One of the important questions of rockburst prevention is the understanding of the mechanism of rockburst source. This question can be effectively studied by direct seismic observations in the rockburst regions. For this purpose, the distribution ofP-wave onset signs and the inversion of first motion amplitudes were utilized as the basic method. In such a way the coal mine regions in Poland (Upper Silesia) and in Czechoslovakia (Kladno coal mine district) were studied as part of the Polish-Czechoslovak rockburst investigation project. More than 250 rockburst events were recorded here in the decade 1977–1983, and analyzed. The results of the statistical analysis of these data allow us to formulate and introduce a model of the rockburst source with an implosion component. The suitability of this conception was verified by laboratory simulation conditions; it was confirmed that the seismoactive displacements with a clear implosive component were recorded in the neighbourhood of a stress concentrator weakened by holes. The results of both the field observations and laboratory tests were in good agreement with the theoretically derived radiation patterns for a combined shear-implosive source and also with the theoretical conception of such a source based on real geometrical configurations of mine excavations and tectonic dislocations. The rockbursts treated exhibit a dominant shear component, the magnitude of the additional implosive component not exceeding 10 percent of the shear component.