We have developed a new imaging technique of subsurface heterogeneities that uses Sp-waves from natural earthquakes. This technique can be used as a first screening tool in frontier exploration areas before conventional active exploration. Analyzing Sp-waves from 28 earthquakes ( 2.0 to 4.2) recorded by two permanent seismic stations, we built an image of the distributions of velocity discontinuities in southeastern offshore Hokkaido, Japan, where intraplate earthquakes in the Pacific plate frequently occur. Our results indicated the presence of three horizontally continuous, distinct discontinuities corresponding to geologic boundaries estimated in a previous study. We also derived the frequency-dependent quality factor for P- and S-waves and use it as a method of characterizing physical properties of subsurface structure. The waveform traces with coherent Sp-phases in the southern part of the study area generally show a constant ratio, and the waveform traces with randomly distributed phases in the northern part show a large variation of the ratio (including several high values).