In October, 1978, sixteen explosions were carried out along a line 400km long at sea off Tohoku, Japan, where many large earthquakes have been occurring. Travel time of first arrivals of seismic waves from these explosions are used to evaluate the accuracy of hypocenter determination. Locations of shot points are computed by using arrival time data observed only at stations of the seismic network of Tohoku University and estimated location of shot points east of the seismic network between 38°N to 40°N is close to real shot points. The difference is less than 15km. However, estimated shot points northeast and southeast of the seismic network is far from real shot points. The largest value of the shift reaches 130km. To simulate hypocenter determination of earthquakes for which both P and S arrival times are used, origin times are restrained in the hypocenter determination of the explosions. In this case, the hypocenters for explosions north of 40°N and south of 38°N are more acculately located. Taking into account the lateral heterogeneity of the velocity structure of the crust and the uppermost part of the mantle, the hypocenter determination is improved to some extent, though improvement is small.
