Abstract In 2004, we started monitoring crustal deformation at Kumano-nada in the Nankai trough using the GPS/Acoustic technique. We observed a large southward seafloor displacement of ∼30 cm associated with the off Kii Peninsula earthquake, which occurred in September 2004, between our two survey campaigns in August and November 2004. The observed seafloor displacement is larger than that predicted from a slip model derived solely from GPS measurements on land. This may indicate the earthquake fault is slightly shallower and extends move to the NW than previously estimated.
A GPS/Acoustic experiment on the southeastern slope of Hawaii Island presented precise seafloor positioning in the condition of large water depth (2.5—4.5 km) and large velocity variations. We estimated sound velocity variations from acoustic ranging, and found that temperature variation can well explain the velocity variation. The effect of daily variation in the sound velocity amounted to +/- 0.7 m on acoustic ranging of 4—7 km with a fixed velocity structure. CTD data observed about every 3 hours could decrease the range residuals to +/- 0.4 m. These large residuals were fairly well canceled in the positioning of the array center of three acoustic transponders. The estimated precision of the array center positioning was about 3 cm in latitude and longitude.
We have developed systems for measuring differential displacements across a fault zone, and examined their resolutions through seafloor experiments at relatively short baselines. A system for a seafloor extensometer makes use of precise acoustic ranging with a linear pulse compression technique. The system has a resolution better than 1 cm in acoustic ranging over a baseline of at least 1 km. The most critical problem is correction for temperature variations, and we estimate that the effect can be corrected with cm-order accuracy in the case of a deep-sea experiment. We have also examined a leveling system on the seafloor using an array of ocean bottom pressure gauges and an ocean bottom gravimeter to detect differential vertical motion. The system is estimated to have a resolution of several centimeters in vertical displacement. These system will be useful for triangulation and leveling on the seafloor, but we need further studies over a longer baseline and to achieve better long-term stability.
Faults related to the 2011 Tohoku earthquake (Mw 9.0) were investigated by using seismic reflection data and submersible seafloor observations before the earthquake (in 2008) and after the earthquake (in 2011). Because the surveyed area includes the region where the largest vertical displacement is predicted to have occurred, the shallow faults here are likely to be directly related to the tsunami characteristics (significant seafloor uplift). On the seismic profile off Miyagi, we identified a large normal fault within the continental crust. The seafloor deformation data demonstrated that dynamic horizontal seafloor displacement across the normal fault is much changed, suggesting that the normal fault could be ruptured during the 2011 earthquake. The seafloor observations after the earthquake demonstrate that several open fissures are developed along the fault traces which could not be observed before the earthquake. Due to large displacement along the plate interface near the trench, geological unit above the plate interface is in tensile state of stress, and the normal faults (and open fissures) should be ruptured during this earthquake. The normal faults as well as open fissures further accelerate the rupture along the plate interface and should induce huge tsunami. Since the geological unit above the plate interface is extension state at the earthquake and compression state during the interseismic period, the stress regime above the plate interface is dynamically changed with mega-earthquake cycle (∼1000 years).
[1] We report an uplift of 5 m with a horizontal displacement of more than 60 m due to the 2011 Tohoku-Oki earthquake. The uplift was measured by an ocean-bottom pressure gauge installed before the earthquake on a frontal wedge, which formed an uplift system near the Japan Trench. Horizontal displacements of the frontal wedge were measured using local benchmark displacements obtained by acoustic ranging before and after the earthquake. The average displacements at the frontal wedge were 58 m east and 74 m east-southeast. These results strongly suggest a huge coseismic slip beneath the frontal wedge on the plate boundary. The estimated magnitude of the slip along the main fault was 80 m near the trench. Our results suggest that the horizontal and vertical deformations of the frontal wedge due to the slip generated the tremendous tsunami that struck the coastal area of northeastern Japan.
