On the Accuracy of Tripartite Method

Abstract

A cooperative observation of microearthquakes was made in the northern part of the Neo Valley Fault in 1964. One of the purposes of this observation was to re-examine various methods for determining the hypocenter. The tripartite observation was carried out at six stations, and 18 earthquakes whose seismograms were available, at least, at 2 stations, were analysed, as for P phase. In 15 earthquakes, the epicenter was determined pretty well, but the focal depth was reasonably determined only in 9 earthquakes. This result suggests that epicenter is pretty reliable, if we take the observatonal error into consideration, but for focal depth, further investigation is necessary. Deviation of both approaching direction and apparent velocity caused by some superficial and underground structure may be meaningful, although the detailed discussion was impossible in this analysis because of insufficient data.