Subducted sediment thickness and Mw 9 earthquakes

Abstract

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 ((Formula presented.)) is greater than 1.3 km in segments where Mw ≥ 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 (Δσ) of large earthquakes (Mw ≥ 7) averaged over a subduction zone segment ((Formula presented.)) is larger in segments where Mw ≥ 9 earthquakes have occurred than in segments without such an event. It has also been shown that (Formula presented.) is linearly related to 1 − λ (λ = the pore fluid pressure ratio in the interplate megathrust). In this study, I revise the previous estimates of (Formula presented.) and λ and show that there is a positive correlation between (Formula presented.), (Formula presented.), and 1 − λ. I present a model that relates Δss to 1 − λ based on the porous flow of H2O in the subducted sediments, which gives a theoretical basis for the correlation between (Formula presented.) and (Formula presented.). The combination of these parameters thus provides a better indicator for identifying segments where Mw ≥ 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.