Estimation of the Orientations of the S‐net Cabled Ocean‐Bottom Sensors

Abstract

The Seafloor Observation Network for Earthquakes and Tsunamis along the Japan Trench (S-net) is a novel cabled ocean-bottom station network covering a broad offshore region east of northeastern Japan. To best use the S-net data, we estimated sensor orientations of all 150 S-net stations, because without this information the orientations of measurements in geodetical coordinates cannot be specified. We determined three parameters of the sensor orientation at each station: the tilt angle of the long axis of the cable, the rotation angle around the long axis, and the azimuth of the long axis. We estimated the tilt and rotation angles by using the direct current components of accelerometers recording the gravitational acceleration. The tilt and rotation angles slightly varied within the range of 0.001°–0.1° for most stations during the period from 2016 to 2018 except for coseismic steps of rotation angles greater than 1° because of the 20 August 2016 Mw 6.0 off Sanriku and 20 November 2016 Mw 6.9 off Fukushima earthquakes. The long-axis azimuths were estimated by the particle motions of long-period Rayleigh waves. We used the accelerometer records in 0.01–0.03 Hz of 7–14 teleseismic earthquakes with Mw 7.0–8.2. The azimuths were constrained with 95% confidence intervals of 3°–12°. After correcting original waveforms based on the estimated sensor orientation, we confirmed coherent waveforms within the whole S-net stations and separation of Rayleigh and Love waves in radial and transverse components. The waveforms were also coherent with those of on-land broadband stations. We provide the estimated sensor orientations and rotation matrix for conversion from the XYZ to east, north, and up components. The estimated orientation can be a fundamental resource for further seismic and geodetic explorations based on S-net data. Supplemental Content: Tables of estimated sensor orientations and of the rotation matrix for converting XYZ to east, north, and up (ENU) components, and figure showing the sensor azimuths estimated from Rayleigh and P-wave polarizations. INTRODUCTION The northeast Japan subduction zone is seismically active and is among the most studied subduction zones in the world. In this region, the Seafloor Observation Network for Earthquakes and Tsunamis along the Japan Trench (S-net) was installed by the National Research Institute for Earth Science and Disaster Resilience (NIED) following the 2011 Mw 9.0 Tohoku-oki earthquake. The deployment of the cable system begun in 2013 was completed in 2017; the data have been publicly available since October 2018 on the NIED’s webpage (see Data and Resources; National Research Institute for Earth Science and Disaster Resilience, 2019b). The new data can possibly not only greatly improve our knowledge regarding various aspects of solid earth science including subsurface structure, regular earthquakes, and slow earthquakes but also show unknown phenomena of the earth system beneath the offshore region along the Japan trench. S-net consists of 150 cabled ocean-bottom stations (Kanazawa et al., 2016; Mochizuki et al., 2016; Uehira et al., 2016; Fig. 1a). The 150 stations cover the offshore region of the northeast Japan subduction zone within 300 km of the coast and for 1000 km along the Japan and Kuril trenches. The station separation is approximately 30 km in the direction perpendicular to the trench and approximately 50–60 km along the trench. The ocean depth range of the S-net stations is 102–7830 m. Stations shallower than 1500 m are routed within grooves approximately 1 m below the seafloor to avoid fishery activities. Each S-net station consists of a geophone (velocity seismometer), strong-motion accelerometer, highand lowsensitivity accelerometer, tilt meter, and water pressure gauge for seismic, geodetic, tsunami, and acoustic observations (Kanazawa et al., 2016; Mochizuki et al., 2016; Uehira et al., 2016). The geophone and accelerometers have three orthogonal components (X, Y, and Z) in a right-handed coordinate system. Although pop-up type ocean-bottom seismometers typically have a gimbal system to maintain a vertical position, doi: 10.1785/0220190093 Seismological Research Letters Volume XX, Number XX – 2019 1 SRL Early Edition Downloaded from https://pubs.geoscienceworld.org/ssa/srl/article-pdf/doi/10.1785/0220190093/4827051/srl-2019093.1.pdf by Tohoku University user on 11 September 2019 the S-net sensors are fixed on cables for long-term stability (Fig. 2). The X component is set parallel, and the Y and Z components are set perpendicular to the cable’s long axis (Kanazawa, 2013; Aoi, 2016). Although the X axis is parallel to the cable route, the cable installation situation on the seafloor has not been confirmed. Thus, the actual sensor orientations on the seafloor are unknown. However, it is necessary to know the orientation of each S-net sensor for detailed seismic or geodetic analyses. The purpose of this study was to estimate the sensor orientations of S-net and provide information for further data analyses based on the S-net data. We define sensor orientation using three parameters: azimuth, tilt angle, and rotation angle (Fig. 2). The azimuth, tilt, and rotation are also termed the yaw, pitch, and roll angles, respectively. When all the angles are zero, XYZ axes are identical to east, north, and up (ENU) axes. All the angles are defined as anticlockwise angles around the rotation axes. 140° 142° 144° 146° 0 100 200 km 140° 142° 144° 146° 34° 36° 38° 40° 42° 44° (a) (b) ▴ Figure 1. (a) Map of S-net. The colored lines show the six cable systems (S1–S6). The black dots indicate the locations of all stations. The associated numbers are the station numbers within each cable line. The station names are composed of the cable number and the station number of the cable (e.g., N.S1N01). The white squares show the locations of the F-net stations used. The two yellow stars represent the epicenters of the two earthquakes: the 20 August 2016 Mw 6.0 off-Sanriku earthquake (northern one) and the 22 November 2016 Mw 6.9 off-Fukushima earthquake (southern one). The dashed contour shows the ocean depth at 1500 m intervals. (b) Sensor azimuths estimated by this study. The colored wiggles show the 95% confidence intervals of the estimated azimuth. The black bars indicate the azimuths of the cable route data. 2 Seismological Research Letters Volume XX, Number XX – 2019 SRL Early Edition Downloaded from https://pubs.geoscienceworld.org/ssa/srl/article-pdf/doi/10.1785/0220190093/4827051/srl-2019093.1.pdf by Tohoku University user on 11 September 2019 The azimuth is the horizontal angle of the X axis (cable’s long axis) from east. The tilt angle is the dip angle of the X axis. The rotation angle is the angle around the X axis. The rotation angle is zero when the Y axis is in the horizontal plane. Here, we first estimated the tilt and rotation angles from the direct current (DC) components of the accelerometer. Then, we estimated the azimuth of the X axis using the polarization of long-period teleseismic Rayleigh waves. Finally, we compared the corrected seismograms of S-net to those of the on-land broadband seismograms of the Full Range Seismograph Network of Japan (NIED F-net; see Data and Resources; National Research Institute for Earth Science and Disaster Resilience, 2019a).