Offshore Postseismic Deformation of the 2011 Tohoku Earthquake Revisited: Application of an Improved GPS‐Acoustic Positioning Method Considering Horizontal Gradient of Sound Speed Structure

Abstract

One of the important issues on the GPS‐acoustic (GPS‐A) observation for sea bottom positioning is how to address the horizontal heterogeneity of the sound speed in oceans. This study presents an analysis method of GPS‐A data in the presence of a sloping sound speed structure. By applying this method and revising the analysis scheme to make full use of existing data, we reevaluated the horizontal postseismic deformations occurring ~1.5–5 years after the 2011 Tohoku earthquake. The revised horizontal movements have more uniform directions and rates between neighboring sites, suggesting enhancement of the array positioning accuracy. The revised displacement rate of the site on the incoming Pacific plate, located ~100 km northeast of the main rupture zone, was decreased significantly; it was only slightly, by 1.4 cm/year larger than the global motion of the Pacific plate, suggesting a relatively small effect of viscoelastic relaxation. The horizontal movements of the near‐trench sites above the main rupture zone were generally landward and were significantly faster than the Pacific plate motion, indicating a viscoelastic relaxation of 5–10 cm/year. The distribution of the fast landward movements peaked near 38°N at an updip of the mainshock hypocenter and extended significantly farther to the north than to the south. This implies the existence of a secondary coseismic slip patch in the northern area in addition to a primary slip patch at ~38°N. The occurrence of episodic slow slip in early 2015 to the north of the main rupture zone was also verified from the GPS‐A analyses.

Shallow slow slip events along the Nankai Trough detected by GNSS-A

“…S1). The GNSS-A seafloor geodetic observation greatly constrained the interplate coupling condition (16,17) along the Nankai Trough subduction zone and detected, in detail, postseismic fields along the Japan Trench (18,19). Recently, we improved GNSS-A technology and upgraded the observation sensitivity (20) to detect a transient crustal deformation caused by large interplate slip.…”

GARPOS: analysis software for the GNSS-A seafloor positioning with simultaneous estimation of sound speed structure

“…Meanwhile, Honsho et al (2019) showed a more general expression for one-dimensional sound speed gradient. As they mentioned, the gradient terms in their formulation correspond to the extracted features in .…”

“…As they mentioned, the gradient terms in their formulation correspond to the extracted features in . The work by Honsho et al (2019) showed the possibility to connect all the GNSS-A configurations into a unified GNSS-A solver. However, due to the limitation in resolving the general gradient structure, an additional constraint was taken for the practical application, which concludes to essentially the same formulation as Yasuda et al (2017).…”

Development of a kinematic GNSS-Acoustic positioning method based on a state-space model

“…The GNSS-A observations enable measurement of seafloor displacements in a global geodetic coordinate system. The system has provided important geodetic observation results including detection of interseismic (e.g., Gagnon et al 2005;Tadokoro et al 2012;Yokota et al 2016), coseismic (e.g., Tadokoro et al 2006;Sato et al 2011;Kido et al 2011), and postseismic (e.g., Watanabe et al 2014;Tomita et al 2015Tomita et al , 2017Honsho et al 2019) deformation associated with earthquake cycles in subduction zones, and plate motions near ridgetransform boundaries (Chadwell and Spiess 2008).…”

“…1 Schematic image of the GNSS-Acoustic observation system. Schematic image of a GNSS-A observation system is shown; inclination angle ( ǫ ), shot angle ( ξ ), and azimuth for the seafloor transponder ( φ ) are used in equations in the main text GNSS-A positioning methods that consider the horizontally graded structure of underwater sound speeds that are persistent over the long-term (over several hours) have recently been developed (e.g., Yasuda et al 2017;Yokota et al 2018;Honsho et al 2019), and have demonstrated some reduction in biased positioning errors. However, it has been found that short-term fluctuations in the horizontal heterogeneity of the SSS can be caused by internal gravitational waves (e.g., Spiess et al 1998;Kido et al 2006;Tomita et al 2015), and these short-term fluctuations have degraded the precision of the kinematic GNSS-A positioning method.…”