諏訪湖湖底の構造調査と環境地質  [in Japanese] A Geological Survey for the Study on Neotectonic Movement and Geoenvironment of Lake Suwa, Nagano Prefecture  [in Japanese]

昭和61年度以降数年にわたる物理探査・地質調査により, 過去2万数千年の地史が解明されてきた。物理探査として, 測深・音波探査, 湖底面状況探査などを行った。湖底面状況調査から, 従来の微凹地としていたものの実態がわかった。その成因は, メタンガス等の湧出によるといわれているが, この小凹地の分布は湖の西部北部に集中し, NE-SW方向にならぶものは諏訪盆地縁辺断層とは斜交する。これは, 浚渫後の湖底にも微凹地がみられ, 現在も微凹地が形成されている様にみえる。湖底地形は, 浚渫地を除いて, 昭和40年以後, 数十cm浅くなっている可能性がある。湖底ボーリングも4点おこない, 1万数千年前までさかのぼることができた。調査項目は, ^<14>C年代測定, 微化石(ケイ藻・花粉), 火山灰分析である。これらの分析から過去1万数千年間の古環境を明らかにするとともに, 年代測定から過去1.3万年の沈降が, 圧密な無視すれば1.6m〜2.2m/千年であり, 陸域での地形の変位を最低0.5m/千年としたものとあわせると, 3.0m/千年近くなる。湖底の火山灰層分析からも1.2〜1.3万年のUG火山灰が推定できた。

The geological history of Lake Suwa during the last twenty thousands years has been clarified by geological and geophygical investigations conducted since 1986. The geological investigations included sounding, sonic surveys and side-scan-sonar surveys. Although the subbottom structure of the lake could not be accurately determined the true nature of the feature which has been called a minor depression ("Kama ana") was determined by the side-scan-sonar survey. The genesis of the depression is considered to have been due to affection of methane or other gas. The minor depressions are concentrated into the northern and western parts of the lake and those trending in a NE-SW direction obliquely cross the faults bordering the Suwa Basin. In addition, the minor depressions are found even over the dredged bottom and thus it seems that the minor depressions are forming at the present time. The depth of the lake, except for the dredged areas, may have become shallower by severaltens of centimeters since 1965. In addition, four drillings from the lake bottom provided dates up to more than ten thousands years ago. ^<14>C age-dating, microfossil (diatom and pollen) analyses, and volcanic ash analysis were made on the drill cores. Age dating of these samples, shows that sediment deposition occurred at a rate of 2.2-2.5 m per one thousand years during thepast thirteen thousands years with a total of about 3.0 m per thousand. Upheaval of the land area is estimated to be at least 0.5 m per thousand years. The UG volcanic ash of about 13,000 to 14,000 years B.P. could be estimated from the analysis of the SL 62-4 volcanic ash layer (GL. -31.60 m〜-31.65 m).