A strong Mw7.0 earthquake struck Pingtung offshore of Taiwan on December 26,2006.It consisted of two major events with an 8-minute interval.The first major shock occurred at 12:26 UTC.Focal mechanism results from Harvard,USGS,and BATS all indicated that the first major shock was a normal fault earthquake and the second one was dominated by strike-slip offsets.The location of the epicenter varied greatly in depth in different analyses.The latest results showed that the focal depth of the first shock was most probably around 40-44 km,placing the epicenter in the lithospheric mantle.However,this is not a location where earthquakes usually occur.To explore the geodynamical mechanism of this event,we carried out 2D finite element method(FEM) numerical experiments.Our primary results indicate that the geodynamical background,as well as the formation of Pingtung earthquake,is a consequence of the collision between Luzon arc and Chinese continental margin.Although Taiwan Island is in the shadow of NW-SE trending compressive collision zone,the existence of ductile lower crust leads to the decoupling between upper crust and lithospheric mantle.As lithospheric mantle subducts to the depth of around 250 km,the upper part of the bending subduction slab puts itself in an extensional state.The extensional stress from bending induced the occurrence of this normal fault earthquake at the critical point.
The viscosity of lower crust of Qinghai-Tibet Plateau on earth should be determined. It has become a predominant problem in quantitative research on geodynamics. Its order of magnitude will have a great influence on the results of quantitative modeling. To obtain the viscosity of lower crust of Qinghai-Tibet Plateau, this parameter was calculated by three methods. The first is based on the estimation on the temperature state of Qinghai-Tibet Plateau in the deep part, and the viscosity of lower crust of northern Plateau was recomputed with strain rate derived from rheology law and GPS observation. Effective viscosity of middle crust in Kunlun region is between 1020 and 1022 Pa·s, and that of lower crust is be- tween 1019 and 1021 Pa·s; the second is based on three kinds of rheological models used to fit the post-seismic deformation recorded by cross-over fault GPS sites set after Ms8.1 Kunlun earthquake in 2001. The viscosity of lower crust obtained by this method is of 1017 Pa·s order of magnitude. However, higher viscosity is required to fit the data of south fault better, and the lower one is required to fit the data of north fault better. The viscosity of lower crust, which was obtained by fitting the cross-over fault post-seismic deformation after Ms7.6 Luhuo earthquake in 1973, is of 1019 Pa·s order of magnitude. Non-linear relationship between effective viscosity and strain rate is ignored in the former research of effective viscosity. This research shows the difference of effective viscosity obtained from laboratory experiment, and shorter and longer time post-seismic deformation after large earthquakes can be explained in phase.
