Foreshocks had been Observed since Two Days before the 2011 off the Pacific
Coast of Tohoku Earthquake, Mar. 11, 2011!
Yoshiaki FUJIIRock Mechanics Laboratory
Hokkaido University, JAPANLong-term predictions have been given for some giant earthquakes as the possibility of occurrence inseveral ten years based on periodicity of earthquake occurrence. Long-term predictions help people
reinforce buildings, construct embankments, prepare evacuation system, etc. but never help people
evacuate "before" giant earthquakes occur. On the other hand, short-range predictions seem difficult
because sometimes we can observe precursors but sometimes we can't. Sometimes earthquakes occur afterprecursors but sometimes they don't. However, obvious foreshocks were observed in Japan before the 2011
off the Pacific coast of Tohoku Earthquake.

Daily rate of earthquakes with JMA (Japan Meteorological Agency) seismic intensity equal to or more than
1 is shown in Fig. 1. Less than ten earthquakes were usually observed in a day in Japan. The daily rate
began to significantly increase on two days before the main shock. Number of earthquake detection per day
for Hokkaido, Iwate, Miyagi, Fukushima, Ibaraki, Chiba, Tokyo, Shizuoka, Niigata and Okinawa before themain shock is shown in Fig. 2. No foreshocks were detected in Okinawa. Significant foreshocks weredetected in Miyagi, Iwate and Fukushima. A few foreshocks were detected in Hokkaido, Chiba, Tokyo,Shizuoka and Niigata.

Precise analyses should be made based on distribution of location of foreshock sources, however,possibility of earthquake occurrence around Miyagi could be suggested even from this kind of very simple
data analyses. Foreshocks were detected also in Iwate and Fukushima. Vastness of the seismic source of themain shock could have been predicted if an effective forecasting system had been developed.

Damage of buildings and infrastructures could have been unavoidable, off course, even if people had been
forced to evacuate until the daily rate recovered to, say, less than 10 earthquakes/day. More than thousands1

of people however could have safely evacuated with their families taking valuables and emergency goods.

Aftershocks are occurring not only off the Pacific coast but also in Tohoku, in Chuetsu, around Mt. Fuji, inTokyo Bay, Tokara Islands and everywhere in Japan relieving tectonic stress. The daily rate showed a peakon the day after the main shock but has not converged yet. Aftershocks should be carefully observed for awhile.

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Earthquakes per day

150
100500-1001020t (day)Fig. 1

Daily rate of earthquakes in Japan with JMA seismic intensity equal to or more than 1 (source:Japan Meteorological Agency Web Site in Japanese, http://www.jma.go.jp/jma/menu/jishin-portal.html).

Fig. 230
Miyagi20

Iwate

10Fukushima

0-10-8-6-4-20

Number of detections per day

Number of detections per day

Time is set at 0 for Mar. 11 (JST) when the main shock occurred.

30
20
10

Okinawa
Shizuoka Chiba

0-10-8-6-4t (day)t (day)

(a)

(b)

-20

Number of detection per day for some prefectures (source: Japan Meteorological Agency Web

Site in Japanese, http://www.jma.go.jp/jma/menu/jishin-portal.html) before the main shock. Time is set at 0for Mar. 11 (JST) when the main shock occurred. (a) Open circle: Hokkaido, solid circle: Ibaraki, solid
triangle: Chiba, solid reversed triangle: Tokyo.

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