Subducted sediment thickness and M w 9 earthquakes

I measure the thickness of subducted sediment (Δss) beneath the décollement in the fore‐arc wedge and show that the average value of Δss over a subduction zone segment ( Δ s s ¯ ) is greater than 1.3 km in segments where M w  ≥ 9 earthquakes have occurred and less than 1.2 km in segments without such large earthquakes. In a previous study, I showed that the stress drop (Δ σ ) of large earthquakes ( M w  ≥ 7) averaged over a subduction zone segment ( Δ σ ¯ ) is larger in segments where M w  ≥ 9 earthquakes have occurred than in segments without such an event. It has also been shown thatΔ σ ¯ is linearly related to 1 −  λ ( λ  = the pore fluid pressure ratio in the interplate megathrust). In this study, I revise the previous estimates ofΔ σ ¯ and λ and show that there is a positive correlation betweenΔ s s ¯ ,Δ σ ¯ , and 1 −  λ . I present a model that relates Δss to 1 −  λ based on the porous flow of H 2 O in the subducted sediments, which gives a theoretical basis for the correlation betweenΔ s s ¯ andΔ σ ¯ . The combination of these parameters thus provides a better indicator for identifying segments where M w  ≥ 9 earthquakes may occur. Based on this, I propose that the tectonic environments where such huge events are likely to occur are (1) near collision zones, (2) near subduction of spreading centers, and (3) erosive margins with compressional fore arcs. Near the Japanese islands, SE Hokkaido is prone to such an event, but the Nankai Trough is not.