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Geophysical Study on the Hotspot Volcanoes in the African Continent.

非洲大陆热点火山的地球物理研究。

基本信息

批准号：
02041008
负责人：
HAMAGUCHI Hiroyuki
金额：
$ 12.8万
依托单位：
Tohoku University
依托单位国家：
日本
项目类别：
Grant-in-Aid for international Scientific Research
财政年份：
1990
资助国家：
日本
起止时间：
1990 至 1991
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-02041008/
关键词：
Volcanic eruption Nyamuragira volcano Zaire Crustal movement Magma chamber Mantle structure East African Rift Valley Hotspot volcano Magma 広帯域地震観測火山活動ホットスポット火山マグマ分化火山性地震

项目摘要

This study is divided into two parts : The one is to study the heterogeneity of the deep mantle and the core-mantle boundary beneath the African plate. The other is the geophysical surveys of active hotspot volcano Nyamuragira and Nyiragongo in the western branch of the East African Rift Valley. The field studies in 1990 were successfully carried out in Zaire, however those in 1991 have been interrupted by the social unrest that occurred on September 23, 1991. The followings are the brief summary of the present study.(1) Observation of broad-band seismometer : The broad-band seismometers (CMG-3) were installed at Lwiro seismographic station (LWI), C. R. S. N., Bukavu, Zaire in 1990 and at Binza meteorological station (BNZ), Kinshasa in 1991. However, the seismometer at BNZ have been operated incompletely because the installation was interrupted by the social unrest. It can not be discussed the results for lack of data. We studied the heterogeneity in the mantle and core by using the da … More ta of WWSSN and GDSN. The following results were obtained ; (a) the core-mantle boundary was bumped several kilometers just beneath the African continent, (b) the S velocity in the D" layer was 3-5% slower beneath Africa than the standard model such as PREM. (3) The temperatures on the surface of the core was estimated to be several ten mili-kelvin higher in the A-hemisphere including the Africa and Atlantic regions than in the P-hemisphere including the Pacific region.(2) Volcanic earthquake and tremor : In addition to the four seismic stations of C. R. S. N., the eight temporary stations were installed in and around volcano Nyiragongo and Nyamuragira in 1990. A moderate size earthquake occurred at the SE part of Nyiragongo volcano on September 20, 1990, which induced a swarm of small earthquakes at the axis of the rift valley. The low frequency volcanic earthquakes accompanying the strombolian eruption at Nyamuragira volcano were located at the depth of 0.2-0.5 km just beneath the active vent. These events were explained by the downward single forces having 10^4-10^5 Newtons and 0.15 sec pulse width at a source.(3) Crustal movements : The geodetic measurements at eight base lines crossing the caldera rim of Nyamuragira revealed 15-36 cm dilatations during the period of 1989 and 1990. This dilatations were due to the opening of the fissure genarated by the 1989 eruption in the caldera. On the other hand, the geodetic measurements at the base lines crossing the rift valley showed no significant changes, suggesting that the recent crustal movements were limited in the active volcano area.(4) Magma activity : Temporal chemical variations at Nyamuragira were examined on two different time scale ; 1976-1899 (seven eruptions) and 23 days during the 1986 eruption. In both cases, lava has become more mafic with time, indicating that a zoned magma body, with less mafic magma at the top, is being sampled at sequentially greater depths over the course of time.(5) Recent eruption : After two years and one month dormancy, Nyamuragira began to erupt on September 20, 1991 from a fissure with the strike of N30E at about 15 km NE from summit. A swarm of earthquakes for 3 days preceded the eruption by one week. The new cone grew up 60-70 meter high with basal dimension of 400 x 300 meters during a week.It should be concluded, from what has been summarized above, that the present study contributed significantly to understand the intraplate hotspot activity in the African continent. Less

本研究分为两个部分：一是研究非洲板块深部地幔和核幔边界的非均匀性。另一个是对东非大裂谷西部分支的Nyamuragira和Nyiragongo活火山热点的地球物理调查。1990年在扎伊尔成功地进行了实地研究，但1991年的实地研究因1991年9月23日发生的社会动乱而中断。以下是本研究的简要总结。(1)宽频带地震计观测：宽频带地震计（CMG-3）安装在Lwiro地震台（LWI），C。R. S. N.，1990年在扎伊尔布卡武，1991年在金沙萨宾扎气象站。然而，BNZ的地震计由于社会动荡而中断安装，因此无法完全运行。由于缺乏数据，无法讨论结果。我们利用数据处理技术研究了地幔和核部的非均匀性 ...更多信息 WWSSN和GDSN的TA。结果表明：（a）核幔边界在非洲大陆下方几公里处发生了碰撞;（B）非洲大陆下方D”层的S波速度比PREM等标准模型慢3-5%。(3)据估计，包括非洲和大西洋地区在内的A半球的地核表面温度比包括太平洋地区在内的P半球高出几十毫开尔文。(2)火山地震和地震：除了C。R. S. N.，这8个临时监测站是1990年在尼拉贡戈火山和尼亚穆拉吉拉火山及其周围建立的。1990年9月20日在尼拉贡戈火山东南部发生了一次中等规模的地震，在裂谷的轴部诱发了一系列小地震。Nyamuragira火山Strombolian型喷发所伴随的低频火山地震发生在活火山口下方0.2- 0.5km处。这些事件可以用向下的单一力来解释，该力在一个源处具有10^4-10^5牛顿和0.15秒的脉冲宽度。(3)地壳运动：在穿过Nyamuragira破火山口边缘的八条基线上进行的大地测量显示，1989年和1990年期间的高程为15-36厘米。这是由于1989年火山口喷发产生的裂缝的开放。另一方面，在跨越裂谷的基线上进行的大地测量显示没有重大变化，这表明最近的地壳运动局限于活火山地区。(4)岩浆活动：在Nyamuragira的时间化学变化进行了检查，在两个不同的时间尺度; 1976年至1899年（7次喷发）和23天，在1986年喷发。在这两种情况下，随着时间的推移，熔岩已经变得更加镁铁质，这表明一个分区的岩浆体，在顶部的镁铁质岩浆较少，随着时间的推移，在顺序更大的深度进行采样。(5)最近喷发：经过两年零一个月的休眠，Nyamuragira于1991年9月20日开始从裂缝中喷发，走向N30 E，距离山顶约15 km。火山爆发前一周发生了一系列地震，持续了三天。一周内，新的锥状体长到60-70米高，基底尺寸为400 × 300米。从以上总结可以得出结论，本研究对了解非洲大陆板内热点活动有重要贡献。少