Study of Lava Lake at Nyiragongo Volcano

位于东非裂谷系统西部裂谷的尼拉贡戈火山在峰顶火山口有一个长寿的熔岩湖，持续了大约100年。我们根据过去的文献和最近的野外观测，回顾了这一熔岩湖的活动。本文的主要研究成果如下：(1)在前人研究的基础上，结合野外观测，总结了近100年来峰顶火山口熔岩湖活动的时间变化。(2)根据对峰顶火山口熔岩湖高程时间变化的观测，估算出峰顶火山口熔岩湖活动的平均供应速率为7000公斤/天。1994~1996年，岩浆总量达8600万立方米。(3)根据火山地震的波形，将火山地震分为两种类型。A型地震活动在尼拉贡戈陨石坑下方约15公里处表现出不太集中的特征。(4)每当观测到火山震动时，熔岩湖的喷发活动总是被目击者重新识别。在熔岩湖最初喷发之后，我们在1994-1996年间观察到至少36次喷发。我们发现，深部A型地震的发生与熔岩湖表面的熔岩喷泉之间存在显著的负相关关系。这一特征可以用管道系统中平流超压的增大来解释。

The Nyiragongo volcano in the western rift valley of the East African Rift System had a long-lived lava lake in the summit crater for about 100 years. We reviewed this lava lake actibities based on the old documents and the recent our field observations. Some results of the present study is summarized as follows ;(1) Based on the old documents and our field observations, the temporal variations of lava lake activity in the summit crater are summarized for the last 100 years.(2) Based on the observatin of temporal changes of lava-lake elevation, the average magma supply rate is estimated to be 7,000 kg/day. Total magma suplly is 86 million cubic meters in the period of 1994-1996.(3) the volcanic earthquakes were classified into two types on the basis of their waveform. The seismic activity of the A-type events shows ratherconcentrated features at the depth of about 15 km just beneath the Nyiragongo crater.(4) Whenever volcanic tremors are observed the eruptive activity in the lava-lake has been always reognized by an eye-witness. After the initial episode of eruption in the lava-lake, we observed at least 36 eruptive episodes in 1994 - 1996. We found that there is a dear reverse correlation between the occurrence of the A-type earthquakes at a depth and the lava fountain on the surface of lava lake. This feature can be interpreted by the increase of advective overpressure in the conduit system.