A seismicity pattern of March 4, 1977 Vrancea, Romania earthquake: An earthquake prediction insight

Abstract

Abstract Recently, the precursor phenomena of seismic nature gained particular attention in earthquake prediction. Among these, the concept of seismicity fluctuations before major earthquakes is the most prominent (Fedotov et al., 1970; Evison, 1977, EOS, 1977; etc.). The occurrence of significant changes in seismicity preceding some earthquakes has been reported by several authors (Evison, 1977; Ohtake, 1977; Usami, 1977), however only for shallow earthquakes. Recently, a relevant anomalous seismicity pattern preceding the deep March 4, 1977 Vrancea earthquake, was pointed out by Mârza (1978). Here, we attempt to present a more detailed description of the phenomenon. The seismicity data of the area clearly show a decrease in seismic activity in the depth range 85 - 130 km, approximately 14) years in advance of the main shock. The depth at which the decrease occurs correlates well with that of the main shock. The observed seismicity pattern (in the depth range 85 - 130 km), is as follows (Figs. 1 and 2) : (A) normal minor seismicity (period 1061 – 1963); (B) first stage of abnormal quiescence (seismic gap) (period 1963 - 1967); (C) a burst of activity (“precursory swarm”) (mainly at the end of 1968); (D) second stage of abnormal quiescence (seismic gap) starting late I968 and lasting until March 4, 1977; when (E) the main shock occurred, just within the above mentioned depth range.