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

Wednesday, 17 December 2014
David W Scholl1,2, Stephen H Kirby3, Roland von Huene2, Holly F Ryan4 and Ray E Wells5, (1)University of Alaska Fairbanks, Fairbanks, AK, United States, (2)U S Geological Survey, Menlo Park, CA, United States, (3)U. S. Gelogical Survey, Menlo Park, CA, United States, (4)USGS, Menlo Park, CA, United States, (5)USGS California Water Science Center Menlo Park, Menlo Park, CA, United States
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.