Abstract:
A technique developed by the authors to monitor variations of geophysical parameters under stress was applied to the precursory period of the 1975 Great Tolbachik Fissure Eruption (GTFE), Kamchatka, to investigate changes in P and S travel times for small earthquakes occurring in the area. The TS/TP ratio between P and S travel times, termed the τ parameter here, was investigated. This parameter is similar to the VP/VS ratio, except that it can be calculated analytically by using an appropriate relation and travel times recorded at a single station. The pre-eruption period was investigated using the τ parameter to yield both a short-term precursor in the shape of a predictive curve χ = F(t) and an intermediate-term precursor represented by summary maps showing the areal distribution of this parameter and its changes in time preceding certain critical points of the eruption process like the growth of new cones, the collapse of the Ploskiy Tolbachik crater, etc. A specially developed algorithm and program were applied to the τ data to determine the stress field and monitor its time behavior. Significant changes in the stress field (type of slip, orientations of compression and tension axes, mean pressure, etc.) preceded individual GTFE phases. The results revealed a straightforward relation between the τ-parameter field and the stress field. The variation of the τ-parameter (TS/TP or VP/VS) was found to have been ultimately caused by changes in the stress field and its components.