PROCESS OF THE 1977 NYIRAGONGO ERUPTION INFERRED FROM THE ANALYSIS OF LONG-PERIOD EARTHQUAKES AND VOLCANIC TREMORS

The lava lake present in the summit crater of Nyiragongo from 1928 to 1977 completely disappeared after the short-lived eruption of January 10th, 1977. The sequence of events during this eruption was elucidated by analysis of seven, long-period earthquakes (LP events) and volcanic tremors recorded at stations LWI, NAI and BUL. The seven LP events observed were composed mainly of Rayleigh waves with large amplitudes and were classified into two groups: one containing three and the other containing four events, based on waveform similarities. It was determined that the former events were excited by the vertical downward single forces and the latter events by the vertical upward ones. The short-period seismograms recorded at LWI and NAI showed that continuous volcanic tremors began roughly 6 min before the occurrence of the first LP event. After the appearance of seven burst-type tremor activities, tremor amplitudes suddenly increased and then gradually decreased. Taking the analytical results of seismic events and observed surface activities into consideration, we inferred the process of the 1977 Nyiragongo eruption to be as follows: (1) quick draining of the liquid lava through the fissures, and disappearance of the long-lived lava lake (less than 6 min); (2) large terraces collapsing onto the bottom of crater, causing the first three LP events during the next 8 min; (3) intermittent vesiculation of lava at the top of magma chamber, exciting the last four LP events during the next 8 min; (4) magma flowing in deeper parts of the conduit, generating the intense tremors measurable at NAI and lasting for roughly 30 min.