Long-term Observation of Seafloor Disturbances by Array of Pressure Gauges

Abstract:

We developed a seafloor array system of pressure gauges to record disturbances on both the oceanic and solid-Earth sides. The array consists of 10 high-resolution pressure gauges (PARO-8B7000-I-005) in a regular triangle configuration with site intervals of 10 km. The targeted disturbances on the oceanic side include infragravity waves, tsunamis and low-mode internal tides. Those on the solid-Earth side include P-wave families and Rayleigh waves, in particular, the Airy phase of suboceanic Rayleigh waves at periods around 10 s. We confirmed using data from the Nankai Trough seafloor network of pressure gauges (DONET) that our system should enable us to retrieve accurately the first-mode internal tides with the signal power four orders of magnitude smaller than the corresponding semidiurnal surface tides. We installed this system around (32.4N, 140.3E, 1500-2200m depths) on the upper slope of the Izu-Bonin Trench in May 2014. The system was recovered in May-June 2015 and reinstalled around (31.2N, 141.7E, 4700-5700m depths) on the lower slope of this trench in a hope to recover in June 2016 (see Figure). The already recovered data contains records of the tsunami earthquake of May 02, 2015, at epicentral distances around 90 km. Its seismic magnitude was only 5.7, yet the tsunami height was 0.5 m at the 170km-distant island. Our pressure records show P-wave families followed by dispersive tsunami wave trains with amplitudes about 200 Pa successively passing through the array. Although P-wave families of comparable amplitudes were recorded by the M5.6 near-trench low-angle thrust earthquake of May 10, 2015, at epicentral distances around 160 km, they were not followed by any visible tsunami signals. These observations imply merits of sea-bottom pressure array, including easiness of comparing amplitudes of seismic waves and tsunamis on the same record and capability of tracing two-dimensional tsunami propagation through the array as a function of period.