The source of natural earthquakes poses an intrinsic problem of loss of precision in a travel-time study. For example, the loss-of-precision problem is, in particular, a result of inherent low accuracies of source parameters of origintime and hypocentral coordinates for an earthquake occurring in a sparse seismological network. Hitherto, such demerits have been overemphasized even if greater seismic wave energy can usually be generated than with an artificial earthquake caused by an explosion.