The ratio of the travel times for S and P at distances less than 10°

The ratios of the S and P travel times, T_s/ T_p, observed at central Australian stations from Banda Sea earthquakes with depths between 100 and 200 km are less than the corresponding ratios for surface explosions in the same distance range. This result is surprising and inconsistent with the increase in the V_p/ V_s ratio with depth inferred from laboratory measurements on upper mantle minerals. Studies from several continental regions, including Japan, and oceanic regions, show similar low values.

Possible reasons for the inconsistency are discussed. Studies of three well-observed earthquakes in the Hokkaido region lead to the conclusion that the depths of earthquakes in the 60 to 200 km depth range are systematically underestimated by ISC and that there is an associated error in the times of origin of about 3 sec. Using corrected origin times, the average value of the T_s/ T_p ratio over the distance range 0° to 8° is about 1.785.