High-Magnitude (>Mw8.0) Megathrust Earthquakes and the Subduction of Thick Sediment, Tectonic Debris, and Smooth Sea Floor

Abstract:

INTRODUCTION: Ruff (1989, Pure and Applied Geophysics, v. 129) proposed that thick or excess sediment entering the subduction zone (SZ) smooths and strengthens the trench-parallel distribution of interplate coupling strength. This circumstance was conjectured to favor rupture continuation and the generation interplate thrusts (IPTs) of magnitude >Mw8.2. But, statistically, the correlation of excess sediment and high magnitude IPTs was deemed “less than compelling”.

NEW OBSERVATIONS: Using a larger and better vetted catalog of instrumental era (1899 through Jan. 2013) IPTs of magnitude Mw7.5 to 9.5 (n=176), and a far more accurate compilation of trench sediment thickness, we tested if, in fact, a compelling correlation exists between the occurrence of great IPTs and where thick (>1.0-1.5 km) vs thin (<1.0-0.5 km) sedimentary sections enter the SZ. Based on the new compilations, a statistically supported statement can be made that great megathrusts are most prone to nucleate at well-sedimented SZs. Despite the shorter (by 7500 km) global length of thick- (vs thin) sediment trenches, ~53% of all instrumental events of magnitude >Mw8.0, ~75% of events >Mw8.5, and 100% of IPTs >Mw9.0 occurred at thick-sediment trenches. No event >Mw9.0 ruptured at thin-sediment trenches, three super giant IPTs (1960 Chile Mw9.5, 1964 Alaska Mw9.2, and 2004 Sumatra Mw9.2) occurred at thick-sediment trenches. Significantly, however, large Mw8.0-9.0 events also commonly (n=23) nucleated at thin-sediment trenches. These IPTs are associated with the subduction of low-relief oceanic crust and where the debris of subduction erosion thickens the subduction channel separating the two plates.

INFERENCES: Our new, larger, and corrected date compilations support the conjecture by Ruff (1989) that subduction of a thick section of sediment favors rupture continuation and nucleation of high magnitude Mw8.0 to 9.5 IPTs. This observation can be linked to a causative mechanism of sediment-effected lateral smoothing of interplate coupling. It is further inferred that lateral smoothing is also effected by the subduction of low-relief seafloor that in combination with subduction erosion promotes IPT events as large as Mw9.0.