Stress heterogeneity on shallow decollement controls Very Low Frequency Earthquakes in Nankai Trough off Kii Peninsula

Variable slips from slow to fast have been observed in subduction plate interfaces. Recently, slow slips and tremor, and very low frequency earthquakes (VLFEs) are observed in shallow portion of subduction plate interface in Nankai Trough. Such variation of slips can be controlled by a rheological heterogeneity, and block in matrix texture in mélange is one candidate to make the heterogeneity. Another possibility is a stress heterogeneity due to the roughness of plate interface although the effect of stress heterogeneity on seismic slips has not been well-defined so far. This study investigates stress heterogeneity on the shallow decollement in Nankai Trough to see the relation to the VLFEs' occurrences.

We used 3D seismic profiles acquired for IODP NanTroSEIZE in the Nankai Trough off Kii peninsula, and produced a 3D surface of the shallow decollement. Dip angle and dip azimuth for each 50 m X 50 m patch of the 3D surface were obtained. With focal mechanisms of the VLFEs in the region, regional stress orientations and stress ratio were estimated. Normal and shear stresses normalized by differential stress on each patch were calculated by the relationship between patch orientations and the regional stress. Slip tendency (Ts) and dilation tendency (Td) were also calculated.

The decollement dips in the convergent direction with low angle 0-5 degree in average. Patches with slightly larger dip (10-25 degree) are identified, distributing in NE-SW direction. The regional stress estimated shows relatively horizontal NW sigma1 and relatively vertical sigma3 with 0.65 of stress ratio. Ts ranges 0.4-0.6 and high or low Ts areas distribute in NE-SW directions repeatedly.Td ranges 0.3-0.7 and the distribution of Td shows a good agreement with that of Ts.

Slips tend to concentrate in the area with high Ts. High Td area corresponds to that with high Ts, which implies that the slip zone could be a fluid conduit. Oppositely, low Ts area has low Td where the fault could be a barrier of fluid. The NE-SW directions of the distribution of high Ts and Td areas shows a good agreement with the direction of VLFEs' elongated distributions. This suggests that the Ts and Td, determined by the regional stress and the geomorphic features on decollement, may have close relation to VLFEs' occurrences.