[S-EM13] 地磁気・古地磁気・岩石磁気

[SEM13-P02] Plaeomagnetic study of the Kusatsu-Shirane volcano, Gunma, Japan

*澤田 渚¹、亀谷 伸子²、川崎 一雄¹、石崎 泰男¹、寺田 暁彦³ (1.富山大学、2.山梨県富士山科学研究所、3.東京工業大学)

キーワード：古地磁気永年変化、草津白根火山、完新世、テフラ

The ring of fire in Japan has 111 active volcanoes. Some have erupted frequently and caused serious loss of life and damages. The Kusatsu-Shirane volcano at Gunma prefecture, Japan is an active volcano and had a sudden phreatic eruption occurred with 12 casualties in 2018. Understandings of past volcanic eruptions are essential for minimizing future losses and therefore accurate age information of past eruptions is important. Paleomagnetic analyses were carried out on 36 volcanic ash specimens and 11 paleosoil specimens from 47 units. Rock magnetic analyses show that the major magnetic mineral in the specimens is likely pseudosingle-domain titanomagnetite. The characteristic remanent magnetization (ChRM) directions of individual specimens were isolated by alternating field demagnetization. The unit mean ChRM directions at which radius of 95% confidence circle is less than 8° were selected for estimating the palaeomagnetic age. The 19 unit mean ChRM directions are compared to the three different composite paleosecular variation curves from either Lake Biwa in Japan or the Tohoku-oki earthquake rupture zone, and found that the BIWA SV-3 curve from Lake Biwa is well-matched for the ChRM of the Kusatsu-Shirane sediments. The results indicate that the observed unit mean ChRM directions likely record the secular variation, giving palaeomagnetic ages for 18 units. The observed paleomagnetic ages show good agreements with radiocarbon dates and provided for previously-undated tephra layers at 11 units. Therefore, the paleomagnetic dating method for the volcaniclastic debris (volcanic ash) deposited on land at the Kusatsu-Shirane volcano is an effective method for directly dating the tephra layers to determine past volcanic eruptions.