Some characteristics of bottom pressure sensors of DONET

DONET, i.e., the dense ocean-floor network system for earthquakes and tsunamis has started its operation in the Nankai Trough, SW Japan in the early of 2010, and followed by the extension to the westward region as DONET2 in near future. DONET observatory is composed of various sensors such as broadband seismometer, seismic accelerometer, tsunami meter, etc. The present study focuses on bottom pressure sensors being used as tsunami meters measuring hydraulic pressure change. Pressure sensors specify their performance of 0.005 % full scale regarding both hysteresis and repeatability. In the present study, pressure sensors' characteristics have been evaluated by using the JAMSTEC's high accuracy pressure standard before deploying into the deep-sea. The high accuracy pressure standard is mainly customized by a piston gauge, a pressure generator/controller, a reference pressure monitor, and a thermal controlled chamber, which virtually reproduces the deep-sea environment. Firstly, we have confirmed hysteresis and repeatability by applying barometric pressure to the full scale pressure range, i.e., ca. 0 MPa to 68 MPa to the sensors. It has been reproduced that all of pressure sensors perform their specified hysteresis and repeatability. Then, long-term sensors' stabilities have been evaluated by applying static 20 MPa hydraulic pressure, which is equivalent to 2,000 meters water depth under the constant temperature of 2 °C for the duration of approximately one month. As a result, sensors' drift varies among pressure sensors, however, which implies that the gradual drift possibly occurs a few centimeters per month in maximum. We also discuss the sensors' drift after deploying the deep-sea by analyzing the long-term in-situ observations, which shows that it continues at a rate of a few to ten centimeters per year in the early stage, then it tends to be reduced gradually. This paper also reports that the tsunami from the Tohoku earthquake on 11 March 2011 could be observed by DONET with those peak amplitudes of ~20 cm.