Instead of L(t)Z(t) in Part I, -∫L(t)Z(t) dt is analyzed in this paper. The positive directions of rectangular co-ordinate (N, E, Z) are taken north, east and upward directions respectively. θ is measured clockwise from the north.
The blockdiagram of the analyzing circuit is shown in Fig.1.
The output of a magnetic tape recorder, three components of seismic waves, are processed through band-pass filter. Then
w(t)=∫L(t)Z(t)dt
where
L(t)=N(t)cosθ+E(t)sinθ
is analogically obtained at θ=0° and 90°, and recorded with a penrecorder.
When the following two wave forms
x(t)=-∫E(t)Z(t)dt
y(t)=-∫N(t)Z(t)dt
in adequate time duration from the initial P phase, are synthesized on the x, y co-ordinate plane, the co-ordinate point (x, y) moves along an orbit in the direction of the epicenter. As easily seen from the above definition, the x and y directions correspond to the east and north directions respectively. The direction of the orbit is measured with the eye, and called the apparent direction of the epicenter. One example of the above procedure is shown in Figs.3 to 4.
Table 1 gives the list of earthquakes and the apparent directions of epicenters obtained in the above-mentioned procedure. The mark (i) means the case where three components of the initial motion of P waves are measured.
Then accumulated frequency graphs for the difference between apparent and true directions of epicenters are shown in Fig.5. The frequency here means the relative frequency (maximum: 100). The conclusions drawn from this figure are as follows:
(1) As for near earthquakes, the differences of both directions are generally small in the case where seismic waves are analyzed in the frequency ranges 1.0 to 2.0 Hz.
(2) As for distant earthquakes, the difference of both directions are generally small in the case where seismic waves are analyzed in the frequency ranges 0.25 to 0.5 Hz.
(3) The lower the frequency of seismic waves analyzed, the greater the number of shocks whose epicenter directions are difficult to be determine. This implies the frequency dependence of the energy of seismic waves.
Fig.6 shows the deflections of the apparent direction of the epicenter to the true direction. From this figure, the following tend- ency is clearly observed.
As for earthquakes whose epicenter directions are northeast from Matsushiro, including shocks of Japan to shocks of Alaska, the apparent directions of epicenters deflect north from the true direc- tion. The deflections of directions turn out to be greatest in cases where seismic waves are analyzed in the frequency ranges 0.5 to 1.0 Hz. The lower the frequency of seismic waves, the smaller the deflection of direction. On the other hand, deflection is small in cases where seismic waves are analyzed in the frequency ranges 1 to 2 Hz.
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