March 11 (Reuters) - Here are some facts about different scales to measure the strength of earthquakes, and related topics.
MAGNITUDE:
- Magnitude measures the size of the earthquake by the energy released at the source of the earthquake, and is determined from readings on seismographs.
- The scale is open-ended. A quake of magnitude 2 is usually said to be the smallest normally felt by humans. The largest recorded earthquake in the world was a magnitude 9.5 in Chile on May 22, 1960.
- There are several different scales to measure magnitude, including "Richter", named after seismologist Charles Richter who first developed the idea in 1935, and also moment magnitude, a relatively new measure created by seismologists.
- But the various magnitude scales should yield approximately the same value for any given earthquake.
- Magnitude readings for a particular earthquake are often updated as the initial figure is usually based on information gathered only by a small number of seismic stations and later revised after additional data become available.
- Magnitudes for the same earthquake can also differ among various agencies that report it due to slight differences in their seismographs and their locations with respect to the tremor's source.
- The scale is based on a logarithmic progression, which means each increase of one whole number means a 10-fold increase in the tremor's magnitude. A 5.0 quake is 10 times more power than a 4.0 quake and a 6.0 quake is 100 times more powerful.
JAPANESE "SHINDO":
- "Shindo", or the Japan Meteorological Agency Seismic Intensity Scale, is a seven-point scale not convertible to magnitude as it measures ground motion at a specific place, not the earthquake's absolute energy.
- Measured with a seismic intensity metre, the figure describes the expected impact and damage on people and buildings caused by the tremor, and differs from location to location for the same earthquake
- A quake with a zero reading is "imperceptible to people", while in a shindo 7 tremor, people would be "thrown by the shaking and impossible to move", according to the agency.
- In the magnitude 7.3 earthquake that struck the western Japanese city of Kobe in 1995 killing over 6,400 people , some spots recorded a "shindo" of 7.
EPICENTRE OR FOCUS?:
- The epicentre is the point on the earth's surface vertically above the focus, or hypocentre, point in the crust where the earthquake is triggered.
- The actual shaking felt by an earthquake of the same magnitude may vary greatly by the depth of the focus and the earth's layer.
(Source: U.S. Geological Survey, Japan Meteorological Agency)
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