Deformation and stress change associated with plate interaction at subduction zones: a kinematic modelling

The interseismic deformation associated with plate coupling at a subduction zone is commonly simulated by the steady-slip model in which a reverse dip -slip is imposed on the down-dip extension of the locked plate interface, o r by the backslip model in which a normal slip is imposed on the locked pla te interface. It is found that these two models, although totally different in principle, produce similar patterns for the vertical deformation at a s ubduction zone. This suggests that it is almost impossible to distinguish b etween these two models by analysing only the interseismic vertical deforma tion observed at a subduction zone. The steady-slip model cannot correctly predict the horizontal deformation a ssociated with plate coupling at a subduction zone, a fact that is proved b y both the numerical modelling in this study and the GPS (Global Positionin g System) observations near the Nankai trough, southwest Japan. It is there fore inadequate to simulate the effect of the plate coupling at a subductio n zone by the steady-slip model. It is also revealed that the unphysical as sumption inherent in the backslip model of imposing a normal slip on the lo cked plate interface makes it impossible to predict correctly the horizonta l motion of the subducted plate and the stress change within the overthrust zone associated with the plate coupling during interseismic stages. If the analysis made in this work is proved to be correct, some of the previous s tudies on interpreting the interseismic deformation observed at several sub duction zones based on these two models might need substantial revision. On the basis of the investigations on plate interaction at subduction zones made using the finite element method and the kinematic/mechanical conditio ns of the plate coupling implied by the present plate tectonics, a synthesi zed model is proposed to simulate the kinematic effect of the plate interac tion during interseismic stages. A numerical analysis shows that the propos ed model, designed to simulate the motion of a subducted slab, can correctl y produce the deformation and the main pattern of stress concentration asso ciated with plate coupling at a subduction zone. The validity of the synthe sized model is examined and partially verified by analysing the horizontal deformation observed by GPS near the Nankai trough, southwest Japan.