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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS013 Oral Presentation 2227
Inversion of viscous properties of crust and mantle from the GPS temporal
series measurements
Dr. Shoubiao Zhu
Geophysics Institute of Crustal Dynamics, CEA IASPEI
Yongen Cai
The viscous properties of earth media are very important in geodynamics. Taking the physical property
variation of focal media after earthquake into account, we propose the method to inverse the viscosities
in lower crust and upper mantle from GPS temporal series measurements. Combining the genetic
algorithm (GA) and Marquardt inversion, we regard the optimum model sought by GA as the initial one
to carry out inversion by Marquardt method, in which the Jacobian matrix is produced by small
perturbation. The areas of focal region are decided by aftershocks and by the velocity changes of
seismic waves before and after the main shock. In inversion, visco-elastic finite element method (VFEM)
is applied to calculate the surface displacements. The differences of the surface displacements in each
time between the computed by VFEM and the observed by GPS are regarded as object function for
optimization. The profile in which the GPS station 5936 in Taiwan, perpendicular to trace of Chelungpu
fault is chosen as our model plane. The model consists of an upper crust overlying a lower crust and
upper mantle. The parameters in the model are chosen as: the study plane is 230 km (west-east
direction) by 120 km (deep from the surface). The boundary conditions are: the nodes on west side is
fixed, the surface ground is free, the nodes on east side move westwards at constant velocity averaged
by GPS measurements, the base of the model is stationary. In VFEM model, the number of triangular 6-
node element is 3384 (6895 nodes). The rheology of the material is assumed to be linear Maxwelllian.
The shear modular, volume modular and density are chosen according to Preliminary Reference Earth
Model. Noticing the substantial changes of both VP and VP/VS anomalies around epicentre before and
after the Chi-Chi earthquake, we model the changes by assuming that the Poisson ratio of the media
around the source region varies with time.Inversion computation and digital experiments show that the
inversion method is high efficient and stable. The preliminary result suggests that viscosities of the
lower crust and the upper mantle in Taiwan is1.21018pas and 3.61019 pas, respectively.
Keywords: visco elastic finite element, frictional contact, viscosity