Bulletin of the GSI (Vol.59)

 The meridian arc length from the equator to arbitrary latitude is of utmost importance in map projection, particularly in the Gauss-Krüger projection. In Japan, the previously used formula for the meridian arc length was a power series with respect to the first eccentricity squared of the earth ellipsoid, despite the fact that a more concise expansion using the third flattening of the earth ellipsoid has been derived. One of the reasons that this more concise formula has rarely been recognized in Japan is that its derivation is difficult to understand. This paper shows an explicit derivation of a general formula in the form of a power series with respect to the third flattening of the earth ellipsoid. Since the derived formula is suitable for implementation as a computer program, it has been applied to the calculation of coordinate conversion in the Gauss-Krüger projection for trial.

 To obtain information about the volcanic activity history of the Io To Island in Ogasawara archipelago, the natural remanent magnetization (NRM) of east coast marine deposits and Kama Iwa deposits, where volcanic sedimentary periods have not been clearly determined, were measured. The two deposits, which may have been deposited in similar environments, showed different magnetic orientations. By comparing the NRM results to the geomagnetism change curves for the past 2,000 years in southwest Japan, it was determined that the sedimentary periods were 1,000 years or more apart.

 This paper presents an overview of crustal deformation caused by the 2011 off the Pacific coast of Tohoku Earthquake, detected by GEONET (GPS Earth Observation Network), the GPS continuous observation system operated by the GSI, and by interferometric SAR (InSAR) analysis using ALOS/PALSAR data. We found that a very wide area of the Japanese Islands, from Hokakido to the Kinki district, was remarkably affected by the crustal deformation caused by the mainshock of the Tohoku Earthquake. We estimated the geometry of the seismogenic fault of the earthquake, as well as the slip model for the plate boundary between the Pacific plate and the North American plate, from the crustal deformation data obtained from GEONET. The fault is estimated to be longer than 400 km, stretching from off Iwate Prefecture in the north to off Ibaraki Prefecture in the south. The largest slip estimate for the plate boundary is more than 56 m in the off-Miyagi region near the Japan Trench. Postseismic crustal deformation is also observed by GEONET, indicating that slow postseismic slip is ongoing along the plate boundary around the main fault zone. Additionally, we detected clear coseismic signals associated with induced inland earthquakes in the Fukushima-Hamadori area from InSAR analysis. The InSAR results show obvious displacement discontinuities, suggesting that rather shallow ruptures occurred. In particular, for the Mj 7.0 event on April 11, the observed displacement boundaries correspond to earthquake surface ruptures that appeared just along the Idosawa and Yunotake faults.