- Open Access
Outline of the 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0)
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2011
Received: 14 April 2011
Accepted: 17 June 2011
Published: 27 September 2011
The Japan Meteorological Agency (JMA) is the sole authority to issue domestic tsunami warnings/advisories to the public in Japan. JMA issues the tsunami warnings/advisories based on hypocentral parameters such as location, depth and magnitude, and the tsunami-simulation database system which stores more than one hundred thousand cases of previously-conducted tsunami-propagation simulation results. This system enables tsunami warnings/advisories to be issued within about three minutes after events. For the devastating tsunami generated by the 2011 off the Pacific coast of Tohoku Earthquake on March 11, 2011, this system successfully functioned and the first warning was issued at 14:49 (Japan Standard Time), which was about three minutes after the occurrence of the earthquake. JMA also monitors sea-level data observed with 172 tide gauges and 12 GPS buoys (there were 11 at the time of the earthquake, and this increased to 12 in May 2011), and they are used for changing grades and areas of tsunami warnings/advisories. This time JMA raised tsunami-warning grades and widened warning areas seven times, and downgraded or cancelled the warnings/advisories four times. Mainly based on tsunami observations, all tsunami warnings/advisories were cancelled at 17:58 on March 13. This was about 2 days and 3 hours after the tsunami warning was issued.
JMA’s tsunami warning/advisory categories.
Forecast tsunami height
Tsunami height is expected to be 3 meters or more.
Forecast heights are specified for every region using values of 3 m, 4 m, 6 m, 8 m and 10 m or more.
Tsunami height is expected to be up to 2 meters.
Same as above, using values of 1 m or 2 m.
Tsunami height is expected to be about 0.5 meters.
2. Tsunami Warnings/Advisories and Observations
The promptly-estimated M 7.9 was based on Mjma, which is the combination of the displacement magnitude for relatively-large earthquakes and the velocity magnitude for relatively-small earthquakes (Katsumata, 2004; Funasaki and Earthquake Prediction Information Division, 2004). It can be calculated within several minutes, but it has a downside to be saturated for a magnitude substantially larger than 8. JMA updated Mjma to 8.4 at 16:00 and determined the moment magnitude (Mw) as 8.8 by CMT analysis at 17:30, which was calculated from overseas broadband seismic data. On March 13, Mw was revised to be 9.0 by CMT analysis using a longer range of filter (Hirose et al., 2011). These revised values were not used for the updates of tsunami warnings, because updates based on sea-level observations had provided a substantially large tsunami-height estimation before these larger-magnitude values were obtained. To obtain Mw earlier should be one of the most important tasks for JMA to solve.
Sea-level observation data presented in Fig. 5 includes broad information of sea-level changes inside the tsunami-wave source region. If the absolute sea level had not changed and only the coastal land and seabed had sank after the event, the tidal record would have shown a positive change. It may indicate that sea water also sank together with coastal land and seabed. Sea-level data in Fig. 5 shows small variations with a period of several minutes just after the event, but it cannot be specified whether they are rippling waves induced by strong ground-shaking or tsunami waves. On the other hand, the record of Ofunato shows a gradual sea-level decline with a longer period. This is considered to be a tsunami wave which was generated by the wide range of seabed deformation relatively close to the station. The causes of these various sea-level changes are to be further investigated.
Maximum tsunami heights for stations located from north (top) to south (bottom). Locations of stations shown in Fig. 3. Measurements are preliminary and will be reviewed and finalized following further investigations.
Tide gauge station
Maximum tsunami height
Observed or disrupted time
Erimo, Hokkaido pref.
Miyako, Iwate pref.
8.5 m or more (disrupted)
Kamaishi, Iwate pref.
4.2 m or more (disrupted)
Ofunato, Iwate pref.
8.0 m or more (disrupted)
Ayukawa, Miyagi pref.
8.6 m or more (disrupted)
Soma, Fukushima pref.
9.3 m or more (disrupted)
Onahama, Fukushima pref.
Oarai, Ibaraki pref.
Tsunami warnings/advisories were cancelled or downgraded four times as tsunami waves attenuated in height and became less destructive. The first downgrade was issued at 13:50 on March 12, followed by the cancellation or downgrade of all “Major Tsunami” warnings at 20:20 on March 12, cancellation or downgrade of all “Tsunami” warnings at 7:30 on March 13, and cancellation of all Tsunami advisories at 17:58 on March 13.
For the 2011 off the Pacific coast of Tohoku Earthquake and tsunami, JMA issued tsunami warnings/advisories, as it usually does, in a timely manner. However, due to the disruption of sea-level data streams, it became impossible to continuously monitor tsunami waves and thus more difficult to decide when to change or cancel ongoing tsunami warnings/advisories. In addition, if Mw could be calculated earlier, the frequency of updates would be decreased and the first update might be issued earlier. To improve the warning system, JMA will be reviewing its tsunami warning/advisory issuing and cancelling methods, processes, and procedures.
Sea-level data are not only from JMA, but also from the Cabinet Office, the Ports and Harbours Bureau of MLIT, the Japan Coast Guard, GSI and local governments. We thank all of these organisations for their efforts in maintaining these observations and providing the data. Some figures were made using Generic Mapping Tools (Wessel and Smith, 1995).
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