Because the frequency distribution of radiated seismic energy changes with earthquake size (e.g., Aki 1967), magnitude scales suffer severe intrinsic limitations, such as saturation (Kanamori 1977 Hanks and Kanamori 1979 Hutton and Boore 1987) and discrepancies between the scales (Gutenberg and Richter 1956). The magnitude by definition quantifies the energy radiated over a particular fixed frequency band, M 0 is estimated seismically from the amplitude of far-field “long-period” seismic radiation (Aki 1966). This enables the wide acceptance of M W as a stable scale, for larger as well as small to moderate magnitude earthquakes (Lay and Wallace 1995). The moment magnitude scale ( M W), as defined by Kanamori ( 1977), has an advantage of not getting saturated for larger earthquakes, unlike the Richter amplitude-based scales (e.g., Hanks and Kanamori 1979 Howell 1981 Ottemoller and Havskov 2003). Based on M 0, moment magnitude M W has been defined by Hanks and Kanamori ( 1979). From then on, seismic moment was considered as a new parameter to specify the size of an earthquake. ( 1969) also suggested that the far-field static-strain field is proportional to M 0. Aki ( 1966) first measured the value of M 0 of the 1964 Niigata, Japan, earthquake. Following Richter, multitude of magnitude scales, each defined in terms of amplitudes recorded on a particular type of seismograph (i.e., over a particular limited spectral band) have been introduced.įrom the study of source mechanism by an elastic dislocation theory, Aki ( 1966, 1967) stated that the amplitude of a very long-period wave is proportional to the seismic moment, M 0, of an earthquake. The Richter local magnitude M L scale (Richter 1935) for an earthquake is still widely used in different parts of the world. Intensity 1: Not felt - Not felt except by a very few under especially favorable conditions.The earthquake magnitude is regarded as the most directly measurable and simple parameter to specify quantitatively the size of an earthquake. Intensity 2: Weak - Felt only by a few persons at rest, especially on upper floors of buildings. Vibrations similar to the passing of a truck. Many people do not recognize it as an earthquake. Intensity 3: Weak - Felt quite noticeably by persons indoors, especially on upper floors of buildings. Sensation like heavy truck striking building. Dishes, windows, doors disturbed walls make cracking sound. Intensity 4: Light - Felt indoors by many, outdoors by few during the day. Intensity 5: Moderate - Felt by nearly everyone many awakened. Some heavy furniture moved a few instances of fallen plaster. Intensity 6: Strong - Felt by all, many frightened. Intensity 7: Very strong - Damage negligible in buildings of good design and construction slight to moderate in well-built ordinary structures considerable damage in poorly built or badly designed structures some chimneys broken. Fall of chimneys, factory stacks, columns, monuments, walls. ![]() Intensity 8: Severe - Damage slight in specially designed structures considerable damage in ordinary substantial buildings with partial collapse. Damage great in substantial buildings, with partial collapse. Intensity 9: Violent - Damage considerable in specially designed structures well-designed frame structures thrown out of plumb. ![]() Intensity 10: Extreme - Some well-built wooden structures destroyed most masonry and frame structures destroyed with foundations. (USGS) I’ve never heard of this intensity scale. Meanwhile, one hypothetical magnitude 7.8 earthquake on the San Andreas fault could send “extreme,” or Intensity 10, shaking across most of urbanized Southern California. So, for instance, a magnitude 8.2 - probably the strongest earthquake that could hit Southern California on the San Andreas fault - would produce an astonishing 178 times more energy than the magnitude 6.7 Northridge earthquake in 1994. Geological Survey has a calculator that can help you make these calculations. The moment magnitude scale measures the movement of rock along the fault, and accurately measures larger earthquakes, which can last for minutes and affect a much larger area the Richter scale did not accurately record such quakes, Jones said. On the so-called Richter scale, a magnitude 8 on a seismogram was 10 times bigger than a magnitude 7.īut the Richter scale was eventually scrapped in favor of what is known as the moment magnitude scale. Magnitude was about how big the waves were on a seismogram at a particular distance from the epicenter. Originally, the definition of magnitude related to seismograms, in which machines used an ink stylus to record rapid motions on a rolling drum of paper that would measure shaking.
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