Roughness analysis of the Nankai Trough subducting plate boundary and comparison with stress distribution

The roughness of a large seismic fault can control a slip of an earthquake in a subduction zone because it results in a heterogenetic distribution of stresses.

Previous studies have suggested that both topographical roughness and physical properties may control seismic activity, but nothing is specified about the causal relationship between the two.

Therefore, we examined a heterogeneous distribution of topography in nature hierarchically by roughness analysis, targeting on the Nankai shallow decollement as an actual subduction plate boundary using 3D seismic survey data. Finally, comparing the roughness map with the stress distribution, their interrelationship was examined.

A roughness map was obtained with variated segment lengths. For small segment lengths, small patches were identified in places, while for large segment lengths, high amplitude regions were distributed over a relatively wide area. The result can suggest that a hierarchical distribution of high amplitude regions was caused by the topography according to the variations in segment length.

To examine the relationship between the distribution of roughness and stress, we used the distribution of slip tendency (Ts) and dilation tendency (Td) in the same decollement (Hashimoto et al., 2022).

Comparing the Ts distribution with the decollement topography, it was confirmed that the region with high or low Ts distribution captured the inflection point of the topography.

Comparing the Ts distribution with the roughness map, the distribution tended to be similar. It is considered that the roughness map of each segment length has a hierarchy due to the difference in the location where the inflection point of the topography was caught. These are also the case in comparison with the Td distribution. Therefore, it is possible that the stress state is greatly affected by the inflection point of the decollement topography. It is also suggested that the inflection point of the topography is important both for roughness and for stress. An examination about the effect of the inflection of the topography on the physical properties using velocity distributions from seismic profiles are needed in the future.