Determination of focal mechanism solution using initial motion polarity of P and S waves

It is important to determine accurate focal mechanism solutions for earthquakes with small magnitudes or those recorded by a small number of seismometers, perhaps with an unfavorable geographic distribution. A method using initial motion polarity of S waves, in addition to P waves, is described. This method can be applied routinely to determine the focal mechanism solution of earthquakes which occur all over Japan where a nationwide earthquake observation network of high sensitivity seismographs at a few tens of kilometers interval will soon be available. We define Q to be a weighted sum of inconsistencies between observed and calculated polarities. Double-couple focal mechanisms are searched to determine the solution that produces the smallest misfit Qmin, and the set of acceptable mechanisms: Q≤Qmin+1.5. The uncertainty of each principle axis is expressed as the maximum angle between the best solution and all the acceptable solutions.