm the obtained slip area we overlaid a map of aftershocks that occurred during one month after
the main shock along with the fault map and stress changes caused by the event. In theory most aftershocks are
supposed to be located along the ruptured fault plane or along secondary faults in the area affected by stress
transfer associated with the main shock. In our case, most aftershocks are located along the main fault plane
without any noticeable clustering in the areas of increased stress.
1m
Latitude (degree)
32.5
32
31.5
*
31
30.5
102
102.5
103
103.5
104
104.5
105
105.5
Longitude (degree)
Fig.1: Horizontal coseismic deformation calculated from the slip model
(red arrows) and correspondent GPS data (blue arrows).
Acknowledgments
This work has been funded by the Fundação para a Ciência e Tecnologia (FCT) of the Ministerio da Ciência,
Tecnologia e Ensino Superior (MCTES, Potugal) through the project PTDC/CTE-GIN/82704/2006
“SISMOD/LISMOT - Modelação de Fontes Sísmicas Extensas por Inversão Conjunta de Dados Sísmicos e
Geodésicos e Movimentos sísmicos fortes na região do Vale Inferior do Tejo”, and the scholarship (BI) for
supporting the first author (ZE).
Referências
[1] Kikuchi, M., and Kanamori, H., 1982, Inversion of complex body waves: Bull. Seismol. Soc. Am., v. 72, p.
491-506.
[2] King, G. C. P., Stein, R. S. y Lin, J, 1994, Static stress changes and the triggering of earthquakes. Bull.
Seismol. Soc. Am. 84,935-953.
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