This plot shows earthquakes near Japan: the horizontal axis is time, the vertical axis is distance from the Fukushima Daiichi nuclear power plant. Different sizes/colors of points indicate different sized earthquakes from magnitude 4.5 to the main 9.0 earthquake of 11 March 2011 (time of this quake indicated by the vertical black line).

This shows the large number and geographic range of aftershocks following the main shock on 11 March. Five of these aftershocks were magnitude 6.5 to 7.2, similar in strength to the Loma Prieta and Northridge earthquakes in California in 1989 and 1994, respectively. One aftershock, 29 minutes later, was magnitude 7.9, similar in strength to the 1906 San Francisco earthquake.

Links:

• Latest Earthquakes in the World--Past 7 days (USGS)

This plot shows the location near Japan of the earthquakes in the above graph. Most of these earthquakes are off the eastern coast of the main Japanese island of Honshu. This region represents part of the area of subduction of the Pacific plate under Japan: the Tohoku earthquake is associated with where the Pacific plate of the Earth's crust slides underneath the plate of the Earth's crust that Japan is on. The contact between the plates is the Japan trench, and from there the Pacific plate drops into the Earth's mantle. The average westward motion of the Pacific plate relative to Japan is about 6 cm/year.

Links:

• Poster of the Great Tohoku Earthquake (USGS)

Largest earthquakes since 1900 (USGS data)

date	location	mag.	lat.	long.
31 JAN 1906	Pacific Ocean, off Ecuador coast	8.8	1.00 N	81.50 W
11 NOV 1922	Chile-Argentina border	8.5	28.55 S	70.50 W
3 FEB 1923	Kamchatka, Russia, USSR	8.5	54.00 N	161.00 E
1 FEB 1938	Banda Sea, off Indonesia	8.5	5.05 S	131.62 E
15 AUG 1950	Assam-Tibet	8.6	28.50 N	96.50 E
4 NOV 1952	Kamchatka, Russia, USSR	9.0	52.76 N	160.06 E
9 MAR 1957	Andreanof Islands, Alaska, USA	8.6	51.56 N	175.39 W
22 MAY 1960	Chile	9.5	38.29 S	73.05 W
13 OCT 1963	Kuril Islands, Russia, USSR	8.5	44.90 N	149.60 E
28 MAR 1964	Prince William Sound, Alaska, USA	9.2	61.02 N	147.65 W
4 FEB 1965	Rat Islands, Alaska, USA	8.7	51.21 N	178.50 E
26 DEC 2004	Indian Ocean, off Sumatra, Indonesia	9.1	3.30 N	95.78 E
28 MAR 2005	northern Sumatra, Indonesia	8.6	2.08 N	97.01 E
12 SEP 2007	southern Sumatra, Indonesia	8.5	4.44 S	101.37 E
27 FEB 2010	Pacific Ocean, off Maule, Chile	8.8	35.85 S	72.72 W
11 MAR 2011	Pacific Ocean, off Tohoku, Honshu, Japan	9.0	38.32 N	142.37 E

This table lists the largest earthquakes by magnitude since 1900: 16 quakes from magnitude 8.5 up to 9.5. The Tohoku earthquake is fourth largest. Of these 16 earthquakes, 4 occurred from 1906-1938, 7 from 1950-1964, and 5 from 2004-2011.

The information above is from the USGS page below on the largest earthquakes since 1900. Note that earthquake magnitudes are sometimes a matter of debate, particularly for historic earthquakes, for reasons including: different definitions of magnitude; lack of available measurements (for older earthquakes); and the fact that measurements for these largest earthquakes are difficult due to both large ground motion and large extent in area.

Links:

• Largest Earthquakes in the World Since 1900 (USGS)

© 2011 by Wm. Robert Johnston.
Last modified 16 March 2011.
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