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thumb The focal mechanism of an earthquake describes the deformation in the source region that generates the seismic waves. In the case of a fault-related event it refers to the orientation of the fault plane that slipped and the slip vector and is also known as a fault-plane solution. Focal mechanisms are derived from a solution of the moment tensor for the earthquake, which itself is estimated by an analysis of observed seismic waveforms. The focal mechanism can be derived from observing the pattern of "first motions", that is, whether the first arriving P waves break up or down. This method was used before waveforms were recorded and analysed digitally and this method is still used for earthquakes too small for easy moment tensor solution. Focal mechanisms are now mainly derived using semi-automatic analysis of the recorded waveforms.〔Sipkin, S.A., 1994, Rapid determination of global moment-tensor solutions: Geophysical Research Letters, v. 21, p. 1667–1670.〕 ==Moment tensor solutions== thumb The moment tensor solution is typically displayed graphically using a so-called ''beachball'' diagram. The pattern of energy radiated during an earthquake with a single direction of motion on a single fault plane may be modelled as a ''double couple'', which is described mathematically as a special case of a second order tensor (similar to those for stress and strain) known as the moment tensor. Earthquakes not caused by fault movement have quite different patterns of energy radiation. In the case of an underground nuclear explosion, for instance, the seismic moment tensor is isotropic and this difference allows such explosions to be easily discriminated from their seismic response. This is an important part of monitoring to discriminate between earthquakes and explosions for the Comprehensive Test Ban Treaty. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「focal mechanism」の詳細全文を読む スポンサード リンク
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