An earthquake is the result of sudden motion of crustal blocks on opposite sides of a fault-plane. The radiation pattern of P-wave amplitude has four lobes of alternating
compression and dilatation (Fig. 3.51). The first motions at the surface of the Earth are either pushes away from the source or tugs toward it, depending on the geometry of the focal mechanism. In contrast, an underground explosion causes outward pressure around the source. The first motions at the surface are all pushes away from the source.
Hence, focal mechanism analysis provides an important clue to the nature of the recorded event. Moreover, an explosion produces predominantly P-waves, while earth-
quakes are much more efficient in also generating surface waves. Consequently, the relative amplitudes of the longperiod surface-wave part of the record and of the short-
period P-wave part are much higher for an earthquake than for an explosion (Fig. 3.51).
Further discrimination criteria are the epicentral location and the focal depth. Intraplate earthquakes are much less common than earthquakes at active plate margins, so
an intraplate event might be suspected to be an explosion.If the depth of a suspicious event is determined with a high degree of confidence to be greater than about 15 km, one
can virtually exclude that it is an explosion. Deeper holes have not been drilled due to the great technical difficulty,e.g., in dealing with the high temperatures at such depths.
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