Background and Scope - Tohoku, Japan Earthquake and Tsunami of 2011 - page 10

The Pacific Tsunami Warning center concurrently posted tsunami watches and
warnings — initially based on an estimated earthquake magnitude of 7.9 — for areas
of the Pacific. After 28 minutes, JMA extended the major tsunami warning to 6
prefectures and increased the expected wave heights to above 10 meters. JMA based
this decision on data received at 26 minutes from a GPS buoy located 18 km offshore
of the port of Kamaishi. This buoy, one of a series of coastal GPS wave buoys in the
area operated by Japan's Port and Airport Research Institute, recorded a maximum
wave height of 6.7 m (Takahashi 2011).
Shortly after a magnitude 7.9 aftershock at 6:15 UTC (29 minutes after the main
shock), communities along the Sanriku coast began to experience the onslaught of
tsunami waves, with one of the first arrivals at Ofunato at 6:18 UTC (32 minutes, see
Figure 3). Waves inundated areas from Miyako to Ishinomaki within 10 minutes of
each other. One hour after the earthquake, tsunami waves had reached as far as
Hachinohe in the North and Soma in the south. Following protocol for nuclear power
plants in Japan, the Fukushima Two (Dai-Ichi) station initiated automatic shutdowns
at the time of the earthquake; however, tsunami waves that overtopped the seawall
about 48 minutes after the main shock interrupted the power supply to the cooling
water system and led to the eventual declaration of nuclear emergency at the plant
(JMA, Tepco 2011).
Figure 3. Ofunato sea level station: observed maximum height of 8.0 m at 06:18 UTC
(JMA 2011)
At the same time tsunami waves were coming ashore in Japan, global tsunami
warning systems received updated seismic data and sea level information from the
DART buoys closest to the event epicenter. PTWC and JMA revised the event
magnitude and extended tsunami warnings. Three hours after the seismic rupture,