Tsunami Report for the earthquake on 11th March, 2011

On March 11th, 2011, at 02:46 PM local time (March 11th, 2011, at 05:46 AM UTC) an earthquake occurred near Japan, with a magnitude Mw 9.0 in a depth of about 10 kilometres. Because of the earthquake magnitude and the relatively shallow location of the epicentre, there was a concrete danger of a destructive tsunami. Therefore, the subscribers of the Tsunami Alarm System were informed of the event immediately after the earthquake. When additional Information was avialable, the warning was expanded to a pacific-wide warning.

The Earthquake triggered a destructive Tsunami. Measurments showed wave heights of more than 4 meters on the japanese coastline near the epicenter. Media reports indicated, that the wave heights even exceeded 10 meters in some cases. The greatest destruction was caused on the nearby japanese coast. Subsequently the Tsunami spread across the Pacific Ocean and reached Chile 20 hours later on March 12th, at about 04:00 AM UTC time. However, outside of Japan, the waves rarely reached heights of more than two meters.

The number of people who died because of the earthquake and the tsunami is not yet known. However, it must be assumed, that several thousand People died in Japan. Outside of Japan there was usually sufficient warning time and most people could be evacuated in time.

By the interaction of the earthuqake and the Tsunami, several nuclear accidents where triggered throughout Japan. The extent of the accidents, and the consequences for the region can not be estimated at this time. The accident in the nuclear power plant Fukushima is considered a level 6 accident by the French Nuclear Safety Authority. This accident level has been achieved so far only two times - 1986 in Chernobyl (level 7) and 1957 in Kyshtym (level 6).

Geological Backround:

The magnitude 9.0 Tohoku earthquake on March 11, 2011, which occurred near the northeast coast of Honshu, Japan, resulted from thrust faulting on or near the subduction zone plate boundary between the Pacific and North America plates. At the latitude of this earthquake, the Pacific plate moves approximately westwards with respect to the North America plate at a rate of 83 mm/yr, and begins its westward descent beneath Japan at the Japan Trench. Note that some authors divide this region into several microplates that together define the relative motions between the larger Pacific, North America and Eurasia plates; these include the Okhotsk and Amur microplates that are respectively part of North America and Eurasia.

The March 11 earthquake was preceded by a series of large foreshocks over the previous two days, beginning on March 9th with a M 7.2 event approximately 40 km from the epicenter of the March 11 earthquake, and continuing with another three earthquakes greater than M 6 on the same day.

The Japan Trench subduction zone has hosted nine events of magnitude 7 or greater since 1973. The largest of these, a M 7.8 earthquake approximately 260 km to the north of the March 11 epicenter, caused 3 fatalities and almost 700 injuries in December 1994. In June of 1978, a M 7.7 earthquake 35 km to the southwest of the March 11 epicenter caused 22 fatalities and over 400 injuries. Large offshore earthquakes have occurred in the same subduction zone in 1611, 1896 and 1933 that each produced devastating tsunami waves on the Sanriku coast of Pacific NE Japan. That coastline is particularly vulnerable to tsunami waves because it has many deep coastal embayments that amplify tsunami waves and cause great wave inundations. The M 7.6 subduction earthquake of 1896 created tsunami waves as high 38 m and a reported death toll of 22,000. The M 8.6 earthquake of March 2, 1933 produced tsunami waves as high as 29 m on the Sanriku coast and caused more than 3000 fatalities.

The March 11, 2011 earthquake was an infrequent catastrophe. It far surpassed other earthquakes in the southern Japan Trench of the 20th century, none of which attained M8. A predecessor may have occurred on July 13, 869, when the Sendai area was swept by a large tsunami that Japanese scientists have identified from written records and a sand sheet.

Continuing readjustments of stress and associated aftershocks are expected in the region of this earthquake. The exact location and timing of future aftershocks cannot be specified. Numbers of aftershocks will continue to be highest on and near to fault-segments on which rupture occurred at the time of the main-shock. The frequency of aftershocks will tend to decrease with elapsed time from the time of the main shock, but the general decrease of activity may be punctuated by episodes of higher aftershock activity. Beyond the ongoing aftershock sequence, the USGS does not believe that the earthquakes in Japan have significantly raised the probability of future major earthquakes. While the probability of future large earthquakes far from northern Honshu has not increased, neither has it decreased and large earthquakes will continue to occur just as we have observed in the past.

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