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摘要
Deviation of earthquakes from the double-couple mechanism is an important, but delicate tool to study their source processes. For assessing the double-couple percentage, the paper suggests to complement the standard least-square moment-tensor retrieval with a hierarchic spatio-temporal grid search, progressively closer to the true source position and time. It enables identification of the double-couple percentage convergence, while its limit is the resulting double-couple percentage value, or range. The so-called double-couple percentage (DC%) versus correlation plots are introduced and difficulties of the double-couple percentage assessment are discussed. It is proved that even close to the true source position, where the strike-dip-rake angles are already stable (within a few degrees), the double-couple percentage may still vary by dozens of per cent. Moreover, even at the optimum spatial position, the double-couple percentage estimate is extremely sensitive (0 to 100%) to small variations of the subevent origin time. This behaviour is explained in terms of the source complexity, implying a time-dependent moment tensor. Therefore, the double-couple percentage of complex events depends on the studied frequency band and, in general, also on the station azimuth. This explains broad variations of the double-couple percentage reports among seismic agencies. Three earthquakes of mutually close epicentres were investigated (Zakynthos, Western Greece, April 2006, magnitudes similar to 5.5) and a strong non-double-couple component of one of them was identified (double-couple percentage of about 20%). Two equivalent models of this earthquake were found: a single-event non- double-couple model, and a double-event model consisting of two double-couple sources with highly different mechanisms.
- 出版日期2008-5
