A new GPS‐acoustic method for measuring ocean floor crustal deformation: Application to the Nankai Trough

Abstract

We developed a new geodetic method of monitoring crustal deformation on the ocean floor. The measurements were conducted at two sites beneath the Pacific Ocean, near the Nankai Trough, where the Philippine Sea plate subducts into the Pacific plate. The first site was at a depth of 700 m in Suruga Bay, and the second one was at a depth of 2000 m in the Kumano Basin. At each survey site, three seafloor transponders, whose positions were repetitively measured, were deployed to define a benchmark unit. An ultrasonic signal was generated from a surface vessel drifting over the benchmark unit that was transmitting and receiving the signal and the replies from the transponders. Both sound speed structure and the benchmark unit positions were determined simultaneously using a tomographic technique. We repeatedly carried out measurements over the two sites. For the Kumano Basin we made six measurements from July 2004 to August 2005. The benchmark unit location's horizontal precision was 5 cm, and its vertical precision was 10 cm. For Suruga Bay we made five measurements, and the positions were less accurate, with horizontal variation of 7 cm and vertical variation of 16 cm. We investigated the reason for the decreased precision in Suruga Bay with a numerical experiment, and the results revealed that the geometry of the measurement, that is, the vessel's path, is one of the most important variables in our method. At the Kumano Basin a 21.5‐cm southward displacement of the benchmark unit was detected just below the site before and after a large earthquake (Mw 7.5). Our observation system therefore proved itself capable of detecting seafloor crustal deformation associated with crustal activities in offshore areas.

Is the Ryukyu subduction zone in Japan coupled or decoupled? —The necessity of seafloor crustal deformation observation

“…As already mentioned, the positioning accuracy is 5 cm in the horizontal component and worse, possibly twofold higher, in the vertical component (Ikuta et al, 2008). In addition, as shown in Fig.…”

“…9 and is similar to that used in previous observations (e.g. Spiess et al, 1998;Tadokoro et al, 2006;Ikuta et al, 2008). Initially, a seafloor unit is freely released from a vessel to settle on the seafloor.…”

“…The technique in determining the seafloor unit is quite similar to the hypocenter determination in seismology. All of the technical details on the observation system and the analytical methods involved are summarized in Ikuta et al (2008). Assuming that we deploy seafloor units off Okinawa Island at Point A (36 km from the trench axis) or Point B (20 km from the axis), then the horizontal displacement would be 32∼35 and 64∼70 mm/yr at Point A and B, respectively, for the coupled width of 50 km.…”

“…These velocity values are derived from the GPS data and plate motion determinations (Table 1) previously discussed. Since the error in determining the horizontal location is 5 cm (Ikuta et al, 2008), the displacement can be measurable within 1∼2 years by the current method of seafloor crustal deformation. However, in the case of a coupled width of 30 km, the displacement at Point A-13-14 mm/yr-would be too small to detect within 1∼2 years.…”

A Precise Positioning Algorithm for Long Baseline Positioning System with Uncertain Sound Speed

“…It has a wide range of applications for precise positioning of underwater objects, such as seafloor geodetic deformation research [1], offshore exploration [2] and ocean engineering [3]. According to the length of baseline, underwater acoustic positioning system (UAPS) can be classified into long baseline (LBL) system, short baseline (SBL) system and ultra-short baseline (USBL) system.…”