Aftershock

Aftershocks of the Tonga quake are continuing, with the largest so far being magnitude 6.0. Most people will believe and tell you that earthquakes are completely random, unexpected and unpredictable, but this isn't strictly true. About a third of the total number of earthquakes that have ever been recorded have been predictable in many aspects, because they are aftershocks. The somewhat obvious reason that they are aftershocks is that they follow a larger mainshock. We know in general that the largest aftershock of an earthquake is usually around one Richter magnitude lower than the mainshock, and that the number of aftershocks will generally increase by about a factor of 10 for each magnitude below that. The aftershocks tend to become less frequent with time, but the individual aftershock magnitudes are more or less random as far as we know today. The strongest aftershock will generally occur close to the mainshock only because more aftershocks occur in the near timeframe than will occur later. However, there is no reason that the strongest aftershock has to be one of the first, so occasionally the largest aftershock can be weeks, months or years later. These relationships are so well known that the USGS turns them into a map of the current California aftershock hazard. This map is updated continuously and represents with "hot" colors areas of the state where damaging aftershocks may be expected to occur. Back to the Tonga quake, I would make a guess that an aftershock of magnitude 6.5 or higher is still in the works within the next few weeks. For more depth on the subject, it's hard to beat this discussion of aftershocks made available on the internet by Susan Hough of the USGS Pasadena.