Investigation for geodetic detection of variation in crustal deformation near the subduction plate boundary

本研究的目的是通过在俯冲板块边界附近进行海底大地测量，研究俯冲板块运动引起的地壳变形的时空变化，并估计海底大地测量的可重复性。基于海面KGPS定位和水声测距的重复精确海底定位已在岩手县和宫城县外进行，并已在福岛县外启动。从福岛附近的地震活动来看，该地区板块边界的地震耦合可能相对较弱。然而，这几年的陆基GPS观测表明，在福岛和宫城附近有很强的耦合。虽然从两年的观测中得到的地壳变形估计误差相当大，但结果表明，福岛附近的地震耦合与宫城附近的地震耦合一样强。福岛附近的耦合可能会表现出时间上的变化。Kii半岛外的GPS/声学（GPS/A）海底定位检测到与2004年Kii半岛外地震（7.4级）相关的30厘米的大同震地壳运动。通过两天的观测获得的定位的可重复性估计约为3-5厘米。这个值比2005年宫城县外地震时GPS/A站点的估计位移值要大。我们证实，更高的GPS采样值可以提高该值。长期海底压力监测是在岩手县外的两个卫星轨道交叉点下进行的。在8个主成分矢量差的平方根和方面，结果与最近的5个1.3 cm范围内的海潮模型一致。虽然我们已经掌握了长期海底压力监测的技术，但要实现长期稳定的海底压力监测，还需要更多的研究工作。

The objectives of this research program are to investigate for spatial and temporal variations in crustal deformation induced by the subducting plate motion through seafloor geodetic observation in the vicinity of the subduction plate boundary, and to estimate the repeatability of the seafloor geodetic observation.Repeated precise seafloor positioning based on sea surface KGPS positioning and underwater acoustic ranging has been carried out off Iwate and Miyagi prefectures, and has been initiated off Fukushima Prefecture. Judging from seismic activities off Fukushima, seismic coupling on the plate boundary in the region can be relatively weak. However, land-based GPS observation in these several years suggests strong coupling off Fukushima as well as off Miyagi. Although the estimated error is fairly large in the crustal deformation obtained from two-year observations, the result shows that the seismic coupling off Fukushima is as strong as that off Miyagi. The coupling off Fukushima may possibly show temporal variation.GPS/Acoustic (GPS/A) seafloor positioning off Kii Peninsula detected a large co-seismic crustal movement up to 30 cm associated with the 2004 earthquake (M7.4) off Kii Peninsula. The repeatability of the positioning obtained from an observation for two days is estimated to be about 3-5 cm. This value was larger than the estimated displacements at our GPS/A sites associated with the 2005 earthquake off Miyagi Prefecture. We confirmed that the value can be improved with higher GPS sampling.Long-term ocean bottom pressure monitoring was carried out off Iwate Prefecture under two crossover points of the satellite tracks for altimetry. The results coincide with recent five ocean tide models within 1.3 cm in terms of root sum square of vector differences for eight principal constituents. Although we have acquired know-how for long-term ocean bottom pressure monitoring, we need more research works to get long-term stability of pressure monitoring.