Many years ago, I tried beta-reading a book for an acquaintance. It was a geological thriller based on earthquakes. I got as far as the end of the first chapter, where one of the characters told another they'd just identified the earthquakes occurring as a foreshock sequence. I never did find a good or elegant way to gently explain to the author that their entire plot was based on an error. So I just quietly ghosted and have remained haunted by it ever since.

Yes, I'm a lily-livered coward.

But here's the thing about earthquake sequences: you don't know what you've got til the shaking's all over.

Foreshocks aren't earthquake that announce themselves by a special seismic signature. You can't tell from looking at them that they're not the main event or the reverberations from it. You won't have to worry about being plopped down in a seismology class, handed a sheaf of seismograms for each individual earthquake in a sequence with date and time stamps removed, and have to put each one in order of foreshock, mainshock, and aftershock. It can't actually be done.

Image is a seismogram showing three different earthquakes. To the far right is a small squiggle in green representing an M 1.7 foreshock. To the left and below it is a large, thick black squiggle representing a M 3.3 mainshock. In the center is a tall but thin black squiggle representing a M 2.9 aftershock.
Seismogram showing the foreshock, mainshock, and aftershock in a small earthquake sequence. Credit: USGS

No, what you need is a full sequence of earthquakes. Then you can start to sort through them all and put them in context. Take, for instance, the most recent earthquake swarm near Valparaiso, Chile, which we interviewed Lori for last month. When the shaking first started, no one knew if this was just going to be a series of wee little tremors with no big shake, or these were foreshocks of a much larger earthquake. It wasn't until April 24th, when a M 6.9 struck, that we could know that yep, these sure were foreshocks.

That April 24th quake is considered the mainshock for now, but here's the possibly shocking thing about foreshocks: even when it seems you've had your mainshock, and there's lots of aftershocks following, and everything has been neatly sorted, it's possible that an even bigger quake will soon come along. And that means that all of your quakes which you thought were a tidy sequence were, in fact, foreshocks, each and every one.

In fact, you may not know for years whether a particular set of earthquakes was actually a series of foreshocks. It's rare, but some foreshocks happen years before the Big One. I don't know about you, but I find that pretty neat.

Not every earthquake comes with a foreshock. If you have a sequence of small earthquakes, those are typically just a swarm, with no clear main event. Some earthquakes, even large ones, never have a foreshock at all – which means that foreshocks don't do much to help us predict major earhtquakes.

Larger earthquakes, ones M 7.0 or greater, are more likely to be preceded by foreshocks. So it would be really useful if, someday, we discovered some subtle thing in the seismic signature that marked foreshocks as unique from any other type of earthquake. Unfortunately, we're nowhere close to being able to determine that with our current instrumentation, and it may turn out that there's really nothing that sets them apart in advance of the mainshock.

And this, folks, is why foreshocks fascinate me, yet give me a tiny frisson of shame each time I encounter them.

If you want to see some very neat videos of the shaking down in Chile, check out this post over at The Trembling Earth.