☆ 1 Magnitude
“Magnitude ” is measured on the Richter Scale , which is named after the American, Charles Richter 1935. The Richter Scale measures the size and power of an earthquake. The scale is logarithmic and has nine levels:
Magnitude 1 is the equivalent power of an explosive power of about 480 g of TNT (TNT is Trinitron toluene).
M 2 is about 31.6 times as large as M 1
M 3 is 〃 M2
M 4 〃 M3
M 5 〃 M4
M 6 〃 M5
M 7 〃 M6
M 8 〃 M7
M 9 〃 M8
M3 is 1000 times as large as M1.
M9 would be about 400 million tons of explosive power of TNT.
The Great North-East Japan Earthquake in 2011 was M9.
☆ 2 Shindo （ Seismic intensity）
Japanese term, Shindo (震度 seismic intensity) was created by the Japan Meteorological Agency in 1949. It measures how we feel the impact of an earthquake. It is more intense closer to the epicenter, and becomes less intense farther from the epicenter.
Now it is measured by a machine, but before 1996, it was measured by how people reported they felt.
Shindo 1 Some people feel shakes slightly.
Shindo 2 Most of people feel shakes.
Shindo 3 Windows rattle.
Shindo 4 Floors shake and things fall from shelves.
Shindo 5 Drawers and bookshelves fall down and stone walls collapse.
Shindo 6 We cannot stand. Wooden houses collapse.
Shindo 7 Landslides occur. Concrete buildings break down.
At the time of the Great North-East Japan Earthquake in 2011, at some places in Tohoku district, it was Shindo 7. Tokyo was roughly Shindo 5. Books fell down from book shelves in my house, and paved roads rose and fell like big waves. It was amazing that after the big waves of trembles, the road returned to its former condition.
☆ 3 Why do earthquakes occur?
The surface of the earth is covered with a crust and is broken up into tectonic plates of 20 to 100 km in thickness. At the east of the Japanese islands, “the Pacific Plate” is moving from the direction of Hawaii towards Japan a few centimeters a year. Here the Pacific Plate hits “the Eurasian Plate” and is forced under the Eurasian Plate.
In places like Japan, where one plate is forced under another plate, earthquakes often occur. The earth’s crust in these places is unstable and it easily distorts.
☆ 4 The major earthquakes in Japan in the last 300 years are :
2016 Kumamoto Aso earthquake Richter scale : M7
2011 Great North-East Japan earthquake and Tsunami Richter scale : M9
1995 Hanshin Awaji earthquake 〃 M7
1923 Great Kanto earthquake 〃 M 7.9
1894 Meiji Tokyo earthquake 〃 M7
1855 Ansei Edo earthquake after the time when the black ships arrived from US
1783 Eruptions of Mt. Asama
1782 Temmei Odawara earthquake
1703 Genroku earthquake 〃 M8.2
In 1596 while Hideyoshi was building a big castle in Fushimi near Kyoto, it was hit by a big earthquake, and the castle collapsed. At this time more than 20,000 people died in Kyoto.
Japan lies on the place where earthquakes often occur, and earthquakes have been a part of Japanese history for a long time.
In a word
“Magnitude” is the power of the earthquake itself, and “Shindo (seismic intensity)” is the shaking that we feel in each place.