Observations of Repeated Seafloor Tilt Events atop the Inward Steep Slope of the Japan Trench Using New High-Resolution Accelerometers

Abstract:

Continuous observations of seismic events and seafloor movements were made from May 2013 to March 2014 atop the steeply rising morphological unit (often called the lower trench slope) located about 50 km landward of the trench axis and 50 km seaward of the epicenter of the great 2011 Tohoku-Oki earthquake, where the seafloor is locally dipping to the southeast at 14-15° .

Measurements were made with a new Acceleration Ocean Bottom Seismometer (AOBS) system using a triaxial array of nano-resolution accelerometers having ranges of 20 m/sec², parts-per-billion sensitivity and good long-term stability. Observations were compared to a nearby system that included a broadband seismometer, differential pressure gauge and ocean bottom tiltmeter.

The AOBS measures seafloor tilting as a change of acceleration in the absolute gravity field and the records show repeated occurrences of approximately unidirectional tilting either to the northwest or southeast. Rapid (tens of second) tilting was always triggered by strong seismic shaking, including a tilt of 0.0016 radians triggered by an Mw7.1 earthquake 145 km distant from the station. Slow (hours) tilting occurs with much smaller amplitude but in a similar dip direction and is most often recovered by opposite tilting at a slower rate. Records of the nearby tiltmeter in their unsaturated cases showed consistent tilt motions.

Traditional broadband seismometers and tiltmeters do not have the range to measure strong seismic events and traditional strong motion sensors do not have the sensitivity or stability to make good long-term geodetic measurements. The AOBS can measure both the statically invariant 1g Earth’s gravity vector and the higher frequency components associated with strong seismic shaking. This invariance, in spite of large offsets in each component axis due to tilts, indicates that the observed offsets are real signals of trench slope-related processes. These observations demonstrate the usefulness of AOBS for high-resolution measurements of submarine earthquakes, tilts, slow-slip and perhaps even longer-term events.