In this paper, the influence of shear deformation happened in the process of the oceanic plate subducting beneath into the continental plate, and the dynamic force from earthquake on the formation of the décollement zone, a special horizontal plate boundary fault, is investigated with a specific numerical tests. The numerical tests are conducted with a static/dynamic soil-water coupling finite element-finite difference method (FE-FD) based on a sophisticated constitutive model in finite-deformation scheme. Particular attention is paid to the change of volumetric strain of the geomaterials in the proto-décollement zone that would become the décollement zone during the plate collision process. It is found that, a significant compressive volumetric strain would happen if the seabed rock has subjected to periodic earthquake loadings during twenty thousand years with a re-occurring period of every two hundred years. Amazingly, in spite of the fact that more than 20% of the volumetric strain happened, the structure of the material within the proto-décollement zone remains undisturbed, which is quite similar to the mechanical behavior of the décollement zone observed in the field. In other words, the cyclic loading might be one of the main reasons to form a fault zone (e.g., the décollement zone) with a much-higher density but keeping the random fabric structure intact.