Wednesday, October 13, 2010

New Zealand Earthquake Aftershock Sequence

As I've always said, something is very weird about this earthquake.  I'm saying it's a 'hard rock' complex earthquake, that rupture in distinct sections.  The maximum section was probably an M6.0, in terms of strong ground motions, which were about 1-2 cm/s on firm ground.  There were no near-field recordings on firm ground.

Since the instrumentation was inadequate to resolve this, we can look at the aftershock sequence.  On one hand, we have this article, which says everything is perfectly normal for an M7.1.   For those of you who believe the official line, you can stop here.  :)

However, for an M7.1, I would expect the aftershock sequence to cascade down from an M6.0.  Here is a graph from the Parkfield M6.0.

This is what I grew up with!  Of course, there must be exceptions, but I've never seen any.  Now, the Parkfield was a very 'clean' M6, which means that it ruptured in a single 'whoosh'.  Many, many large (M8 or more) earthquakes have also ruptured in distinct segments, and that requires very good instrumentation to reconstruct the sequence.  The destructive M7's on blind thrust faults have also ruptured clean.

As usual, I'm just relying on my experience, and don't want to engage in any academic hissy fights.  :)

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