On Mohorovi_??_i_??_Wave appeared in Tazima-Earthquake

Abstract

Some seismograms of the destructive earthquake occurred on May 23rd, 1925 in North Tazima district, show distinctly so-called the Mohorovi_??_i_??_ wave _??_. One of them is shown in Fig. (4) magnified from the seismograms of Wiechert's 200kg. seismograph installed at the Central Meteorological Observatory in Tokyo. (Epicenter distance about 450km.)
The author of the present paper has drawn the hodograph for the longitudinal wave P from the results of seismic observations of the meteorological stations in this country as well as the time difference between P and _??_ from seismograms of these stations in case when it revealed on it. Moreover the curve of _??_ is also drawn on the same diagram and is shown in Fig. (6).
Nextly, the author has assumed that, to interprete the time curve the discontinuity layer exists not so far from the surface in the earth crust. Following numerical values are obtained:-
Velocity of the longitudinal wave
above the discontinuity layer, v₁=5.56km. per see.
Ditto under the discontinuity layer, v₂=7.50km. per see.
Epicenter distance of stations where P
and _??_ waves arrive simultaneously Δ₀-120km.
Difference of T(P)-T(_??_), shown in Fig. (6) is 6.2 sec.
From these values, the depth of the discontinuity layer and that of the seismic foeus of this earthquake is determined. He has also compared the time of occurrence with those of the other two great earthquakes which have occurred on Jan. 15th, 1924 and Sept. 1st, 1923 respectively in Fig. (1); and ascertained that the geographical effect is not so great in these time-curves that the following conclusion may hold good in its main figure, even in any earthquake which occurred everywhere in this country.
Followings are main results obtained in this paper:-
(1) So called “initial movement” takes place with sudden change in its magnitude, and sometimes in direction, at the place about 100-200km. distant from the epicenter. Fig. (5).
(2) The epicenter distance above mentioned depends chiefly upon the depth of seismic focus, therefore the depth may be estimated by this means.
(3) I_??_itial movement must be very weak at the stations which lie within the epicenter distance, from 200km. to 1000km. approximately.
(4) For the determination of the direction of epicenter from that of the initial movement of longitudinal wave, it is better to use _??_ phase than P and this will be done with much accuracy and easiness.
(5) Depth of the discontinuity layer and that of the focus of Tazima-earthquakes are calculated and their values have been found to be about 42km. and 32km. respectively.
(6) S-phase of the transverse wave begin to appear gradually o_??_ seismogram and with increasing epicenter distance it becomes more distinct.
(7) Irregular form of isochronal curves may be considered as the effect of the discontinuity layer whose depth may not be definite in a part of the earth-crust of this part of the globe.
(8) Well known “Omori's formula” which expresses the relation between the duration of preliminary tremor and epicenter distance, may hold good with _??_ and _??_ phases for the region of small epicenter distance and with P and L phases at distant places.
The present paper is a preliminary report and the more complete studies will follow in a near future.