How long will the aftershocks continue?
All earthquakes strong enough to do damage are followed by aftershocks. Extensive experience with aftershock sequences and seismic networks of high enough quality to record them allow us to make some generalizations. Most aftershock sequences follow two trends: 1) the number of aftershocks decreases exponentially, and 2) the maximum magnitude of the aftershocks decreases, with time after the initial shock. We could refine this generalization more by comparing the current aftershock sequence with one from a previous Hawaiian earthquake.
Since our modern seismic network was installed in about 1960, only one earthquake similar in depth and probable mechanism has occurred on April 26, 1973 - just off Honomu on the Hamakua coast - and was located at a depth of 39 km. The graph below shows the number of aftershocks per day having magnitudes greater than M1.8 that followed.
The number of aftershocks per day decreases approximately exponentially; more than four weeks later, the aftershocks were occurring infrequently. On the same time scale, the graph below shows the daily number of aftershocks (as of November 12) for the M6.7 Kiholo Bay earthquake that occurred on October 15, 2006, at a depth of 39 km.
The number of aftershocks is decreasing rapidly, but they haven't stopped, just as in 1973. Using 1973 as an analog, we might expect the current aftershock sequence to continue at low levels for a few months to a year.
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Updated: 12 November 2006 (pnf)