Subduction Zones Launch the Highest Magnitude Earthquakes Where Thick, Laterally Continuous Masses of Trench Sediment Enter the Subduction Channel

Ruff (1989, Pure and Applied Geophysics, v. 129) found that the insertion of a thick section of trench-floor sediment into the subduction channel separating the upper and lower plates of subduction zones would favor the formation of a "homogeneous and strong contact zone". A smooth, lateral distribution of coupling would in turn set up conditions for the nucleation of large megathrust earthquakes (eqs). The observation that ~50 % of all recorded eqs of Mw = to/> 8.2 occurred at sedimented trenches established a probable link with high magnitude. Rea and Ruff (1996, EPSL, v. 140) found no relation between sediment flux (through put) and large magnitude eqs.

We reexamined the observation that large eqs are associated with excess sediment subduction. Improved databases of sediment thickness and its lateral continuity, and occurrence of megathrust earthquakes support Ruff's conjecture. We observe the occurrence at sediment-dominated SZs of:

48 % of all eqs = to/>Mw 7.5 (36 of 75)

43 % of all eqs = to/>Mw 8.0 (18 of 42)

45 % of all eqs = to/>Mw 8.2 (13 of 29)

63 % of all eqs = to/>Mw 8.5 (10 of 16)

67 % of all eqs = to/>Mw 9.0 (4 of 6)

100 % of all eqs = to/>Mw 9.1 (3 of 3).

Key to this compilation is a matching of earthquake magnitude to trench fills of >1-1.5 km and along-trench continuity of >200 km. For all eqs great than Mw = 9, the lateral continuity is >750-1000 km. The insertion of a thick, laterally expansive sediment mass into the subduction channel evidently favors rupture prolongation and generation of large megathrust eqs. It is also clear that, except for the largest earthquakes, factors other than sediment ingestion may serve to localize events of Mw = 7.5 to 9.