How Are Earthquake Magnitudes Measured?

The Richter Scale

There are a number of ways to measure the magnitude of an earthquake. The first widely-used method, the Richter scale, was developed by Charles F. Richter in 1934. It used a formula based on amplitude of the largest wave recorded on a specific type of seismometer and the distance between the earthquake and the seismometer. That scale was specific to California earthquakes; other scales, based on wave amplitudes and total earthquake duration, were developed for use in other situations and they were designed to be consistent with Richter’s scale.

Unfortunately, many scales, such as the Richter scale, do not provide accurate estimates for large magnitude earthquakes. Today the moment magnitude scale, abbreviated MW, is preferred because it works over a wider range of earthquake sizes and is applicable globally. The moment magnitude scale is based on the total moment release of the earthquake. Moment is a product of the distance a fault moved and the force required to move it. It is derived from modelling recordings of the earthquake at multiple stations. Moment magnitude estimates are about the same as Richter magnitudes for small to large earthquakes. But only the moment magnitude scale is capable of measuring M8 (read ‘magnitude 8’) and greater events accurately.The Moment Magnitude Scale

Magnitudes are based on a logarithmic scale (base 10). What this means is that for each whole number you go up on the magnitude scale, the amplitude of the ground motion recorded by a seismograph goes up ten times. Using this scale, a magnitude 5 earthquake would result in ten times the level of ground shaking as a magnitude 4 earthquake (and 32 times as much energy would be released). To give you an idea how these numbers can add up, think of it in terms of the energy released by explosives: a magnitude 1 seismic wave releases as much energy as blowing up 6 ounces of TNT. A magnitude 8 earthquake releases as much energy as detonating 6 million tons of TNT. Pretty impressive, huh? Fortunately, most of the earthquakes that occur each year are magnitude 2.5 or less, too small to be felt by most people.

Recent Large Earthquakes Magnitudes in Kefalonia

We will shortly publish further articles giving more detail on these and other significant seismic events that impacted Cephalonia.

Sources – cbs.ca & iris.edu

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