約160万年前に噴出した玄武岩溶岩が，浸食・採掘されてできた洞窟．顕著な柱状節理をもち，マグマの冷却過程を想像することができる．1807年，江戸の儒学者・柴野栗山が，多角形の節理が亀の甲羅に似ていることから，霊獣・玄武にちなんでこの洞窟を「玄武洞」と名付けた．1884年，東京大学の小藤文次郎が，この玄武洞にちなんで「basalt」を「玄武岩」と名付けた．1929年，京都大学の松山基範が，玄武岩の残留磁化を測定し，地球磁場の反転を示し（Matuyama, 1929），古地磁気学の発展やプレートテクトニクスの確立へ大きな貢献を果たした．約258–77万年前の逆磁極期は「松山逆磁極期」と呼ばれ，最後の磁極反転はチバニアン期の始まりを示すことで知られる（Suganuma et al., 2021）．2022年，ユネスコ世界露頭100選にも選ばれ，日本の地質学にとって重要な露頭である．玄武洞公園の許可を得て，360度カメラで撮影¹⁾

（写真・説明：河合研志・佐藤友彦・庄司真史　2023年5月15日撮影）

注

1) https://s.insta360.com/p/529abe323a3fe013f80e522cc25376f0?e=true&locale=en-us [Cited 2023/9/26].

 The Ansei-Tokai Earthquake occurred at around 10:00 A.M. on December 23, 1854. The seismic intensity of the earthquake tended to be greater where Shizuoka and Aichi prefectures face the Pacific Ocean. However, it is pointed out that the Kofu Basin, which is located inland, was also hit by strong motions equivalent to seismic intensity 7. A detailed seismic intensity distribution map of the Kofu Basin has been revised several times. However, the historical records are biased toward the western part of the Kofu Basin; as a result, seismic intensity tends to be higher in the western part of the basin. New historical records clarify damage to houses at each village in the eastern part of the Kofu Basin. Seismic intensity is estimated from the damage rate of houses identified. As a result, two new IDPs (intensity data point) with seismic intensity 7 and six new IDPs with seismic intensity 6-7 are added. It is observed that not only the western part of the Kofu basin but also the eastern part of the basin was hit by strong seismic motions. Regional bias of seismic intensity within the basin is updated, which allows a reexamination of the distribution of seismic intensity.

 The Fujikawa-kako fault zone (FKFZ) is one of the most active fault zones constituting the northwest border of the Izu collision zone between the Honshu and Izu arcs. Although FKFZ has the highest vertical slip-rate in Japan, its structures in the seismogenic zone have not been sufficiently elucidated. In order to clarify the detailed structures, we carry out two stages of integrated seismic exploration. The first stage named FIST2012 is for revealing shallow to deep structures from the Philippine Sea Plate (PHS) across FKFZ to the Itoigawa-Shizuoka Tectonic Line (ISTL) at the northwest border of the Izu collision zone. A gravity anomaly analysis is also performed. The second stage aims to elucidate the overall structure of the Omiya and Iriyamase faults from the surface to the bottom for presenting the outcomes of the first stage. The results of the first stage confirm that the upper surface of PHS, from a depth of 2,000 m to 20,000 m in the FIST2012 depth section, dips at about several degrees westward from a depth of 2,000 to 2,500 m, and about 25 degrees below 2,500 m. A branching fault is generated from the seismogenic zone at a depth of approximately 2,500 m from the upper surface of PHS, and reaches an area a few hundred meters below the surface trace of Omiya fault, which is one of the eastern marginal faults of FKFZ. Although this suggests a close relationship between the branching fault and the Omiya fault, it is not revealed clearly because the specifications of FIST2012 are not set for such very shallow targets. ISTL dislocates P-wave velocity contours in a high angled west-dipping reverse fault sense. It shifts the P-wave 5 km/s contour from 3,000 m below sea level to the east of ISTL up to nearly sea level. The relationship between ISTL and the upper surface of PHS is not revealed, because of sparce shot and receiver points along the FIST2012 seismic line across the Jizo Pass where ISTL runs. Volcanic bodies composed of the middle Pleistocene Iwabuchi volcanic rocks of the Ihara group are recognized as Bouguer anomaly highs in FKFZ. They are considered to control the traces of both the eastern and the western marginal faults of FKFZ.

 At the beginning of the 19^th century, Ino observed the apparent altitudes of stars crossing the meridian to calculate latitude. Then, he revised the values using astronomical refractions provided in Rekisho Kosei Kohen, a Chinese version of a European astronomy book published in 1742. Before starting a field survey, he prepared an inventory of the most recently observed altitudes of stars at his home observatory, which was located at a latitude of 35 degrees 40.5 minutes. During a field survey, he also observed the altitudes of stars at a survey site. The difference in latitudes between the site and his observatory could be obtained from the difference in altitudes of the same star. Using this method, he fixed the latitudes of 1,220 sites on the Japanese Archipelago. It is worth noting that he did not use a catalog of stars, but an inventory of star altitudes he had prepared, although he obtained star declinations for the year, which were derived from declination and precession in the Gisho Kosei catalogue of stars, a Chinese version of a European astronomy book published in 1752.