Seafloor Crustal Deformation Research Articles

Interseismic seafloor crustal deformation immediately above the source region of anticipated megathrust earthquake along the Nankai Trough, Japan

We monitored seafloor crustal deformation using the GPS/acoustic seafloor geodetic observation technique at three sites on the Kumano Basin, off southwestern Japan, which is located immediately above the source region of the anticipated Tonankai megathrust earthquake. We directly measured landward crustal movements of ∼40 mm/y in the ∼N80°W direction with respect to the Amurian Plate on the seafloor. The directions were found to be the same as those measured at the on‐land GPS stations. The magnitudes of the velocity vectors indicated significant crustal shortenings of approximately 10–20 mm/y between the Kumano Basin and the southeastern coast of the Kii peninsula of the Japanese Islands. The present observational results show strong and direct evidence for interplate locking during the interseismic period. Coupling ratios were roughly estimated at ∼0.6–0.8 on the plate interface up to at least 10 km in depth.

Accuracy assessment of GPS/Acoustic positioning using a Seafloor Acoustic Transponder System

The accuracy of GPS/Acoustic positioning is crucial for monitoring seafloor crustal deformation. However, the slant range residual is currently the only indicator used to evaluate the precision of positioning seafloor transponders. This study employs a unique Seafloor Acoustic Transponder System (SATS) to evaluate the accuracy of GPS/Acoustic seafloor positioning. The SATS has three transponders and an attitude sensor in a single unit, which provides true lengths of transponder baselines and true attitude of the SATS to ensure assessment reliability and validity. The proposed approach was tested through a GPS/Acoustic experiment, in which an off-the-shelf acoustic system was used to collect range measurements. Using GPS/Acoustic geodetic observations, the positions of three transponders on the SATS were estimated by an optimization technique combined with ray-tracing calculations. The accuracy of the GPS/Acoustic seafloor positioning is assessed by comparing the true baselines and attitude with the results derived from the position estimates of the three transponders. A sensitivity analysis is conducted to investigate the robustness of the GPS/Acoustic positioning results to changes of sound speed. Experimental results demonstrate that the use of the SATS can help to assess the validity of the GPS and acoustic travel time measurements in the GPS/Acoustic seafloor positioning.

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

Abstract The 2004 Sumatra-Andaman earthquake of M w 9.3 occurred in a region where a giant earthquake seemed unlikely from the point of view of tectonics. This clearly implies that our current understanding of strain accumulation processes of large earthquakes at subduction zones needs to be reexamined. The Ryukyu subduction zone is one such zone since no large earthquake has been anticipated there for reasons similar those pertaining to the Sumatra-Andaman arc. Based on our analysis of historical earthquakes, plate motion, back-arc spreading, and GPS observation along the Ryukyu trench, we highly recommend monitoring seafloor crustal deformation along this trench to clarify whether a large earthquake (M w>8) could potentially occur there in the future.

Estimation of Interplate Coupling in the Kumano Basin along the Nankai Trough Based on Observation of Seafloor Crustal Deformation:The Current Status and Foresight

Estimation of Interplate Coupling in the Kumano Basin along the Nankai Trough Based on Observation of Seafloor Crustal Deformation:The Current Status and Foresight

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

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.

Repeated Observation of Seafloor Crustal Deformation along the Suruga-Nankai Trough, Japan

Repeated Observation of Seafloor Crustal Deformation along the Suruga-Nankai Trough, Japan

Observation of coseismic seafloor crustal deformation due to M7 class offshore earthquakes

We have developed an observation system for seafloor crustal deformation measurement combining the two following techniques: the precise acoustic ranging and kinematic GPS (Global Positioning System) positioning. On July 14, 2003, we installed the units for this observation system at the seafloor of the Kumano Basin, Japan (33.58°N, 136.60°E). We performed repeated observation six times on 1) July 15–16 and 21, 2004, 2) November 9–10, 2004, 3) May 18 and 20, 2005, 4) July 19–20, 2005, 5) August 18–19, 2005, and 6) August 29–30, 2005. The present observation system can stably determine the seafloor position with an RMS repeatability of ±3 cm in both horizontal components. Two M7 class offshore intraplate earthquakes occurred only 60 km from the Kumano Basin site on September 5, 2004. We observed the coseismic horizontal displacement of 18 cm at the Kumano Basin site using the presently developed seafloor observation system. Our observation system can provide seafloor crustal deformation associated with crustal activities in offshore areas.

Error evaluation in acoustic positioning of a single transponder for seafloor crustal deformation measurements

The observation of seafloor crustal deformation is very important to understand plate motions, nucleation processes and mechanisms of great interplate earthquakes as well as the activities of submarine volcanoes. We have been developing an observation system for seafloor crustal deformation. This system consists of two main components; (1) kinematic GPS positioning of an observation vessel and (2) accurate acoustic measurements of distances between a transponder attached on the side of the vessel (onboard station) and one located on the ocean bottom (seafloor station). In this study, we performed numerical simulations to estimate measurement errors with acoustic positioning assuming acoustic velocities in the sea water and the distribution of observation points around the single seafloor station. We found that the position of the seafloor station which we can obtain by analyzing travel-time data might have around 18-cm discrepancy with respect to its “true” position. Colombo et al. (2001) reported that the position of the vessel can be determined with about 10-cm error by kinematic GPS positioning. These results indicate that the system should be able to detect seafloor crustal deformation much larger than 28 cm, including pre-, co-, and post-seismic slips due to the large earthquakes at subduction zones, slow and silent earthquakes, etc. Therefore, we emphasize the importance of continuous observations with a nationwide geodetic observational network for seafloor crustal deformation.

Centimeter-level positioning on the seafloor

This paper reports the activity in the development of the observation system of seafloor crustal deformation for understanding mechanism of interplate earthquakes at subduction plate boundaries around Japan. Currently, the method for the seafloor geodesy is under development in order to overcome several difficulties. The position of the sea-bottom transponders at the vicinity of the Nankai trough and the Japan Trench is observed using a GPS/Acoustic measurement system. Early results from processed data show a scatter of 4cm standard deviation for repeated horizontal positioning from drifting survey vessel. This paper describes two techniques developed to realize precise acoustic ranging, automatically detecting and correcting the Doppler-shift of acoustic propagation signal due to ship's drifting and attitude motion, and theoretically forming a pulse of one wavelength in cross correlation between transmission and reception of long-coded signals.

次の南海トラフ巨大地震に備えて 熊野トラフにおける長期地殻変動観測技術の高度化

With a view toward understanding the mechanism of interplate earthquakes at subduction plate boundaries around Japan, remarkable progress was made in the long-term observation system of seafloor crustal deformation. The method for the seafloor geodesy has been exploited in order to overcome several difficulties. The position of the sea-bottom transponders on the Kumano Trough was repeatedly observed using a GPS/Acoustic measurement system in May and August 2000. Early results from processed data show about 2 cm differences between May and August with a scatter of 4 to 10 cm in horizontal positioning. This paper describes techniques developed to realize precise acoustic ranging, automatically detecting and correcting the Doppler shift of acoustic propagation signal due to ship's drifting and attitude motion, and theoretically forming a pulse of one wavelength in cross correlation between transmission and reception of long-coded signals. Furthermore, by using GPS surface positioning, acoustic ranging and sound speed profiling data, the temporal changes of sound speed profile were estimated with sufficient accuracy for acoustic positioning.

Seafloor positioning system with GPS-acoustic link for crustal dynamics observation—a preliminary result from experiments in the sea—

Many active plate boundaries, such as mid-ocean ridges and trenches, are under the sea. Seafloor crustal deformation data will contribute significantly to understanding the nature of the tectonic processes at these plate boundaries. We have developed a seafloor positioning system with a GPS-acoustic link. This system consists of two main components; (1) the surface positioning by differential GPS to on-land reference and (2) the precise acoustic ranging using the M-sequence between the surface and seafloor references. The position and attitude of the surface GPS-acoustic link unit are determined from the GPS observations. Simultaneously, the acoustic ranging between the surface unit and seafloor references are carried out. The positions of the seafloor references are determined from these observations and a sound-speed structure model of the seawater. We performed preliminary seafloor positioning experiments. In these experiments, simple 1-D structure models are assumed for the sound-speed in the sea. The results show that the positions of the seafloor references are estimated with an accuracy on order of 10 cm. The residuals for acoustic ranging imply that there are systematic differences between the assumed and real sound-speed structure. It is necessary to reduce the uncertainties of sound-speed structures for more accurate positioning.