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As defined, an earthquake magnitude scale has no lower or upper limit. Sensitive seismographs can record earthquakes with magnitudes of negative value and have recorded magnitudes up to about 9.0. (The 1906 San Francisco earthquake, for example, had a Richter magnitude of 8.25.)
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Read More »Earthquake magnitude is a measure of the “size,” or amplitude, of the seismic waves generated by an earthquake source and recorded by seismographs. (The types and nature of these waves are described in the section Seismic waves.) Because the size of earthquakes varies enormously, it is necessary for purposes of comparison to compress the range of wave amplitudes measured on seismograms by means of a mathematical device. In 1935 the American seismologist Charles F. Richter set up a magnitude scale of earthquakes as the logarithm to base 10 of the maximum seismic wave amplitude (in thousandths of a millimetre) recorded on a standard seismograph (the Wood-Anderson torsion pendulum seismograph) at a distance of 100 km (60 miles) from the earthquake epicentre. Reduction of amplitudes observed at various distances to the amplitudes expected at the standard distance of 100 km is made on the basis of empirical tables. Richter magnitudes M L are computed on the assumption that the ratio of the maximum wave amplitudes at two given distances is the same for all earthquakes and is independent of azimuth. Richter first applied his magnitude scale to shallow-focus earthquakes recorded within 600 km of the epicentre in the southern California region. Later, additional empirical tables were set up, whereby observations made at distant stations and on seismographs other than the standard type could be used. Empirical tables were extended to cover earthquakes of all significant focal depths and to enable independent magnitude estimates to be made from body- and surface-wave observations. A current form of the Richter scale is shown in the table. Richter scale of earthquake magnitude magnitude level category effects earthquakes per year less than 1.0 to 2.9 micro generally not felt by people, though recorded on local instruments more than 100,000 3.0–3.9 minor felt by many people; no damage 12,000–100,000 4.0–4.9 light felt by all; minor breakage of objects 2,000–12,000 5.0–5.9 moderate some damage to weak structures 200–2,000 6.0–6.9 strong moderate damage in populated areas 20–200 7.0–7.9 major serious damage over large areas; loss of life 3–20 8.0 and higher great severe destruction and loss of life over large areas fewer than 3 At the present time a number of different magnitude scales are used by scientists and engineers as a measure of the relative size of an earthquake. The P-wave magnitude (M b ), for one, is defined in terms of the amplitude of the P wave recorded on a standard seismograph. Similarly, the surface-wave magnitude (M s ) is defined in terms of the logarithm of the maximum amplitude of ground motion for surface waves with a wave period of 20 seconds. As defined, an earthquake magnitude scale has no lower or upper limit. Sensitive seismographs can record earthquakes with magnitudes of negative value and have recorded magnitudes up to about 9.0. (The 1906 San Francisco earthquake, for example, had a Richter magnitude of 8.25.)
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Read More »On , the most powerful earthquake in recorded history—magnitude 9.5—struck southern Chile. The rupture zone stretched from estimates ranging from 500 kilometers (311 miles) to almost 1,000 kilometers (621 miles) along the country's coast.
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