Re-examination of Travel Time Tables for Local Earthquakes

Abstract

   The travel time table which was proposed by Ichikawa and Mochizuki (1971) and in use at the Japan Meteorological Agency for hypocenter determination since 1973 was revised. The table was primarily based on the observational results of explosion seismic experiments carried out in and around Japan. It consists of two parts, namely one part is the travel time of surface focus which was derived by averaging observational results of explosion seismic experiments and other parts are travel times for other depths which were computed from the velocity structure which was primarily derived from the results of explosion seismic experiments. However, investigation revealed that the velocity structure was inconsistent with the travel time from the surface focus.
   This inconsistency might be due to a process in which the velocity structure was created using the Herglotz-Wiechert method. Some errors in computation were also seen in the table. These problems frequently caused unstable hypocenter solution and minus depth solution for shallow focus earthquakes.
   Considering heteorogeneous crustal structure in and near the Japanese Islands and anisotropic P wave velocity distribution reported by explosion seismic experiment, it doesn't seem to be always meaningful to make a total revision of the travel time tables in the present circumstances. Considering several velocity structures reported as a standard model for the crustal structure of the Japanese Islands, the velocity structure of the table in use doesn't seem to be unusual, although velocity around the Mohorovicic discontinuity seems to be slightly small. Therefore, the revised travel time tables given in unit of 0.01 second and called 83A were computed from the same velocity structure.
   An attempt to make new travel time tables which are more convenient for hypocenter determination was also made. On the basis of the travel time curve for surface focus of Ichikawa and Mochizuki (1971)'s table (I-M table), a new velocity model for crustal layer was computed. The new structure was combined with the velocity structure of Jeffreys and Bullen (1958)'s table (J-B table) by three different ways to compute new tables (83B, 83C and 83D). Applications of these new tables to hypocenter determination showed that some of these tables are generally more convenient than the revised table 83A for hypocenter determination. For the sake of continuity in the routine work, however, the author concluded that the revised travel time table 83A should be used for a while.