Earthquakes　accompanied　by　tunamis　occurring　under　the　sea　off　the　Islands　of　Japan

INTRODUCTION

　　　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.
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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.
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20阿武隈川

　　　(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[37].　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.
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21房総
　　　(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　[15]　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　[24].　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.
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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[19].　The　most　predominating　period　of　each　mareogram　was　from　20　to　40　minutes.
　　　(10)The　tunami　of　May　23,　1938.　—This　earthquake[35]　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.
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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.
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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.
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35師崎　36知多

(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
　[10].　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　[34]　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

　　　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　[13].
　　　(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　[27].
　　　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
　　　m＝aM＋b,
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
　　　m＝2.5M−17.0.
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＝4.0M−28.2,
and　also
　　　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).

————
37理科年表

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　[8].　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.

CONCLUSION

　　　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.

ACKNOWLEDGMENT

　　　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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