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.
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.
The central Philippine Fault Zone is found to be the locus of great earthquakes, a transition zone with slow slip and creep activity. This is based on the analysis and correlation of seismic historic data and detailed documentation of recent seismic events in the region. Based on this study the Guinyangan fault is defined to be the northern locked portion with recurrence interval of as short as 65 years. The Masbate fault is the central part with large and medium earthquakes accompanied by unusually large ground rupture. The north Central Leyte fault and the south Central Leyte fault, on the other hand, are characterized by aseismic creep and medium-sized events, usually with clusters of foreshocks, respectively. Unusual seismic activity both on the Masbate fault and Central Leyte fault somehow correlates well with the behavior of known slow events and creep activity. Further investigation of this region could lead to deeper understanding of impending major earthquakes, especially along the Guinyangan fault, which usually produces larger damaging events, and for further understanding of the impact of slow events and creep on the adjoining active structures.
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