抄録
Only the earthquakes of normal depth in the seismological reports issumed by the five stations in Japan during the I. G. Y. are used. The gradient of time-distance curve of seismic P wave is estimated directly from the observations, not from the smoothed curve of t-Δ.
The difference of arrival times due to the characteristics of seismometers and the foundations of stations are considered by reducing them to the standard instrument and station.
The earthquakes near the bisector of any pair of stations are rejected from the calculation, because the indefinite determinations of epicenters of these earthquakes bring about considerable errors to the result. Errors of dt/dΔ due to the focal depth, distance interval (dΔ), and the reference time-distance tables are also considered.
Comparison of stations.
Finally, calculated values from each earthquakes are superposed. But the earthquakes adopted at this stage are restricted to the ones near the great circle which involves Honshu (Japan), in order to avoid the errors due to the indefinite determinations of epicenters relative to the pair of stations.
