As is generally known, strong earthquakes occurring off deep-water coasts are sometimes followed by violent seismic sea waves, i.e. tunamis, which may rush inland several kilometers, over beaches, sweeping over villages and cities along the coast, casting ships ashore, and consequently causing great damage and a heavy loss of life. The tunamis associated with strong earthquakes are frequent in Japan. The more destructive tunamis have been reported by many investigators[1,2,3,4], whereas smaller tunamis have not received the same attention from investigators, and consequently full knowledge of the type of earthquake which is accompanied by tunamis has not been developed.
By experience we know that the crustal deformation on land is frequently caused by strong earthquakes. It may be thought that earthquakes centering entirely under the sea also cause a vertical displacement of the earth's crust forming the sea bottom and producing a violent motion of sea water with the result that there are sea waves over a comparatively long period. This vertical displacement of sea floor is considered to be the origin of tunamis associated with earthquakes, as suggested by theory[16,17,31,32,33]. Submarine land-slides started by earthquakes and submarine faulting are also considered to be the cause of tunamis[5,6]. Of course, tunamis arise not only from such seismic changes in the earth's crust, but also from submarine volcanic action and from strong atmospheric storms.
A great number of after-shocks following strong earthquakes in the area of crustal deformation on land have been investigated[7,36]. It is therefore im-portant to investigate the relation between after-shocks and the generation of tunamis. The present paper covers an investigation of all reported earthquakes and tunamis that from ancient times to date have occurred under the ocean bed in the vicinity of Japan.
TUNAMIS ASSOCIATED WITH EARTHQUAKES
To investigate tunamis which follow earthquakes, all existing mareographic records of seismic sea waves were collected and are discussed below.
(a)Our first concern is with tunamis associated with earthquakes that occurred in the Sanriku region for the period from 1924 to 1943.
Since earthquakes accompanied by tunamis have occurred most frequently off the Pacific Coast of northeastern Japan (Sanriku1 region), we first investi-gated the relation between earthquakes and tunamis in this region. The mareo-graphic records of from ten to fifteen stations distributed along the Pacific Coast of northeastern Japan were examined for the period of 20 years, from 1924 to 1943.
We first fully examined the tunamis and secondary undulations of oceanic tides, both seismic and meteorologic registered at stations such as Hachinohe,2 Kesennuma,3 Onagawa,4 Ishinomaki,5 Tsukihama,6 Shiogama7(Ojima8 and Hana-buchihama9), Onahama,10 Choshi,11 and in part those at Hakodate,12 Muroran,13 Kushiro,14 Nemuro,15 Ayukawa16 and Minato17 (Iwaimachi18). The geographic distribution of these mareographic stations is shown in Fig.1.
Then we examined the records of earthquakes which had occurred off the coast of Sanriku for the same period as appearing in Kisho-yoran,19 published by the Central Meteorological Observatory to ascertain whether or not tunamis really occur in association with earthquakes. We picked out only the tunamis of seismic origin. In this way we found that the tunamis occurring in connec-tion with earthquakes in Sanriku region numbered 17 (described below) during that 20-year period, as given in Table1 where earthquake epicenters(λ,φ) and earthquake magnitudes are given together with tunamis data such as magnitude and the largest of the maximum amplitude recorded at each station to see the scale of tunami.
1三陸 2八戸 3気仙沼 3女川 5石巻 6月浜 7塩釜 8尾嶋 9花淵浜 10小名浜 11銚子 12函館 13室蘭 14釧路 15根室 16鮎川 17港 18祝町 19Monthly Geophysical Review (in Japanese)気象要覧
(1)The tunami of August 6, 1927 (Fig.2).‐The earthquake related to this tunami occurred at 6h 14m off the Pacific Coast at the mouth of Abukuma-gawa20 river. It was a remarkable earthquake followed by a number of after-shocks occurred within the seismic area, as given in Table2.
The tunami waves were so small as to be traced only on the mareographic record, some samples of which are shown in Fig.2 and the mareographic data on which are given in Table3.
(2)The tunami of August 19, 1927(Fig.3).—The earthquake with which this tunami is associated occurred at 4 h 28 m off the Pacific Coast of Boso.21 This has already been studied by Wadati. A moderate shock, it was followed by a number of after-shocks, as given in Table4.
Tunami waves as traced on the mareographic record are reproduced and shown in Fig.3 from which we can note that a second tunami came about 85 minutes after the occurrence of the first. If we can assume that the second tunami followed the after-shock which occurred at 5 h 7 m, it took about 80 minutes from the start of the earthquake motion for the tunami waves to arrive at Choshi Station, whereas it took about 34 minutes for the first tunami to travel, and all after-shocks recorded were not felt. Wadati suggested that this second tunami was a reflection of the first. All pertinent data are given in Table5.
(3)The tunami of May 27, 1928 (Fig.4).—This earthquake occurred at 18 h 50 m off the northeastern coast of Miyako22 in Iwate23 Prefecture. It also was a remarkable shock followed by a number of after-shocks, as given in Table6. Some samples of the mareograms are reproduced in Fig.4 and the mareographic data are given in Table7.
(4)The tunami of March 9, 1931 (Fig.5).—The earthquake with which the present tunami was associated occurred at 12 h 49 m off the Pacific Coast at the mouth of Mabuchigawa24 river. This remarkable earthquake was followed by a number of after-shocks, as given in Table8. Three samples of the mareographic records are given in Fig.5 and the data of tunamis observed are given in Table9. The predominating period of each mareogram was from 20 to 30 minutes.
(5)The Sanriku tunami of March 3, 1933.—This earthquake occurred at 2 h 31 m off the Pacific Coast of Sanriku. It was a great regional earthquake followed by numerous after-shocks, numbering about 1,700  in total 54 of which were felt and 1,645 recorded but not felt. Some of these after-shocks were strong, their magnitude exceeding 7, but no tunamis were observed, even on the mareographic records, as already reported . The tunami following the main shock was so strong that great damage was done and many investi-gators [1,3,18] have studied this in detail.
22宮古 23岩手県 24馬淵川
(6)The tunami of June 19, 1933. —This earthquake occurred at 6 h 37 m off the eastern coast of Kinkazan25 in Miyagi26 Prefecture. It was a remarkable earthquake followed by a number of after-shocks, as given in Table10. Some samples of mareograms are given in Fig.6 and pertinent data given in Table11.
(7)The tunami of July 19, 1935. —This earthquake occurred at 9 h 50 m off the coast of Kashima27 in Ibaraki28 Prefecture. It was a remarkable earthquake followed by a number of after-shocks, as given in Table12. A sample of the mareographic record at Onahama is given in Fig.7 and pertinent data con-tained in Table13.
(8)The tunami of October 13, 1935. —This earthquake also was a remarkable shock that occurred at 1 h 45 m off the northeastern coast of Miyako in Iwate Prefecture. It was followed by many after-shocks, as given in Table14. The mareograms recorded at stations such as Onagawa, Tsukihama, and Hachinohe are given in Fig.8 and all pertinent data concerning the tunamis as recorded at several stations included in Table15.
(9)The tunami of November 3, 1936.—The earthquake occurred at 5 h 46 m off the coast of Kinkazan in Miyagi Prefecture. It was felt most severely on the Pacific side of northeastern Japan, and was followed by a number of after-shocks. as given in Table16. Table17 contains tunamis data obtained from mareograms which were studied by Miyabe. The most predominating period of each mareogram was from 20 to 40 minutes.
(10)The tunami of May 23, 1938. —This earthquake was a remarkable shock that occurred at 16 h 18 m off the coast of Shioyasaki29 in Fukushima30 Prefecture, but strong enough to cause some slight damage in Fukushima Prefec-ture, followed by a series of shocks, as shown in Table18 and Fig.9. Some samples of collected mareograms are given in Fig.10 and pertinent tunamis data given in Table19. The predominating period was about 20 minutes for every station.
25金華山 26宮城県 27鹿島 28茨城県 29塩屋崎 30福島県
(11) The tunami of November 5, 1938.—As already reported [12, 23, 25, 26], this tunami followed the remarkable earthquake that occurred at 17 h 43 m off the Pacific Coast of Fukushima. It was quite strong and could be felt along the Pacific Coast from the Kwanto region to northeastern Japan. There followed a number of after-shocks, as given in Table 20 and Fig.11. Some of these after-shocks were accompanied by tunamis, see (12) to (17) following. Pertinent data of these tunamis are given in Tables21-27. The predominating period of each mareogram was from 15 to 30 minutes.
(12) The tunami of November 5, 1938 [12, 23, 25, 26].—This earthquake occurred at 19 h 50 m, in almost the same place and with equal severity as (11). The shock was considered to be an after-shock of the main shock (11). Tunami data as determined from the records of several stations are given in Table22. The predominating period of each mareogram was almost the same as (11).
(13) The tunami of November 6, 1938 [12, 23, 25, 26].—This earthquake occurred at 17 h 54 m, somewhat northeastward of the epicenter of (11). The severity of the shock was almost equal to that of (12). Tunami data determined from the records of several stations are given in Table23. The predominating period of each mareogram was about 10 minutes.
(14) The tunami of November 7, 1938 [12, 23, 25, 26].—This earthquake oc-curred at 6 h 39 m the northeastern side of Sanriku. Tunamis data from several station are given in Table 24. The predominating period of each mareogram was about 10 minutes.
(15) The tunami of November 14, 1938.—This earthquake of rather small magnitude occurred at 7 h 31 m, southwest of the epicenter of (11). The tunamis were recorded at the stations such as Kesennuma, Onagawa, Onahama, Choshi, as shown in Fig.12. Tunamis data determined from the records of several stations are given in Table25. The predominating period of each mareogram was from 10 to 15 minutes.
(16) The tunami of November 22, 1938.—This earthquake occurred at 10 h 14 m, somewhat to the southeast of the epicenter of (11). The tunamis were recorded at Onahama and Onagawa stations as shown in Fig.13 and tunamis data determined from the records of several stations are given in Table26. The predominating period of each mareogram was from 5 to 10 minutes.
(17) The tunami of November 30, 1938.—This earthquake occurred at 11 h 30 m, at almost the same place as (16). The tunamis recorded at Onahama and Onagawa stations are shown in Fig.14 and sample data taken from the mareo-grams of several stations are given in Table27. The predominating period of each mareogram was from 10 to 20 minutes.
A study of the above data proves that several after-shocks were accompa-nied by tunamis as if they themselves were independent earthquakes. In this, the after-shocks are defined generally as these shocks in the seismic area im-mediately following the initial shock. As seen in Fig.11, there are many shocks immediately following the initial shock usually diminishing in frequency with time-lapse, but some increasing frequency at certain times as if some new shocks had occurred. In any event, it can be concluded that tunamis are associated only with earthquakes which are followed by many after-shocks.
(b) Our second concern is with tunamis associated with earthquakes originating at sea during the same period between 1924 to 1943 in other regions.
Tunamis associated with earthquakes which had occurred in oceanic bed regions other than in Sanriku were investigated. We selected six tunamis from various reports [14, 20] and Kisho-yoran as listed in Table28 where the largest of the maximum amplitudes of tunamis observed at each station is also given. These tunamis occurred off the coasts of eastern Hiuga31 on Kyushu, the Japan Sea at Shakotan32 in Hokkaido, Oga33 Peninsula, and the northern Miyako Island of the Ryukyus.34 The earthquakes with which these tunamis are associated were all followed by many after-shocks, as seen by Tables29-34.
31日向 32積丹 33男鹿 34琉球
(c) Next we list tunamis associated with earthquakes that occurred off all the coasts of Japan during the period from 1944 to 1955.
The tunamis data numbering seven associated with earthquakes during the 12-year period from 1944 to 1955 were compiled from various reports [2, 4, 11, 21, 28, 29, 30] and Kisho-yoran. A report of the examination of the mareographic records at Morozaki35 station near the extremity of Chita36 Peninsula in Aichi Prefecture is given as a supplement to include the tunamis off the coast of South Central Honshu. The list of tunamis thus obtained is given in Table35 in which the largest of the maximum height of tunami observed at each place as well as their magnitudes are shown. Earthquakes accompanied by tunamis are always followed by many after-shocks, as given in Table35.
(d) Finally we come to those tunamis associated with earthquakes that occurred in the sea all around Japan before 1924.
Various reports [1, 3, 9, 10, 34] were used to compile and investigate tunamis that occurred in Japan during the historical period before 1924. Early data on tunamis are listed in Table38 together with up-to-date reports. Table38 reveals that approximately 106 tunamis have visited the coasts of Japan throughout the historic times before 1924, the earliest recorded one occurring in 684 A.D.
MAGNITUDE OF TUNAMIS
The magnitudes of tunamis have been separated into five classifications ac-cording to maximum wave-height and damage done, as reported by Imamura
. In this classification, the maximum wave-height is reduced by one-half with the decrease of magnitude of unity.
Magnitude 0 ; The First Classification includes tunamis with wave-height of the order of one meter ; no damage.
Magnitude 1 ; The Second Classification includes tunamis with wave-height of the order of 2 meters ; house-damage along the coast, ships washed ashore.
Magnitude 2 ; The Third Classification includes those tunamis with wave-height of the order of 4-6 meters ; some destruction of houses, considerable loss of life.
Magnitude 3 ; The Fourth Classification comprises those tunamis with wave-height of 10-20 meters ; damaged area along the coast running about 400 kilometers.
Magnitude 4 ; The Fifth Classification of tunami is those of the severest with maximum wave-height of more than 30 meters; damaged area along the coast running more than 500 kilometers.
The total tunamis which have occurred during 1,270 years of Japanese history, classified according to magnitude and locale are listed in Table36.
A close study of these classifications proves that the severest tunamis in-cluded in Classification Five occurred most frequently along the Pacific Coasts of northeastern and southwestern Japan and that the tunamis in Classification One caused no damage but occurred most frequently only in northeastern Japan (Sanriku), whereas there were fewer to visit the coast of southwestern Japan (Tokaido and Nankaido). Tunamis of the Second Classification occurred along the Japan Sea Coast, especially along the western coast of Hokkaido. They occurred less frequently in other regions, such as the Kwanto, eastern and southern Hokkaido, Kyushu, and the Ryukyus. The origins of these tunamis are plotted in Fig.15 for the period before 1883 according to Takahasi  included with epicenters of earthquakes accompanied by tunamis for the period from 1884 to 1955. In Fig.15 the radius of the circle indicates the magni-tudes of tunamis and numerals outside the circles are the serial numbers in Table38. From Fig.15 we can easily determine the active region of tunamis. Further we can see also that tunami activity has not been the same throughout historic time, as shown in Fig.16. The most active period is from 1600 to date.
MAGNITUDE OF EARTHQUAKES ACCOMPANIED BY TUNAMIS
Array The magnitudes of earthquakes accompanied by tunamis were determined as follows :
(1) The magnitude M of each earthquake under consideration for the period from 1924 to 1955 was derived from Mk determined by means of intensity-distance curve as adopted by Kawasumi .
(2) The magnitude M of each earthquake for the time before 1924 was ac-cording to the Rika-nempyo,37 edited by the Tokyo Astronomical Observatory and other report .
The magnitudes of earthquakes thus obtained are given in Table38. As shown in Table38, it can be noted that the smaller earthquake of a magnitude under 6 was accompa-nied by tunami. The greatest magnitude of any earthquake under consideration was 8.6. Conse-quently the magnitude of an earth-quake accompanied by tunami is restricted to a range between about 5.5 and 8.6.
In order to study the relation between the magnitudes of earth-quakes and tunamis, the magnitude of tunami has been plotted against that of the earthquake, as in Figs.17 (for all investigated data) and 18, according to the region of occurrence. From this we find that in general the greater the earthquake, the greater is the tunami and thus the relation may be expressed by
in which m is the magnitude of tunami, M the earthquake magnitude, and a, b the constants depending on the region of occurrence as well as on the nature of the earthquake. The values of a and b can be obtained from the slope of a straight line representing the relation under consideration.
For instance, as a equals 2.5 and b equals — 17.0 for the curve in Fig.17, which represents the relation between the magnitude of earthquake and that of tunami by all data investigated, the relation may be expressed by
From Fig.17 it can be seen also that all the points representing the relation under consideration are included within the domain between the two straight lines ; m＝2.1M−11.5 and m＝6.0M−48.6. As a＝4.0, b＝−28.2 for the curve in Fig.18 b for the tunamis in Sanriku region, we have
m＝2.6M−18.0 for the tunamis in Tokaido and Nankaido region,
m＝2.5M−16.1 for the tunamis in Japan Sea,
m＝4.0M−29.2 for the tunamis in Sagami and Boso region.
In the same way we get the following results with less accuracy :
m＝5.4M−39.5 for the tunamis in southeastern Hokkaido,
m＝2.0M−142.0 for the tunamis in eastern Kyushu (Hiuga-nada).
THE CHARACTERISTICS OF EARTHQUAKES ACCOMPANIED BY TUNAMIS
In general, there are two occurrence types of earthquakes that originate under the ocean bed in the vicinity of Japan. One is a successive-occurrence type of earthquake i.e. a series of shocks following the initial earthquake and the other is single-occurrence type of earthquake, i.e. no shocks following im-mediately the initial earthquake even when it is quite strong. The earthquake accompanied by tunamis is not of the single-occurrence type but is of the suc-cessive-occurrence type. This can be seen from Table38 in which are given the shocks immediately following the initial shock occurring since 1924. The number of shocks following the initial main shock seems to be due to the grade of the earthquake magnitude as well as the degree of crustal deformation which has taken place in the seismic area. At any rate, it must be marked that all earth-quakes accompanied by tunamis are always followed by a number of after-shocks. On the other hand a great number of earthquakes occurred under the ocean bed off the entire coast of Japan not followed by tunamis, as shown in Fig.19-21, which were obtained from the Kisho-yoran. The figures represent epicenters of many earthquakes occurring within a ten-year period, including earthquakes that were followed by a series of after-shocks but not accompanied by tunamis. However the earthquakes followed by tunamis are quite few, as shown in Fig.15. We can therefore say that earthquakes succeeded by after-shocks are not always accompanied by tunamis and that earthquakes accompa-nied by tunamis are always followed by a number of after-shocks.
Attention must now be directed to the fact that some after-shocks defined as general, were accompanied by tunamis. As one example, the series of earth-quakes which occurred in 1936 under the sea bed off the eastern coast of Fuku-shima had the after-shocks of November 5, 6, 7, 14, 22, and 30. We do not yet know whether these shocks are independent of the earthquakes which they seem to follow, because it is difficult to distinguish between earthquakes related to after-shocks and earthquakes to which the following shocks are not related. We can only say that the mechanisms of both earthquake-occurrences on November 5 were the same ; whereas those of earthquake-occurrences on November 6 and 7 were quite different. This may also be noted from the initial direction of tunamis as shown in Tables21-27. This same fact can be noted in earth-quake-occurrences on November 14, 22, and 30. Further, the magnitude of the initial earthquake which occurred at 17 h 43 m on November 5 is almost the same as that of the earthquakes which occurred at 19 h 50 m on November 5 and also the one at 17 h 54 m on November 6. Hence, some of these after-shocks might be rightly considered independent earthquakes. If we take the strongest earthquake of November 5 as a principal shock, those which followed this initial shock may also be called after-shocks based upon our definition as usual. We can therefore conclude that after-shocks can be accompanied by tunamis. This fact must be taken into consideration in any study of the nature of earthquakes in connection with tunamis.
Further, we can consider the mechanisms of earthquake-occurrences from the results of the geographic distribution38 of sense of initial motion of these tunamis which were almost the same as those of earthquakes and also from the ex-istence of the predominating long periods which were observed in most seismic records of these earthquakes.
As another example, we may take the earthquake of August 10, 1901. This shock was considered to be an after-shock of an initial shock which, however, occurred on August 9.
It can be surmised from the results above given that the same deformation of the earth's surface took place in comparatively large areas of the sea bed in the same manner as on land.
CENTRAL AREA OF TUNAMIS
To determine the central area of tunamis we must consider the length of time usually needed for the tunami waves to reach the mareograph station, as given in Tables7, 9, 11, 15, 19, 21-26. In this case, the average error that might be involved will be 5 minutes at the maximum. Taking the station as imaginary wave-source, from which long waves are propagated in all directions with the velocities of √gh (g is acceleration of gravity and h the depth of sea), we can obtain a curve of the imaginary wave front after a time-lapse equal to the ob-served travel time needed by the actual wave to reach each station. Drawn accordingly, the curves pertaining to the various stations are indicated by the dotted or broken lines in Figs.22-24. The central area of each tunami is given by the domain enveloped in curves of imaginary wave fronts39 starting from several station points and includes the epicenter of each earthquake. The central area thus obtained is from about 50 to 120 kilometers in linear dimensions. The wave length of tunami waves calculated from the period of its initial oscil-lation as given above and the depth of the sea is roughly equal to the linear dimension of this central area. In the same way we also obtained the central area of other earthquakes given in Table37, together with the results [14, 18, 19, 20, 21, 22, 30] calculated up to the present time. The central area has a linear dimension ranging from several hundred kilometers to about fifty kilometers. This area is considered to be the region within which the crustal deformation has taken place.
The relation between the linear dimension of the central area of tunamis and the magnitudes of their respective earthquakes is given in Fig.25. As seen in Fig.25 it can be concluded that the greater the earthquake, the larger is the central area of tunami. This suggests too that the greater the earthquake, the larger is the area of deformation of the earth's surface on the sea-bottom. A marked upheaval or a subsidence or any discontinuity of the earth's crust is supposed to have happened after an earthquake accompanied by tunami.
38 The relation was already shown in the case of Oga earthquake on May 1, 1939 . We studied the present case in the same manner as previously.
39 We used the wave front charts which were obtained from the curves of imaginary wave fronts starting from the stations distributed along the coast of Sanriku region, as shown in Fig.1.
Earthquakes accompanied by tunamis were investigated and a compilation made of tunamis that occurred in Japan up to the present date.
Total tunamis accompanying submarine earthquakes which occurred in the vicinity of Japan number 136 from historic times to including 1955. Earthquakes accompanied by violent tunamis occurred off the Pacific Coasts of northeastern and southwestern Japan principally, that is in the Sanriku and Tokaido-Nan-kaido regions. Tunamis follow not only great submarine earthquakes but also small ones with magnitudes under 6. Tunamis always follow the submarine earthquakes with magnitudes over 8.
Earthquakes accompanied by tunamis are always followed by many after-shocks, that is, the earthquake under consideration is of a successive-shock-occurrence type. However every earthquake succeeded by after-shocks is not necessarily accompanied by tunamis. Another marked point is that some after-shocks as generally defined are accompanied by tunamis even though their magnitudes are small. These seem to be important in tunamis warning.
The relation between the magnitude of earthquake and that of tunami can be expressed by m＝aM＋b, in which m, M represent respectively the magni-tudes of tunami and earthquake, and a, b the constants. In general this shows that the greater the earthquake, the greater is the tunami and also that the greater the earthquake, the larger is the area of deformation on the sea-bottom which is comparable to the central area of tunami as obtained from the relation between the linear dimensions of the central area of tunamis and the magnitudes of the respective earthquakes.
Tunamis which accompany submarine earthquakes are attributed to vertical displacement of the sea floor within the seismic area where crustal deformations such as a marked upheaval or subsidence, or a discontinuity such as a fault may have taken place on the sea-bottom.
The author expresses his sincere appreciation to the officials of the Civil Engineering Sections of the various Prefectures Ibaraki, Fukushima, Miyagi, Iwate, Aomori, and Aichi and also to those of Hokkaido for a great deal of valuable data furnished throughout the present investigation.
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