| Popis: |
Major earthquakes are always followed by aftershocks, but the occurrence of large ones which could themselves increase the damage, is never systematic. Using the NEIC data set of focal parameters and hypocenter determinations over a period of 13 years and some corresponding values of the magnitude Ms inferred from a very broadband seismographic station, we characterise major earthquake sequences according to the faulting mechanism of the main shock, i.e, reverse (1), normal (2) and strike slip (3), and make comparisons between them. We show that the slope of the linear regression curve, log M0 = f(Ms), is greater in (1) than in (3). As a consequence, the stress drop in (1) is lower than in (3). This result implies that: (a) — the magnitude difference between the main shock and the largest aftershock is smaller in (1) than in (3); and (b) — the duration of the aftershock sequence is longer in (1) than in (3). Examples are shown for worldwide earthquakes and for some Californian events. They confirm that generally, the aftershock activity is more important in the case of reverse faulting earthquake than in a strike-slip one. We do not infer any conclusion concerning (2). |
