Automatic Determination of Focal Mechanism Solution Using Initial Motion Polarity of P Waves

Abstract

   An automatic method of determining the focal mechanism solution is proposed. This method includes picking initial motion polarity of P waves and judging reliability of the solution. The method of picking initial motion polarity of P waves is as follows. First, Bessel band-pass filter is applied. Then, by using an AR model, arrival time of P wave initial motion is detected and AR filter is applied. To determine a focal mechanism solution, a grid search method is used. For judging reliability of the solution, it is evaluated synthetically, for example, from stability of the solution and focal mechanism solutions of earthquakes which occurred about there. It is shown that about 70 percents of initial motion polarity picked by Japan Meteorological Agency (JMA) can be picked by applying this method. Moreover, the ratio of inconsistent polarities between those picked in this study and picked by JMA is less than 3.5 %. Judging from this, automatic picking of initial motion polarity works with sufficient accuracy. Furthermore, by applying this method, more than 2.8 times focal mechanism solutions obtained by JMA are determined with sufficient reliability. Occasionally, focal mechanism solutions of inland shallow earthquakes, which are less than 2.0 in magnitude, or of deep earthquakes, whose magnitude can not be calculated because of their smallness, can be determined with sufficient reliability. Applying this method, focal mechanism solutions can be effectively determined with sufficient reliability.