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.

An Evaluation of Strong-Motion Parameters at the S-net Ocean-Bottom Seismograph Sites Near the Kanto Basin for Earthquake Early Warning

“…To improve earthquake early warning (EEW) for offshore earthquakes, it is important to understand the characteristics of strong motions in the offshore area. It is also crucial to examine the properties of the recorded motions at the ocean-bottom sites for various seismological and engineering applications by comparing with the data recorded by seismometers installed firmly on the ground or borehole because the waveforms at the ocean-bottom sites are likely to be contaminated by various factors such as tilting of the sensors due to strong motions and poor coupling between the sensor houses and the seabed sediments (e.g., Nakamura and Hayashimoto 2019;Takagi et al, 2019).…”

Stagnant forearc mantle wedge inferred from mapping of shear-wave anisotropy using S-net seafloor seismometers

“…We use three-component 100-Hz-sampled waveforms obtained from the S-net OBSs with a natural frequency of 15 Hz. We rotate waveforms to the geographic directions (up, east, and north) based on sensor orientations 24 (see Methods). We use practically the same procedure employed by Nakajima et al…”

“…The accuracy of the azimuth of the seismometers are estimated to be 3-12°2 4 . Takagi et al 24 also found that some large earthquakes resulted in the rotation of the observation pressure vessel. The M W 6.0 off-Sanriku earthquake on August 20, 2016 and the M W 6.9 off-Fukushima earthquake on November 22, 2016, caused rotation with 1 degree or more for 1 station and 3 stations, respectively.…”

“…Since we need horizontal components to perform shear-wave splitting analysis, we rotated the waveforms to East, North, and Up (ENU) directions. The rotation matrix which is estimated from the gravity and teleseismic long-period Rayleigh waves observed by accelerometer 24 was used for the conversion from the XYZ to ENU components. The accuracy of the azimuth of the seismometers are estimated to be 3-12°2 4 .…”

“…1a). Seismic records from these instruments not only help understand seismicity and megathrust slip in the shallow subduction zone [23][24][25] , but also expand the study of mantle wedge shear-wave anisotropy into a vast virgin territory. In the following, we will document the densest systematic mapping of the shear-wave anisotropy of an offshore forearc to date, and we will demonstrate that the results indicate a lack of anisotropy in the offshore forearc mantle wedge and, in conjunction with previous findings based on onshore networks, provide a clear and simple picture of mantle wedge dynamics.…”