A method for earthquake source studies based on a combined use of polarity and amplitude observations is proposed. It makes use of the impulses of the observed body waves to avoid the introduction of additional source parameters besides the orientation angles and seismic moments.
A statistical model is proposed and the method is applied to five small earthquakes in Sweden recorded by 3 to 5 stations in the distance range of 20 to 100 km. The seismic moments of these earthquakes are in the range of 0.6 to 10 × 1011 N-m (1 N-m = 107 dyne-cm).
The resulting mechanisms are statistically significant and well defined in spite of the fact that for four of the earthquakes, only two or less polarities are used. Furthermore, the solutions are close to the mechanism of the major (Mo = 3.5 × 1015 N-m) earthquake in the Gulf of Finland 1976. They all indicate horizontal principal compressive stresses in the sector E-W to SSE-NNW.
The successful use of amplitude observations presented in this paper shows that source mechanism inversion can be made based on recordings at only a very few close stations.