Solomon Islands Earthquakes: Foreshocks, Mainshocks and Aftershocks

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More earthquakes in the Santa Cruz Islands, in the Solomon Islands area – and they’re still pretty large. Are these foreshocks, aftershocks, or what?

Forshocks, mainshock and aftershocks in teh Solomon Islands, February 2013. Image credit: USGS

Forshocks, mainshock and aftershocks in teh Solomon Islands, February 2013. Image credit: USGS

It’s a restless Earth we live on. Certainly it must seem like that for residents of the Solomon Islands. This week they’ve experienced what must feel like a lifetime of earthquakes. Over a dozen quakes exceeding magnitude 5 (≥M5.0) between the first and the fifth of February could be deemed significant activity; but these were followed by a tremor of M8.0. Statistically speaking, you might expect only one or two of this magnitude in a year.

And it goes on. Earthquake after earthquake followed, and continues to follow. At the time of writing at least 80 aftershocks of ≥M5.0 have been recorded.  So what are all these earthquakes and what, if anything, do they tell us?

Foreshocks, Mainshocks and Aftershocks

In the sometimes-complicated language of earth science, the concept of shocks is among the easiest to understand. It’s self-defining. In any sequence of earthquakes, the mainshock is the largest. Those which come before it are foreshocks and those which come after it are aftershocks – and we don’t know which is the mainshock until the sequence of earthquakes is completed.

That’s as simple as it gets. Earth scientists have evolved complicated laws to show that (generally speaking) there’s a decrease in the size of aftershocks over time (Omori’s Law) and a relationship between the size of the mainshock and the largest aftershock (Bath’s Law) but even these don’t always hold true. Yeats refers to ‘sets ‘ of large earthquakes occurring in the Solomon Islands and just last year the two largest earthquakes of 2012, an M8.6 and an M8.2, occurred within hours of each other in the Indian Ocean – making the M8.2 a very large aftershock indeed.

Earthquakes: Patterns of Aftershocks

Graph of aftershock patterns. Image creidt: USGS

Graph of aftershock patterns. Image credit: USGS

Again it seems self-evident that the mainshock, by definition, releases most of the accumulated strain which eventually triggers the earthquake, but not all. It is generally true that the larger the earthquake, the larger, and greater in number, the aftershocks will be (the 2011 Tohoku-Oki earthquake, an M9.0, had aftershocks of M7.9, M7.7, M7.1 and M7.0, in that order though interspersed with numerous smaller tremors).

Although most aftershocks occur relatively close to the epicentre of the mainshock and the area of the fault which has been ruptured, Liu and Stein note that aftershocks following larger earthquakes can be more widely dispersed and also that seismic waves can trigger tremors at even greater distances: these, too, should be regarded as aftershocks.

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