Seismicity and source parameters of earthquakes in the region of the large tolbachik fissure eruption

This study discusses the focal mechanisms of 61 earthquakes and dynamic parameters of 97 earthquakes (m_i, ≥ 2.0 and focal depths 0–20 km) occurring in the region of the large Tolbachik fissure eruption in 1975–1978. Variations of these parameters during the different eruptive stages are examined. Three stages can be distinguished: 1) June 27–July 2, 1975 (from the beginning of the seismic preparation to the large earthquakes of July 2nd withM=5.0); 2) July 2nd-July 6th, 1975 (before the onset of eruption); 3) From July 6th, 1975 to the end of 1976 (during the eruption). During the second stage the 90° reorientation of the compression stress axes was observed; during the third stage the compression stress system returned to the initial strike. No variation in the orientation of the tension stress axes was noted during all these stages. Simultaneously with the reorientation of the compression stress axes in the volcanic earthquake foci during the second stage, the stress-drop values decreased strongly, whereas during the first stage these values were anomalously high. During the third stage the stress-drop values continued to decrease and approached the average regional values indicative of a calm period. All the estimates of parameters variations were made at 0.1 significance level. The above-mentioned temporal variations in the volcanic earthquake focal parameters are discussed in connection with the probable model of preparation and course of eruption.