19 Current Research in Coastal and Marine Processes (Posters)

Sunday, 17 March 2019
1:30 PM - 5:30 PM
Casco Bay Hall

THE SEDIMENTARY AND ISOTOPIC SIGNATURE OF THE 2011 TOHOKU EARTHQUAKE AND TSUNAMI AND PRIOR EARTHQUAKES, JAPAN TRENCH

Paper number: 19-11
Program: Theme Sessions
Time: 1:30 PM – 5:30 PM

Authors

Abstract

The subduction of the Pacific Plate under the Okhotsk microplate has resulted in numerous, high magnitude earthquakes and tsunami. The 2011 Tohoku earthquake (Mw9.0) and subsequent tsunami were record breaking geologic catastrophes for Japan in part because the Japan Trench, where subduction is taking place, is only 200 km away from the coast and because a Mw9.0 earthquake had not occurred for almost 1 ky and was not expected in that region at that time. Our understanding of these long-recurrence earthquakes is limited to the instrumental and historical records. We are developing methods to document paleoearthquakes as part of the emerging field of submarine paeloseismology in order to better understand the magnitude range of possible earthquakes, their recurrence intervals, and fault segmentation. The data were obtained by surveying the seafloor with high-resolution sub-bottom profiles, multibeam bathymetry and piston cores.

During the 2013 R/V Natsushima and 2016 R/V Sonne expeditions to the Japan Trench Margin, we collected in 850-8000 m of water depth, thirty-five up to 10 m long piston cores from the upper forearc slope, a prominent quasi-continuous terrace on the forearc slope and by the Trench. We discovered thick sediment layers that were suspended and remobilized downslope by gravity during the 2011 Tohoku event and two prior earthquakes. The sediment samples were analyzed for short-lived radioisotopes, grain size distribution, core x-ray radiographs and elemental ratios. We measured their excess (xs) 210Pb enrichment and the 137Cs and 134Cs radioisotope concentrations. These later radioisotopes were emitted during the Fukushima nuclear reactor accident making them excellent tracers of the 2011 earthquake and tsunami. The Tohoku remobilized sediment extends spatially for ~260 km along the strike of the Japan Trench and for ~150 km in the trench. We identified three main facies and several subfacies associated with the 2011 Tohoku and two prior events in the mid- and early 1900s. These events deposits can be tracked synchronously over tens of kilometers. Synchronous depostion over large distances can be used as evidence of a large megathrust rupture. These findings will help to better recognize megaquakes in the stratigraphic record and advance the field of submarine paleoseimology.

Geological Society of America Abstracts with Programs. Vol. 51, No. 1

doi: 10.1130/abs/2019NE-327862

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