Abstract
Teleseismic travel times of P waves to northeastern Japan, in the distance range from 24° to 85°, have been studied using data from 19 high-sensitivity stations of the micro-earthquake observing network operated by the universities. About 2, 800 arrival times from 267 seismic sources were used. Most of the seismic sources are earthquakes in the subduction zones of the western Pacific and the Southeast Asia. Mean travel times calculated for data at 2° intervals were fitted by a polynomial to determine a mean travel time curve. The standard deviations of travel times at the intervals are on the average about 0.7s. Travel times from two Nevada Test Site explosions have been utilized to estimate a correction of -2.39s for the absolute level of the travel time curve. The shape of the travel time curve shows broad agreement with the result of other works (e. g. HERRIN et al., 1968), though it is characterized by earlier times around 30°, which is probably due to the effect of the subdcting plates beneath the Japanese Islands. Differences from the mean travel time curve are 2 seconds at maximum, and mostly range within 1 second. Clearly they depend on the azimuth, suggesting the lateral heterogeneity in the lower mantle and/or the upper mantle in the source region. For a group of sources in a small limitted area, the standard deviation of travel times reduces to a value less than 0.5s. Especially the travel times from 12 Kazakh nuclear explosions, whose epicenters are probably limitted within 10km, give a standard deviation of 0.18s. This is as small as that expected from the reading error of P phases for teleseismic events.
The relative travel-time residuals of each station are consistent with both the station corrections determined from an inversion of P-wave travel times from local earthquakes and the Moho time terms derived from a set of Pn phases from events in northeastern Japan. Therefore the relative travel-time residuals for the teleseismic events are estimated to be affected mainly by the inhomogeneity in the crust and the uppermost mantle beneath the station network.
