Décollement geometry controls on shallow very low frequency earthquakes

Recent studies have documented the occurrence of shallow very low frequency earthquakes (VLFE) in subduction zones. The heterogeneity of the materials or stresses that act on the plate interface results in the variable slip rate. Stress on the décollement can be controlled by the décollement geometry and the regional stress, which is also able to control the material properties. We determined the distribution of stress along the shallow portion of the décollement in the Nankai Trough using a three-dimensional (3D) seismic survey and regional stress analysis to construct maps of normalized slip tendency ( T s ′) and dilation tendency ( T d ). Alignments of VLFEs trend parallel to the trends of T_s^^' and T_d . On the other hand, very low T_s^^' and T_d areas probably act as barriers that limit the number of VLFEs that can migrate towards the trench. Because the T_s^^' and T_d distributions are derived only from the décollement geometry and the regional stress without incorporating any data on sediment properties, the consistency between the trends suggests that the décollement geometry is the primary control on VLFE activity.