In order to study the form ation of the hydraulic jump, several numerical experiments are conducted about the three-dimensional flow of two layers over an isolated obstacle. As a clue to under-, standing the results of the numerical experiment, the criterion for the existence of a steady flow over an obstacle is considered using the two-dimensional flow in linearized formulation. The results of six out of eight nume r i cal experiments performed in the present study are examined: three cases do exist in the steady regime, the other three in the unsteady regime. In the cases of the steady regime the continuous flow is obtained throughout the whole region, but in the cases of the unsteady regime a bow-shaped jump appears in the leeside of the obstacle, as is observed in the single fluid system. In the steady cases the internal surface tends to change more remarkably than the external surface; in the unsteady cases a reverse tendency holds.
Storm surges on the Pacifi c and the East China Sea coasts of West Japan are investigated by use of hourly readings of the tidal records for the ten years 1953 through 1962. Remarkable storm surges on these coasts are caused mainly by atmospheric pressure fall and the wind set-up associated with the passage of large typhoons. On the coast of the Pacific Ocean the rise of sea level caused by the wind set-up is relatively small compared with that due to atmospheric pressure fall, because there are only narrow strips of shallow water between the coast and the deep ocean. The severest storm surge reaches about 100 cm height near the path of a large typhoon. In storm surges along the coast of the East China Sea, the rise of sea level due to atmospheric pressure fall is most conspicuous, and the rise caused by the wind set-up is somewhat larger than on the coast of the Pacific Ocean. In Kagoshima Bay, th e effect of wind set-up is small owing to the deepness of the bay, the maximum depth of which is more than 200 m. But occasionally remarkable storm surges occur in the bay, because the surge invading the bay from the outer ocean grows remarkably by the influence of the bay topography. In Ariake Bay, especially at the head, the contributions of both wind and atmospheric pressure fall to the storm surge are great, and sometimes an abnormal elevation of sea level takes place associated with the storm surge. The contribution of the invading surge through the mouth is less in Ariake Bay than in Kagoshima Bay. But it must be remembered that Ariake Bay extends off Tachibana Bay and the surge invading Ariake Bay has already been amplified under the topographical influence of Tachibana Bay.
As one of the countermeasures against the damages caused by sea-salt particules at the time of a seasonal wind in seaside districts, we measured sea-salt particles produced by seasonal winds and floating in the air, at the coasts of the Tokyo Bay, Sagami Bay and the Japan Sea. The size distribution of airborne sea-salt particles at shoaly beaches was small in its range, and most of the particles were under 10 la cubic (2.2 × 10-9 gm). On the other hand, the size distribution range at other beaches ranged around 20 u cubic (18.0 × 10-9 gm). The total amount of airborne sea-salt particles is i n fluenced by the state of the sea surface, and the state of the sea surface is related to the topography of the seaside and wind velocity. Therefore we classified the total amounts of sea-salt particles according to the featuers of the seaside and Beaufort's wind scale. The sea-salt particles produced at the surface are mostly lost on the land within 300 meters from the bearch, by sticking to other objects or falling out. Only about one-fifth of the sea-salt particles are carried far inland. Their size is mostly less than 5 u cubic (2 × 10-10 gm). The decreasing rate of the sea-salt particles with the trans p ort distance was also studied within 1 km from the beach.
Aftershocks of the Tokachi Earthquake of 1968 and other shocks in Northeast Honshu and Hokkaido regions, recorded by the broad-band large dynamic range seismograph at Urakawa, were studied. The following is concluded from the observation and analyses: (1)I A delay in arrival time of high-frequency energy both in P and S, compared with low-frequency, was seen in comparatively distant Tokachi aftershocks of Δ=1.5°∼3.5° (Fig.3). (2) Such delay time becomes longer with increasing epicentral distance (Fig.4). (3) Earthquakes, which showed this effect of pseudo dispersion, occurred in the mantle as well as in the crust. (4) Two distant but deep aftershocks in the delayed group showed no such effect (Fig.6). (5) Earthquakes which gave the pseudo-dispersion effect also showed large attenuation of high frequency energy without exception. (6) The region in which earthquakes with this effect occurred is limited to areas off the Pacific coast of Northeast Honshu at some distance from the coast. Other earthquakes of the same order in epicentral distance but occurring in other parts showed normal arrival and abundant high-frequency energy (Fig.9). These observed facts seem to suggest that there is off the Pacific coast of Northeast Honshu an unusual region in the crust and the uppermost part of the mantle. It is suspected that some scatterers in the region cause the high-frequency energy to make a detour. The ordinary attenuation of Q-type is also larger in this region. This unusual region is located at the northernmost part of the Japan trench, where it bends to the east and continues as far as the Kuril trench, and the focal mechanism in this region seems different from that in surrounding regions. This region may possibly have some relation to the large-scale tectonics currently in discussion, but in a very complicated way.
The present observations and a nalyses form the third and final experiment for the study on “Noise Attenuation in Shallow Holes ”(T. HIRONO, S. SUYEHIRO, M. FURUTA and K. SATO,1968 and 1969), the first and second experiments having been made at the Meteorological Research Institute, Tokyo and the Onahama Weather Station on the Pacific coast of Northeastern Japan, respectively. The final test site was at the Niigata Meteorological Observatory on the Japan Sea coast of Northeastern Japan, where the geological founda tion is very weak and the microseisms are very large in winter. The final depth of the bore-hole was 200 m, and observations were made at depths of 26,45,105,146 and 200 m. The instrumentation and method of analyses were basically the same as in our previous experiments. The results are summarized in the following: 1) The observatory premises are in a recently reclaimed ground. The high frequency portion of both background noise and earthquake motions are already much absorbed by the soft foundati on. The shortlived man-made noise, which is generated locally and mainly of high frequencies, is also attenuated very rapidly by depth for the same reason. 2) The background noise in Niigata predominates at a frequency band of 4 to 2.5 cps at the surface. The amplitude of this freq uency band, however, becomes very small in the ground by a rapid a ttenuation (see Figs.2,3 and 4). 3) In our previous tests, almost no attenuation was observed. even at a depth of more than 100 m in a frequency band of microseisms. In the present experiment, however, the amplitudes of a frequency band of 0.4∼0.25 cps is attenuated down to about half at a depth of 200 m. This is probably because of the surface layer being very soft ground, which was recently reclaimed (see Fig.3) 4). Amplitudes of earthquakes become smaller with increasing depth, and this effect is larger at higher frequencies as recognized in our previous experiments (see Fig.5). 5) The improvement of signal to noise ratio, which depends on the depth attenuation of both background noise and earthquake motion, is greater with increasing depth. At a depth of 200 m, the theoretical lower limit of earthquake detectability is expectantly lowered by 0.5 in M. If it is taken into account that the seismogram interpretation is made much easier because of the elimination of confusing background noise, the lower limit is effectively lowered by 1 in M as shown by a routine test (see Fig.7). 6) Due to large microseisms in winter, it is very difficult to identify earthquakes in seismograms. To eliminate those disturbing microseisms, a low-cut (FL=1 cps) was connected. Clear seismograms were obtained, which are free both from artificial background noise and natural microseisms (see Fig.8)1. 7) By the three experiments since 1967, the bore-hole seismograph has been proved effective in improving the signal to noise ratio for local earthquakes and already, employed in the JMA routine operation. It has also been shown that this noise attenuation becomes much greater when the, depth reaches a certain hard layer. To study the local geology near the surface is therefore important to bury at an optimum depth.