Sensing of upslope passages of frontal bores across the trench slope break of the Japan Trench

Abstract

AbstractThe circum‐Pacific trench slope system is one of the most spectacular topographic features of the Earth. We report the relatively frequent occurrence of upslope passages (∼50 times a year) of internal bores in the Japan Trench system. Observations were made using a triaxial accelerometer, a tiltmeter, and a current meter at the seafloor, ∼3400 m deep, during 10 months from 2013 to 2014. We detected 42 tilt events from the records of the accelerometer and confirmed their consistency with the tiltmeter records. The tilt occurred always landward by 1–10 µrad with a rise time on the order of 1000 s and return to the original with a much longer recovery time. The current and temperature records available for the first two events indicated that landward tilting was associated with an upslope current with a speed of several cm/s and a temperature drop of several tens of m °C. The temperature remained cold even after the current had diminished. All of these observations implied the frontal passages of upslope advancing bores, which caused the instrument to be tilted landward. The observational site was located at the trench slope break, separating the trench slope (sloping supercritically or near critically for the M2 internal tide) from the fore‐arc basin (sloping subcritically). This unique locality suggested the internal tide origin of the observed bores, although other possibilities could not be precluded. Bores generated on the trench slope may play a role for upslope transportation of suspended sediments stirred up by deep submarine landslides.