Study of Crustal Activity in the East African Rift Valley

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(1) After the initial episode of eruption of Volcano Nyiragongo, eastern Zaire, we monitored the temporal levnl changes of lava lake in the summit crater by using the electro-optical distance measuring (EDM) deuce. The surface of lava lake elevated 118 meter in the period from August, 1994 to September 1996. The lava volume supplied in this period was 86 mmllion cubic meter.(2) We observed at least 37 eruptive episodes in the period from June, 1994 to February, 1995, during which volcanic tremors were intermittently recorded at the nearby stations. The A-type volcanic earthquakes were located at the depth ranges of 10 - 15km beneath the summit and around the NE flank. We found the reverse correlation between the occurence of earthquake and that of tremors.(3) We summarized the volcanic and seismic activities at the Virunga volcanic area, based on the geophysical, geochemical and geological published data.(4) We cooperated with Kenya Institute of Surveying and mapping and made the GPS networks crossing the Rift Valley at the northern Kenya. We measured the seven locations in 1997 and 1998.(5) We analyzed PKP,S and ScS seismic phases recorded by the GDSN networks in order to clarify the lateral heterogeneity in the mantle ant the core : (A) We found that large positive residuals of ScS-S differential travel times greater than 10 seconds locate 2 % low-velocity regions beneath south and northwest Africa : (B) The geographical pattern of the PKP differential travel time (BC-DF) residuals was found to be a manifestation of degree one heterogeneity in the outermost 100 to 500km of the inner core. The P velocities in the inner core are fast in the quasi-eastern hemisphere with a peak value of +0.29 sec and slow in the quasi-western hemisphere with that of -0.66 sec.