Journal of Natural Disaster Science, Volume 23, Number 2, 2001, pp83-88

The 869 Jogan tsunami deposit and recurrence interval of large-scaletsunami on the Pacific coast of northeast Japan
K. MINOURA
Institute of Geology and Paleontology, Graduate School of Science,
Tohoku University, Sendai 980-0845, Japan
F. IMAMURA
Disaster Control Research Center, Graduate School of Engineering,Tohoku University, Sendai 980-8579, Japan
D. SUGAWARA
Institute of Geology and Paleontology, Graduate School of Science,
Tohoku University, Sendai 980-0845, Japan
Y. KONO
Department of Civil Engineering, Tohoku-Gakuin University,
Tagajo 985-8537, Japan
T. IWASHITA
Institute of Geology and Paleontology, Graduate School of Science,
Tohoku University, Sendai 980-0845, Japan(Received for 11 Mar., 2002)

ABSTRACTThe fore-arc region of northeast Japan is an area of extensive seismic activity and tsunami generation. On July13, 869 a tsunami triggered by a large-scale earthquake invaded its coastal zones, causing extensive deposition ofwell-sorted fine sand over the coastal plains of Sendai and Soma. Sediment analysis and hydrodynamic simulation indicate that the tsunami inferred to be triggered by a magnitude 8.3 earthquake spread more than 4 km
inland then coast. We postulate that the sand layer was developed by the tsunami’s first wave. Traces of largescale invasion by old tsunami as recorded in the coastal sequences of the Sendai plain show about a 1000-year
reoccurrence interval. We suggest that the Jogan tsunami was much larger than tsunami generated by normal
earthquakes in the subduction interface.

INTRODUCTION
An earthquake offshore of northeast Japan on July 13, 869(Usami, 1987) produced a large-scale tsunami that damaged the
low-lying coastal zones of northeast Japan. This 869 tsunami,
named the Jogan tsunami after the reign of then emperor, is unusual because of its widespread flooding.
The historical document Sandai-jitsuroku, which gives adetailed history of all of Japan for 1200 years, describes the Jogan
earthquake and subsequent tsunami as follows: “The large earthquake was accompanied by a luminous phenomenon, and coastal
areas were illuminated in the dark. Some time after severe seismic
shocks, a gigantic tsunami reached the coast and invaded entire
Sendai plain. Rising seawater flooded an old castle town (Tagajo;
Fig. 1A), causing the loss of 1000 lives.” There is no historicalevidence of co-seismic subsidence of the plain (Usami, 1987),
therefore the prolonged period of flooding indicates that waves
from the Jo gan tsunami sequentially invaded the coastal areas.
Destroyed structural foundations that date from the 8th and 9th
centuries, discovered in the ruins of Tagajo, are overlain by sediment layers containing artifacts from the middle 10th century. The

committee studying the remains considers that exposed structuresin the castle town collapsed owing to erosion by the Jogan tsunami(Board of Education, Tagajo City, 2000).
More than a century has passed since scientific observations
were begun in northeast Japan. During that time no tsunami has
penetrated more than 2 km inland (Watanabe, 1998). On the basisof the Tagajo findings remains, seawater inundation by the Jogan
tsunami is thought to have reached 4 km or more inland. Is thisdeep penetration of seawater evidence of the occurrence of an
unprecedented large-scale tsunami?
The Pacific coast of northeast Japan is known for repeatedtsunami invasions (Fig. 1B; Watanabe, 2000). The Sendai plain,
however, has not been struck by such a large tsunami since the
Jo gan event. Urbanization has rapidly advanced to the coastal
area, and most of the land inundated by the Jogan tsunami is now
developed. An understanding of the cause and effect of theregion’s extensive invasion by the Jogan tsunami is important, notonly for disaster prevention, but to gain an understanding of fore
arc tectonic processes. We studied Jogan tsunami deposits by sediment analyses and numerical hydrodynamic model to clarify theorigin of that tsunami.

KEY WORDS: Jogan earthquake, Historical tsunami, tsunami deposits, northeast Japan arc, numerical simulation

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K. MINOURA, F. IMAMURA, D. SUGAWARA, Y. KONO AND T. IWASHITA

Fig. 1 Stratigraphic section of coastal sequences (C) on the Sendai plain (A). Well-sorted fine sand layers are intercalated in marsh deposits at three horizons. Layer 2 is interpreted as having been developed by the AD 869 Jogan
tsunami. Layer ②. Overlying felsic tephra are traceable inland 4.5 km or more from the shore. Historical andobserved tsunami (B), which struck the Sendai plain, mostly originated in the region offshore Sendai Bay(Watanabe, 1989).

SEDIMENT LAYERS LEFT BY THE JO GAN TSUNAMI
Historical documents record that the Jogan tsunami invasion
turned the flood plain into a broad expanse of water (Watanabe,1998). We used trenching and coring to obtain traces of tsunami
invasion on the coastal plains of Sendai and So ma (Fig. 1A).
Results of sediment facies analysis and a stratigraphic correlationof the deposits are shown in Figures 1C and 2 as sectional views ofsubsurface sequences. Sediment layers consisting of well-sorted,
fine arkosic sand are intercalated with nonmarine black organic
mud that includes fossil plant roots. A grayish-white felsic tephra
underlies the Sendai plain. Below this tephra, we found three 2- to
15–cm-thick layers of well-sorted sand exposed at approximately
40cm intervals on our trench walls (Fig. 1C). The sand layer justbeneath the tephra (layer 3 in Figure 1C) was traceable inland
more than 4.5 km and showed evidence of landward tapering andfining. In the coastal sequences of Soma a 1-cm-thick felsic tephra
and underlying 2-cm-thick sand layer are intercalated in organic
mud at the highest horizon. The absence of sediment grading
within the sand layers of Sendai and Soma suggests rapid sedimentdeposition.

We examined fossil diatoms in order to determine the sourceof the sediments. Dominant species in the sand layers are marineor brackish-water diatoms, or both, whereas the underlying or
overlying organic mud has abundant fresh-water diatoms (Fig. 2).
In the sand layer of the Soma section, more than 60 % of the floral
assemblage is marine. Because of the abundance of saltwater
diatoms in the flora assemblages and excellent grain sorting, thearkosic sand must have originated in a shoreface or foreshore environment. The organic mud contains an abundance of deciduous
pollen and fossil grass roots, indicative of deposition in a flood
plain environment.
Landward tapering and sediment fining indicates that arkosic
grains were transported inland from the coast. We conclude thatthe tsunami waves that penetrated the coastal zone of Sendai
formed fast-flowing currents associated with rapid lateral translation of water and the suspension of sediments of marine origin.
Storm surges along the coast of northeast Japan generally areagents of erosion and do not produce regionally extensive deposits
of marine sand on the flood plain (Minoura et al., 1987; Minoura etal., 1993). The transport of marine materials therefore is best
explained by in the sand layer having been produced by a tsunami

THE 869 JOGAN TSUNAMI DEPOSIT AND RECURRENCE INTERVAL OF LARGE-SCALE TSUNAMI
ON THE PACIFIC COAST OF NORTHEAST JAPANFig. 285

Stratigraphic and paleontological correlation of the sedimentary sequences of Sendai (Trench 1) and Soma.
Fossil diatoms in the sediments indicate that sand layers of marine to brackish water origin were deposited in aback marsh environment. The thin layer of grayish-white felsic tephra at Soma corresponds mineralogically andchemically (Fig. 1C) to that of Sendai. It follows that the sand layer just beneath the tephra is a trace of the invasion of the Jogan tsunami. The Sendai plain was opened to agriculture about 400 years before by a feudal load.
Agricultural disturbance of the subsurface layers reaches a depth of 10 to 20 cm at Trench 1, resulting in the formation of an unconformity just below the uppermost sand layer. Some historical documents report that cultivation was interrupted by invasion of the 1611 Keicho tsunami. The calendar periods of the sand layers are estimated by the 14C ages of wood fragments and indicate that the largest-scale tsunami struck the Sendai plain atintervals of about 1000 years.

(Bourgeois and Minoura, 1997; Minoura and Nakaya, 1991).
Although we do not have definite stratigraphic evidence that thesand layer of Soma has extensive distribution regionally, the deposition of marine materials in fluvial environments shows that thatsand has characteristics similar to the Sendai samples. Taking into
account the landward thinning of the layer in the limited distribution found at the trench site, we conclude the sand is tsunami
derived, as is the sand layer in Sendai.

Table 1.

EPMA chemistry and refractive index (RI) values ofglass shards in felsic volcanic products from geological sections at Sendai (Trench 1) and Soma.

TSUNAMI DEPOSIT AGE
Felsic volcanic activity to the north of Sendai about 1100
years ago resulted in the deposition of an extensive tephra layerover northeast Japan (Jogan tephra; Akojima and Danbara, 1991).
Yamada and Shoji (1981) performed radiocarbon dating on woodchips from this tephra, which indicated an age range corresponding
to the years 870-934 A.D. The reflective–index values of the glass
shards from this tephra range from 1.494 to 1.503 (Yamada andShoji, 1981; Akojima and Danbara, 1991). Refractive-index measurements and EPMA geochemistry results for glass shards in thetephra from Sendai and Soma are consistent with published data
(Table 1). The chemical constituents of the glass shards from the
Sendai tephra are very similar to those of the Soma tephra. Except
for the Jo gan tephra, no extensive deposition of felsic volcanic

products has occurred during the last 3000 years (Akojima andDanbara, 1991). We conclude that the felsic layers present in the
coastal sequences correspond to the tephra produced by the 870934 volcanic eruption. Two thin arkosic intercalations are present
in the Soma section at a depth of about 100 to 150 cm. It follows
that the 2-cm-thick sand layer just beneath the Soma tephra corre-

K. MINOURA, F. IMAMURA, D. SUGAWARA, Y. KONO AND T. IWASHITA

86

sponds to sand layer ③ at Sendai (Fig. 1).
We conducted radiocarbon dating on wood fragments fromthe organic mud at Sendai (Trench 1) with a scintillation measuring device. The results shown in Figure 2 are calibrated to calendar years. The range for the date of the sand layer just beneath the
Sendai tephra is 790-950 (1 s range). This includes the date estimated from the age of the tephra (870-934). From archaeologicalevidence and geochemical findings, we conclude that the Jogan
tsunami invaded the Pacific coast of northeast Japan and depositedmarine sand over extensive areas of the flood plains of Sendai andSo ma just before deposition of the Jogan tephra.
SEISMOTECTONICS OFF THE SENDAI COAST
The mechanism that triggers earthquakes along the arc-trenchboundary of northeast Japan, discussed in various previous studies,is closely related to the westward subduction of the Pacific plate
beneath the northeast Japan arc (Hasegawa et al., 1978; Minoura

Fig. 3and Hasegawa, 1992). Numerous large-scale earthquakes haveoccurred in the region offshore Sendai Bay (Usami, 1987). Recent
GPS observations (Nishimura et al., 2000) show that the plateboundary is being shortened by east-west compression and strainenergy accumulating offshore Sendai. Some past earthquakes havegenerated tsunami with waves several meters high (Watanabe,1998). Hatori (1987), who studied many old documents recording
historical offshore earthquakes, estimated that the magnitude of the1793 Kansei earthquake was about 8.2. He judged that the 1611
Keicho tsunami that invaded the coastal area of the Sendai plain(Fig. 1C) was caused by an 8.1 magnitude earthquake. The type ofcrust deformation and mode of fault movement depend on the tectonic processes. If two sets of fault movement are linked in theaccretionary wedge, it is probable that the magnitude will be 8.0 or
more offshore Sendai (Earthquake Research Committee of theJapanese Government, 2000).
Considering the hydraulic characteristics of long-wave propagation over the entire area of northeast Japan, the probable source

Results of the numerical simulation of the Jogan tsunami. Based on historical documents and proposed fault parameters of the Jogan tsunami (Sato, 1989), a model ofthe tsunami source was established. The computational region covers the area 37°N
to 39°N and 143°E to 144°30’E. An interval of 2 s and spatial grid size of 1 km
were used. The simulation covers wave propagation for 5 hours. The maximum
water-elevation contours and the time history of the water levels at Sendai andSoma are shown for 2 hours. After 30 min, a tsunami with waves 8 m high struck
the Sendai plain. Thin dotted lines show submarine contours in meters of depth.

THE 869 JOGAN TSUNAMI DEPOSIT AND RECURRENCE INTERVAL OF LARGE-SCALE TSUNAMI
ON THE PACIFIC COAST OF NORTHEAST JAPANof the Jogan tsunami was shoreward of the continental shelf. The
seismic damage and wave heights described in historical documents have maximums along the coast of the Sendai plain(Watanabe, 2000), indicative that the Jogan earthquake took placein the overlying arc-crust, probably within the accretionary wedge,
not in the subduction interface far from land. Focal mechanisms ofmodern earthquakes and the displacement of major faults in theaccretionary wedge off northeast Japan (Zhao et al., 1990) lead tothe inference that the fault movement of the Jogan earthquake wascaused by WNW-ESE compressional tectonics.
Based on reported structural damage, Watanabe (1998) concluded that the seismic intensity of the Jogan earthquake was 5 ormore at Tagajo. If the seismic intensity region of 5 to 6 is within aradius of r km, the relationship between the radius r and earthquake magnitude M is expressed by the equation log r = 0.50M 1.85 (Muramatsu, 1969). Estimating that the epicenter was 150200 km offshore Sendai, we calculated the earthquake magnitude
as about 8.3.

ORIGIN AND NUMERICAL RECONSTRUCTION OF THE
JO GAN TSUNAMI
Information on the scale and geometry of fault movements
facilitates accurate reproduction of tsunami. The distribution ofobserved aftershocks within the accretionary wedge forms a seismic plane, indicative of a thrust fault dipping around 45° westerly(Hino et al., 1996). For this model, we conducted a numerical simulation of tsunami generation and propagation using TUNAMI-N2
which is based on a shallow-water theory consisting of nonlinear
long-wave equations (Minoura et al., 1997, Minoura et al., 2000).
In this tsunami model, sediment transport and its energy dissipation are not included because their effects are not significant in thesea region (Takahashi et al., 2001). Bathymetric data were derived
from submarine topographic maps (scale 1:250000) published by
the Meteorological Agency of Japan. The area was overlaid with agrid of cells, the side of each cell being 1000 m. A grid size of 100
m was used near the coastline. Using the fault parameters presented by Sato (1989) and comparing the simulation results of several
fault models and the estimated tsunami heights in the present
study, we established a composite-fault model for the most plausible source of the Jogan tsunami. The proposed tsunami source islocation: lat. 37° to 39°N, long 143° to 144°30’E; magnitude: 8.3;
fault size: 200 km long; 85 km wide; 1 km deep; fault angle: 25°
strike, 45° dip, 90° slip; vertical displacement of the sea bottom 5.6
m. Figure 3 giving results of the numerical simulation of the
Jogan tsunami, shows that a train of waves with a maximum heightof 8 m reached the shores of Sendai Bay 30 minutes after the earthquake occurred.
DISCUSSION AND CONCLUSIONS
Despite the moderate wave height (~8 m) scale inferred fromthe numerical model, the extensive deposition of well-sorted
arkosic sand suggests that waves of the Jo gan tsunami reachedinland areas of the Sendai plain.
Matsumoto (1985), who studied the Holocene evolution ofcoastal sequences in northeast Japan, concluded that development

87of the Sendai plain resulted from the seaward progradation of fluvial systems. He made clear that progradation has continued
steadily for the last 5000 years. Using inversion analysis of geodetic data (1966-1995), El-Fiky and Kato (1999) estimated that the
Sendai plain subsides at the rate of 6-7 mm/year. This findingshows the Sendai plain to have been susceptible to vertical displacement. We believe that this coastal subsidence was canceledby a following uplift.
Due to agricultural disturbance of subsurface layers (Fig. 2),
the depth of the Jogan tsunami deposit varies from 10 to 40 cm(Fig. 1C). On the assumption that the surface elevation was 50 cm
lower than at present, we made a numerical simulation of tsunami
wave propagation for the on-land region. Results show that thefirst tsunami wave triggered by an earthquake of magnitude 8.3
would have spread 4 km inland from the then coast. Synthetic
marigrams of the shore of Sendai show that the first wave would
be the biggest, subsequent waves reaching the coast at about 7.5min intervals (Fig. 3). The relationship between the surf similarity
parameter, ξ, and the classification of tsunami-induced sedimentation (Imamura et al. 1997) also provides information about
whether marine sand was deposited by the tsunami on the plain in
the area. We estimated a ξ value of 1.4 x 10-2 and a coastal slopeof 0.001 using the calculated results for the first tsunami, height 5
meters and wave period 10 minutes, indicative of marine sandbeing deposited in the area. Run-up simulation that includes theinteractive model for wave and sediment is required for the further
study of sedimentation. Our simulation suggests that marine sediment was transported by the first wave and that the rapid regulararrival of following waves prevented a rundown of the waters by
the first wave, resulting in the flooding of the area.
Two thin layers of arkosic sand that underlie the Jogan tsunami deposit can be traced inland from the coast for more than 4 km
(Fig. 1). Continuity landward tapering, and the sediment-fining
pattern indicate a tsunami origin for these sand layers. We believethat the scale of the two early tsunami suggested by sand layers ④
and ⑤ is equal to that of the Jogan tsunami and that they also wereassociated with widespread flooding. The depositional ages
inferred from 14C dating suggest that gigantic tsunamis occurred
three times during the last 3000 years (Fig. 2). The respective calendar age ranges of the lower two layers are BC 140 - AD 150 and
ca. B.C. 670-910 (1σrange). The recurrence interval for a largescale tsunami is 800 to 1100 years. More than 1100 years havepassed since the Jogan tsunami and, given the reoccurrence interval, the possibility of a large tsunami striking the Sendai plain ishigh. Our numerical findings indicate that a tsunami similar to the
Jogan one would inundate the present coastal plain for about 2.5 to
3 km inland.
ACKNOWLEDGMENTS
This research was supported by the Ministry of Education,
Culture, Sports, Science and Technology, Japan. We thank Dr.
E.L. Geist and two anonymous reviews for their helpful comments.
Drs. H. Watanabe and A. L. Moore offered much useful advice on
the historical documents and sedimentological interpretation of the
Jogan tsunami.

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