Discussion of tripartite microseismic measurements*

Abstract There has been a tendency to overemphasize the accuracy of a tripartite station's determination of the bearing of a microseismic wave. Such a determination is based on the assumption that the time of passage of the crest of a specific advancing wave is uniquely observed at each instrument of the network. This is not the case, in general, where waves are approaching from more than one direction. There is compelling evidence that they are doing this in many, if not all, storms. Specific cases are discussed of waves of the same period and waves of different periods crossing a tripartite network. It is concluded that routine averaging of intervals from time marks to the nearest crest or trough can lead to serious errors if the pattern and character of groups at the three stations are not taken into account. Velocity determinations for microseisms are open to question when they are based on records from horizontal components only, which do not permit distinguishing between Rayleigh and Q waves.