We had a relatively strong earthquake of M6.5 yesterday at 13:39 local time (04:34GMT), 125km offshore Miyagi and some 40km below seabed. The report said that there were no immediate casualties or damage and no tsunami warnings from our Metrological Agency. No special reports here in Japan on this earthquake, but some on Allvoices and from other news medias. You might have wondered why no damage and no tsunami as the magnitude was rather strong as 6.5. This is because even though the magnitude that is the scale of earthquake energy may be strong, the SEISMIC INTENSITY does not always correspond to the magnitude scale. Christ Church earthquake is a good example. The February 22 earthquake recorded M.6.3, which is smaller than that of 7/23, but caused huge damage and killed 181 people. Christchurch is located on the swampy and sandy soil, which resulted causing liquefaction and devastated the area dreadfully. If we apply here so called SEISMIC INTENSITY, Christchurch earthquake was like Shindo 7 or 8. Yet, they don’t use this Shindo scale in New Zealand. The 3/11 earthquake in Japan, on the other hand, recorded Shindo 9, the severest in the Japanese earthquake history. For the more detailed explanation, I have copy pasted from Wikipedia as below.
The Japan Meteorological Agency seismic intensity scale is a seismic scale used in Japan and Taiwan to measure the intensity of earthquakes. It is measured in units of shindo (震度, seismic intensity, lit. "degree of shaking"). Unlike the moment magnitude scale (formerly Richter scale), which measures the energy released by the earthquake, the JMA scale describes the degree of shaking at a point on the Earth's surface, and is analogous to the Mercalli intensity scale. The intensity of an earthquake is not totally determined by its magnitude, and varies from place to place; for example, a quake may be described as "shindo 4 in Tokyo, shindo 3 in Yokohama, shindo 2 in Shizuoka".
The JMA operates a network of 180 seismographs and 627 seismic intensity meters and provides real-time earthquake reports to the media and on the internet.
And you may also ask me how this Shindo, Seismic intensity scale be calculated. Sorry, this is beyond my capacity to explain to you. There is a site in the Japan Meteorological Agency home page how the scale is determined mathematically. Unfortunately it’s written in Japanese only but for your reference I attach the URL here.http://www.seisvol.kishou.go.jp/eq/kyosh
Also you may have some idea how Japan was shook by the said 7/23 earthquake of Saturday at each locations.
http://www.jma.go.jp/en/quake/2011072313
When only Magnitude is applied, there seem causing misunderstandings. Probably international geological authority should adapt the similar SEISMIC INTENSITY scaling to clarify this measurement.
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