Seismic Moment 2
Seismic Moment 2 : An earthquake parameter recendy introduced in seismology is that of the seismic moment (Mo). The seismic moment is related to fundamental parameters of the faulting process resulting from earthquakes in the following way: The amplitude of every long period seismic wave is proportional to the surface area of the fault, the average displacement on the fault plane, and the rigidity of the material at the fault. The result of these three quantities is the seismic moment, Mo, which after some corrections, can be calculated from measurement of long-period seismic waves. The seismic moment is the most suitable parameter representing the size of an earthquake because, unlike magnitude obtained from surface waves, it does not saturate for larger shocks. Another advantage of the seismic moment is that it represents, in addition to the size of the earthquake, the overall deformation at the source. The seismic moment is determined by the Fourier analysis of seismic waves of such long period that the details of the rupture are smoothed out, with the effect that the entire fault may be considered to be a point-source. The periods at which the seismic moment are determined increase with the size of the fault. If the fault is based on such long-period waves, the slip from unruptured to ruptured state appears to be instantaneous. The actual pattern of the seismic radiation emitted by the instantaneous rupture is mathematically equivalent to the theoretical radiation pattern emitted by two hypothetical torque couples embedded in an unruptured elastic medium. The torque couples, rotating in opposite directions, deform the medium thus radiating elastic waves in a pattern identical with that in which an earthquake source radiates seismic waves. The moment can be calculated from this model. Earthquake magnitudes calculated by the use of seismic moment for large historic earthquakes would be greater than the assigned Richter magnitudes. For example, the 1964 Alaskan earthquake's assigned magnitude (Ms), was 8.4 while the magnitude (Mw) based on the seismic moment was 9.2. Similarly, for the 1960 Chilean earthquake, Ms was 8.3 whde Mw would be 9.2. Scientists have concluded that comparison of earthquake magnitudes obtained from surface waves and seismic moments agree quite well for earthquakes with smaller fault dimensions, but do not agree for earthquakes of larger fault dimensions
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