Study on subsurface structure by using geoidal height from GPS/leveling and gravity in the southern part of central Japan

大地水准面的确定是大地测量学的基本课题之一。利用空间技术将传统大地测量数据与新的大地测量数据相结合，对其进行研究是必要的。GPS技术的发展使得通过比较椭球面高度和正形高度精确确定大地水准面高度成为可能。有人认为，菲律宾海板块与欧亚板块的碰撞产生了富士山等火山和日本中部南部的明石山等构造山。通过重力测量和地震勘探，对该区的地下构造进行了研究。本文通过将大地水准面资料与重力和地震勘探资料相结合的方法对这些问题进行了研究。我们在基准附近通过GPS测量精确确定了大地水准面高度，结果表明，在明石山区部分站点测得的大地水准面高度与重力数据测得的大地水准面高度相差超过1 m。通过对该地区重力资料的分析，明确了伊豆半岛西北方向不存在高密度俯冲板块。我们还指出，莫霍面要比在赤石山脉下的地震勘探所估计的深度要深。人们认为富士山没有均衡根源，因为布格异常的轮廓在那里没有扰动。然而，根的重力效应非常小，即使根存在，也很少影响布格异常。我们制作了均衡异常图，表明富士山附近的重力异常可以解释富士山有均衡根和没有均衡根两种情况。

Determination of the geoid is one of the fundamental subjects of geodesy. Its study is necessary to combine traditional geodetic survey data and new geodetic data by applying space technique. Development of GPS technique makes precise determination of geoidal height possible by comparison between ellipsoidal height and orthometric height.It is thought that the collision of the Pilippine Sea plate to the Eurasia plate produced volcanoes such as Mt.Fuji and tectonic mountains such as Akaishi mountains in the southern part of central Japan. The subsurface structures of this region have been investigated through gravity surveys and seismic explorations. In this study they are investigated by adding geoid data to gravity and seismic exploration data.We performed precise determination of geoidal heights by GPS measurements near benchmarks, and the geoidal heights obtained at some stations in Akaishi mountains area have more than 1 m differences from those obtained from gravity data.We analyzed gravity data in the area concerned, and made clear that there is no subducting slab with high density to the northwest direction from Izu Peninsula. We also pointed out that the Moho discontinuity is deeper than that estimated from seismic explorations under Akaishi mountains.It was thought that Mt.Fuji has no isostatic root because the contour of Bouguer anomalies has no disturbances there. The gravity effects of the root, however, are so small that the root, if it exists, seldom affects to Bouguer anomalies. We made isostatic anomaly map and showed that gravity anomalies near Mt.Fuji can be explained in both cases that Mt.Fuji has an isostatic root and not.