A double couple can be viewed as "equivalent to a pressure and tension acting simultaneously at right angles". If a second couple of equal and opposite magnitude is applied their torques cancel this is called a double couple. In mechanics (the branch of physics concerned with the interactions of forces) this model is called a couple, also simple couple or single couple. If the pair of forces are offset, acting along parallel but separate lines of action, the object experiences a rotational force, or torque. A pair of forces, acting on the same "line of action" but in opposite directions, will cancel if they cancel (balance) exactly there will be no net translation, though the object will experience stress, either tension or compression. If it has sufficient strength to overcome any resistance it will cause the object to move ("translate"). The simplest force system is a single force acting on an object. An early step was to determine how different systems of forces might generate seismic waves equivalent to those observed from earthquakes. The study of earthquakes is challenging as the source events cannot be observed directly, and it took many years to develop the mathematics for understanding what the seismic waves from an earthquake can tell us about the source event. These had M s magnitudes of 8.5 and 8.4 respectively but were notably more powerful than other M 8 earthquakes their moment magnitudes were closer to 9.6 and 9.3. A particular problem was that the M s scale (which in the 1970s was the preferred magnitude scale) saturates around M s 8.0 and therefore underestimates the energy release of "great" earthquakes such as the 1960 Chilean and 1964 Alaskan earthquakes. Additional scales were developed – a surface-wave magnitude scale ( M s) by Beno Gutenberg in 1945, a body-wave magnitude scale ( mB) by Gutenberg and Richter in 1956, and a number of variants – to overcome the deficiencies of the M L scale, but all are subject to saturation. At greater depths, distances, or magnitudes the surface waves are greatly reduced, and the local magnitude scale underestimates the magnitude, a problem called saturation. The local magnitude scale was developed on the basis of shallow (~15 km (9 mi) deep), moderate-sized earthquakes at a distance of approximately 100 to 600 km (62 to 373 mi), conditions where the surface waves are predominant. (This scale is also known as the Richter scale, but news media sometimes use that term indiscriminately to refer to other similar scales.) He established a reference point and the now familiar ten-fold (exponential) scaling of each degree of magnitude, and in 1935 published what he called the "magnitude scale", now called the local magnitude scale, labeled M L . Richter then worked out how to adjust for epicentral distance (and some other factors) so that the logarithm of the amplitude of the seismograph trace could be used as a measure of "magnitude" that was internally consistent and corresponded roughly with estimates of an earthquake's energy. The initial step in determining earthquake magnitudes empirically came in 1931 when the Japanese seismologist Kiyoo Wadati showed that the maximum amplitude of an earthquake's seismic waves diminished with distance at a certain rate. History Richter scale: the original measure of earthquake magnitude Īt the beginning of the twentieth century, very little was known about how earthquakes happen, how seismic waves are generated and propagate through the earth's crust, and what information they carry about the earthquake rupture process the first magnitude scales were therefore empirical. Subtypes of the moment magnitude scale (M ww , etc.) reflect different ways of estimating the seismic moment. Geological Survey for reporting large earthquakes (typically M > 4), replacing the local magnitude (M L ) and surface wave magnitude (M s ) scales. It has become the standard scale used by seismological authorities like the U.S. It is more directly related to the energy of an earthquake than other scales, and does not saturate – that is, it does not underestimate magnitudes as other scales do in certain conditions. Moment magnitude (M w ) is considered the authoritative magnitude scale for ranking earthquakes by size. Despite the difference, news media often says "Richter scale" when referring to the moment magnitude scale. Similar to the local magnitude/Richter scale (M L ) defined by Charles Francis Richter in 1935, it uses a logarithmic scale small earthquakes have approximately the same magnitudes on both scales. It was defined in a 1979 paper by Thomas C. The moment magnitude scale ( MMS denoted explicitly with M w or Mw, and generally implied with use of a single M for magnitude ) is a measure of an earthquake's magnitude ("size" or strength) based on its seismic moment.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |