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◎ Mechanisms ◎Scale of Earthquakes There are four tectonic plates in the area around the Japanese archipelago and each plate moves slightly over the course of many years. When they do, a great deal of pressure is brought to bear both at plate boundaries Magnitude (earthquake size) is and within the plate; when plates are displaced, it generates an earthquake. a measure of the amount of energy released by the earth- Oceanic trench quake. (trough) Magnitude Reverse Gal is a unit of measure that expresses the strength of the shaking of an earthquake nu-

Strike-slip merically in terms of accelera- fault Oceanic plate tion (cm/sec). Gal In general, the greater the Gal Continental plate number, the greater the seis- mic intensity.

Shallow land earthquake Earthquake inside a subducting plate (normal fault) 1891 Mino-Owari Earthquake Shindo is the Japanese mea- 1995 Great Earthquake sure of the strength of shak- 2004 Chuetsu Earthquake ing of the earthquake at an ob- 2008 Iwate-Miyagi Nairiku Earthquake, etc. Earthquake inside a subducted plate (obtuse fault) servation point on a decimal 1994 Offshore Sanriku Earthquake scale from 0 to 7. There are some 4,200 observa-

Shindo tion points across Earthquake at a plate boundary (acute reverse fault) monitored by the Japan Me-

1923 Great Kantō Earthquake (seismic intensity) teorological Agency. 1944 Tōnankai Earthquake 1946 Nankaidō Earthquake Deep earthquake inside a subducted plate (horizontal fault) 1968 Tokachi Earthquake The 2011 Great East Japan Earthquake was 1993 Hokkaidō earthquake 2003 Hokkaidō Earthquake a magnitude of 9.0 and the fault stretched 2011 Great East Japan Earthquake some 450km long by 200km wide. ◎Active Faults This refers to a fault that has been active repeatedly in recent geological history and may be active again in the future.

5-2-10 Sources: Former Nuclear and Industrial Safety Agency, The Seismic Safety of Nuclear Power Plants, and Japan Meteorological Agency homepage ©JAERO