The MT calculation from the P and SV waves is based on the displacement of the vertical components of the first inputs of the P wave and the vertical components of the first inputs of the S wave. At the same time, it counts the solutions: full, trace null and double couple. In this method, MT calculates from the vertical and horizontal components of the signal.
Counting with the method of the first inputs of wave P and wave S is selected when the Old method option is unchecked (Fig. 26, 3). Then, in the seismogram window (5), pairs of vertical and horizontal velocity and displacement seismograms are displayed for stations with marked P or Pg waves on the L component oriented in the radial direction from the source of the rotated seismogram, S or Sg waves on the Q component in the negative-radial direction of the rotated of the seismogram and with the exit and ascent angles calculated for these waves (see 3D vibration analysis).
Fig. 30. Determination of the maximum displacement of the S wave
For each station's seismogram, determine the displacement amplitude of the first wave P, if possible, of the SV wave by indicating the beginning and end of the wave, i.e. the point on the velocity seismogram where it starts and the point where it crosses the zero value, which means maximum displacement as in (Fig. 27, Fig. 30). The beginning and the end of the wave are marked with blue lines on the seismogram. The beginning marked with a dashed line is assumed by default as the time of the P or S wave. It can be changed by right-clicking on the velocity seismogram with the Ctrl key pressed. The end of the wave marked with a solid line is defined by right-clicking on the velocity seismogram. The end of the wave should coincide with the intersection of the zero axis of the velocity signal and the maximum of the displacement signal. Sometimes a drift or an offset may be superimposed on the signal, in which case the Shift key must be pressed to remove the offset during the integration, or the Shift and Alt keys to remove both the offset and the drift. Then, a blue line appears on the graph, defining the area of integration (Fig. 28). The displacement signal is integrated using the trapezoidal method and the integration result is marked on the velocity seismogram in the form of two blue horizontal lines. There may be slight differences between these lines and the seismogram, due to the sampling of the signal and the fact that the marked end and beginning of the wave may be between the signal samples. Larger differences require correcting the start or end of the wave.
After selecting the waves and pressing Computing, three seismic moment tensors are calculated and their result is displayed in the form of "beach balls" in position (6 - Fig. 26). Pressing Ok accepts the result.
For MT calculations using the P and SH wave method, a number of at least 7-8 P and S waves is required, with at least 5 P waves. At least 8 waves are recommended for full solution MT and 7 waves for trace null solution. Otherwise, the quality of the result is poor. In such a situation, after pressing Ok, a message appears (Fig. 29) and the analyst has the opportunity to make a decision. Double couple MT is always added to the development.