Holocene climate variability as deduced from the organic carbon and diatom records in the sediments of Lake Aoki, central Japan

ln order to understand the climate ｖariability of the Holocene period, two sediment cores were extracted at the northeastern　part of Lake Aoki,an intermontane freshwater body located near the northern Japanese Alps, Central Japan. The sediments are mainly composed of silty clay with some interealation of event sediments, including a tephra known as the Kikai-Akahoya (K-Ah) that dates 7,250cal BP. The sediment chronology yields the sedimentation rate of ca. 0.16 and 0.37 mm yr⁻¹ above and below the tephra, respectively. The sediments were investigated at 0.5cm interval, providing a time resolution of 13-29 years, for total organic carbon (TOC) and total nitrogen (TN) contents. The record of the organic contents along with the diatom abundance at 50 horizons is used as limnological monitors and climate proxies. The diatom record suggests that the lake has been oligotrophic and alkaline. TOC content and diatom abundance yield close correspondence with both short- and long-term fluctuations, shifting between low-and high-values. Climate, which control surface-water temperature, appears to be the main factor to affect the variability in the proxy records in Lake Aoki. The climate is dynamic, abruptly switching from various degrees of cool to warm Conditions in centennial to millennial scale. Warm conditions prevail from ca. 750-1300, 1750-3050, 4000-5250, 6150-7250, and 8800-10000calBP, and cooling occurs during 150-750, 1300-1750, 3050-4000, 5250-6150, and 8350-8800cal BP. The period between 7250-8350cal BP is rather Auctuating.Climatic events such as the Little Ice Age (LIA), the Medieval Warm Period (MWP), the Kofun cold stage, and the Holocene Optimum (HOP) are evident, and some other warm and cool events not well recorded before in Japan and abroad are also recognized. The lake also provides a valuable window into the recent climate change and the modern hydraulic changes in the lake system.青木湖は北アルプスの山麓,標高822mにある貧栄養湖である.表層堆積物の粒度分布を検討し, 1954年以降の電源開発のための人為的水利用が堆積物の性質と堆積速度に大きな影響を与えていることを明らかにした.青木湖の北東部から採取した2.2m長のコア試料は,おもに粘土質シルトから構成されており,年代的には約1万年間をカバーしていた.この柱状試料中の有機炭素･窒素含有率と珪藻殻含有数の増減はよく一致しており,湖の生物生産性の増減が有機炭素･窒素含有率の増減に反映している. 1954年以前の青木湖の生物生産性は,気温と連動した水温の変化に支配されていた可能性が高いので,有機炭素含有率の時代的変遷を基にして完新世の気候変動を復元することができた.完新世にはそれぞれ5つの温暖･冷涼期と1つの変動期が認められた.それらの多くは,小氷期,中世温暖期,完新世最温暖期などの汎世界的な寒暖変動とはぼ一致する.climatic variability, Holocene OptimumArticle地質学雑誌 108(4): 249-265(2002)journal articl

Himalayan strain reservoir inferred from limited afterslip following the Gorkha earthquake

The magnitude 7.8 Gorkha earthquake in April 2015 ruptured a 150-km-long section of the Himalayan décollement terminating close to Kathmandu The earthquake failed to rupture the surface Himalayan frontal thrusts and raised concern that a future Mw ≤ 7.3 earthquake could break the unruptured region to the south and west of Kathmandu. Here we use GPS records of surface motions to show that no aseismic slip occurred on the ruptured fault plane in the six months immediately following the earthquake. We find that although 70 mm of afterslip occurred locally north of the rupture, fewer than 25 mm of afterslip occurred in a narrow zone to the south. Rapid initial afterslip north of the rupture was largely complete in six months, releasing aseismic-moment equivalent to a Mw 7.1 earthquake. Historical earthquakes in 1803, 1833, 1905 and 1947 also failed to rupture the Himalayan frontal faults, and were not followed by large earthquakes to their south. This implies that significant relict heterogeneous strain prevails throughout the Main Himalayan Thrust. The considerable slip during great Himalayan earthquakes may be due in part to great earthquakes tapping reservoirs of residual strain inherited from former partial ruptures of the Main Himalayan Thrust