Abstract: In this paper, the method and theory relevant to inversion for source parameters by using long-period Rayleigh waves on IDA accelerograms (International Deployment of Accelerograph) are discussed. For earthquake sources deeper than 30 km, all of the five elements of the momemt tensor can be retrieved. For shallow (h30 km) earthquakes, two of the five elements become unresolvable1,2,3. Taking the Nan Huang Hai (South Yellow Sea) shallow earthquake of May 21, 1984 as an example, the difficult problem of how to extract more source information from shallow earthquakes is studied. The main shock fault mechanism is determined as following: strike N35W, dip 52, slip 54, seismic moment 1.21025 dyne. cm. The compression axes is in the direction of N74.4E. It turns out to be a reverse fault striking NNW with left-lateral slip. The distributed source model problem is also discussed. The results indicate that it does not make large difference for inversion results of moderate earthquakes, such as the South Yellow Sea earthquake, by utilizing the model of distributed source or point source. Analysing the finiteness of the source, an explicit expression for performing correction is presented.