Great Earthquake-Generated Events - Tsunami - page 119

for tsunami warnings, issued a series of bulletins beginning
4 min after the main earthquake (Earthquake Engineering
Research Institute 2011 ). However, the initial warning
assumed an earthquake with a magnitude of 7.9 and the
Agency forecast 3-6 m high tsunami along the adjacent
coast. This information was disseminated to emergency
officials and to the public via television, radio, and cell
phones. For the next 4 h, succeeding bulletins expanded the
warning areas, and increased the expected water height;
however, by then the damage was done. The arrival time of
the tsunami was highly dependent upon local bathymetry
and topography. The first tsunami wave arrived 29 min
afterwards in the Chiba Prefecture and 55 min at Sendai
Airport (Fig. 6.17 ). Generators at the Fukushima Daiichi
Nuclear Plant stopped at 3:41 PM, 55 min after the earth-
quake. In northern Miyagi and southern Iwate Prefectures
the tsunami arrived at the coast 25-30 min after the earth-
quake. Run-up heights were extraordinary; depths of flow
inconceivable (Fig. 6.16 c, Table 6.7 ) (Earthquake Engi-
neering Research Institute 2011 ). Over 5,400 water level
measurements have been collected along 2,000 km of the
Japanese coastline (T ¯hoku Earthquake Tsunami Joint
Survey Group 2011 ; National Geophysical Data Center
2013 ). The highest run-up of 38.9 m was measured at
Aneyoshi Bay south of Miyako City in Iwate Prefecture,
although a higher value has been reported. Flow depths
exceeded 20 m in most populated coastal areas in Iwate and
northern Miyagi Prefectures. On the coastal plain south of
Sendai, peak water heights averaged 8-10 m. The highest
run-ups were measured north of the epicenter because of a
narrower continental shelf and steeper onshore topography.
However landward extent of inundation was greatest where
topography was flatter, especially on the Sendai Plain where
the tsunami travelled 4.0-4.8 km inland (Abe et al. 2012 ;
Chagué-Goff et al. 2012 ). This agrees well with a penetra-
tion limit of 4.2 km predicted by Eq. 2.14 for an initial 8 m
tsunami traversing a flat, farmed surface. Flow velocities
modeled from sediment characteristics ranged from 2.2 to 9.
0ms -1 , but were strongly dependent on the choice of
Manning's n defining surface roughness. Highest velocities
occurred in a 300-m wide zone landward of forested coastal
dunes (Jaffe et al. 2012 ). The tsunami also entrained sig-
nificant amounts of mud and debris to the extent that it
behaved as a debris flow (Kain et al. 2012 ).
The tsunami also propagated into the Pacific Ocean.
Travel times are shown in Fig. 2.7 . Its height as it spread
eastwards into the North Pacific Ocean is well documented
because of the number of DART buoys placed there fol-
lowing the 2004 Indian Ocean Tsunami. On the three buoys
operated by the Japanese Meteorological Agency closest to
the epicenter, the wave height exceeded 6 m in 5,195 m
depth of water (National Geophysical Data Center 2013 ).
The wave had a height of 1.86 m, 550 km away in 5,500 m

Table 6.7 Tsunami run-up heights along the Japanese coast for the
T¯ hoku Tsunami, March 11, 2011
Location Height (m)

Hachinohe 2.7

Hirono 5.5

Kuji 18.6

Noda 17.0

Fudai 28.0

Iwaizumi 6.8

Miyako 37.9

Yamada 10.0

Otsuchi 19.0

Kamaishi 9.0

Ofunato 23.6

Rikuzentakata 19.0

Kesennuma 22.2

Minamisanriku 15.5

Ishinomaki 16.0

Higashiatsusmima 10.4

Matsushima 2.4

Shiogama 4.0

Shichigaharna 10.0

Tagaiyo 7.0

Sendai 9.9

Natori 12.0

Iwanuma 7.6

Watari 8.0

Yamamoto 5.0

Shinchi 7.9

Soma 8.0

Minamisoma 4.5

Source Earthquake Engineering Research Institute ( 2011 )

depth of water (Fig. 6.16 d). North of Hawaii, 6,180 km
away, this height was 0.31 m; and by the time the wave
reached Washington State in the United States, it only had a
height of 0.18 m in 3,500 m depth of water. The wave thus
had minimal impact on distant shores (Fig. 2.7 ) . It was not a
significant teleseismic tsunami event. The distribution of
run-up heights on tide gauges around the Pacific Ocean
supports this fact (Table 6.8 ). Heights, above 2.0 m were
due to bathymetric focusing discussed already in Chap. 2 .
The greatest height of the tsunami at shore, outside of
Japan, was 2.5 m at Klamath River, California and Severo,
Kurilskiye, Russia. Sites further than Hawaii had more than
8 h warning because the earthquake was detected by the
Pacific Tsunami Warning Center soon after it occurred. This
did not prevented one death in California and minor damage
to marinas and coastal structures (Earthquake Engineering