Why do aftershocks occur in different places

Darfield's shaky legacy has continued with many more notable quakes - including the 5. Aftershock activity also wasn't a case of only neighbouring faults shifting loads between each other.

A significant aftershock could change stresses on a fault far from the epicentre with the seismic waves it sent propagating through the earth. This week, GNS scientists said they couldn't rule out the possibility that passing seismic waves from the 7. The scale of all of these changes, however, could be related back to the scale of the initial earthquake.

It was also possible to say some places tended to generate more aftershocks than others. This was especially true for the Marlborough Fault Zone, where seismic activity had been ever more heightened by the 6.

At GNS Science's offices in Wellington, GeoNet officers have been mapping the Kaikoura Earthquake's aftershock sequence and feeding the data into statistical models that calculate forecasts. GeoNet's quakesearch tool shows how the aftershocks have been clustered relatively tightly around those places in Marlborough where faults - the Kekerengu, the Hundalee, the Hope - violently ruptured.

The area nearest those faults had a probability of 80 per cent or more for damaging shaking in the next 30 days, while the probability of damaging shaking in the Wellington area was put at less than 10 per cent. While this probability was considerably lower than in other regions, it was possible for shaking similar to what occurred last Monday to happen again in the capital.

The GeoNet probabilities also pointed to a virtual certainty of many more aftershocks large enough to be felt, although numbers were decreasing with each new forecast issued. The latest gave a 99 per cent chance of an average 21 quakes between 5. In this case, geologists rename the first earthquake, calling it a foreshock, and the aftershock becomes the primary earthquake. Also, there are usually other faults nearby that have built up strain over the years.

A nearby earthquake may push these faults over the edge. These events are not considered aftershocks, however, because the added stress from the earthquake was just the tipping point that triggered the fault to release its pent-up energy, resulting in a new earthquake.

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Skip to main content Skip to main menu Skip to search Skip to footer. Search for:. Manage subscription. Subscribe to the Monitor. Monitor Daily current issue. Monitor Weekly digital edition. Community Connect. Lots of maps, not many photos, right? That's because not a lot's going on there.

Most of it's concealed below the surface, and what's been exposed doesn't look much like a fault. Unless you're a professional, the photo of the fault at this Missouri Department of Natural Resources article doesn't exactly stand out. Ed Yong says ,. In other words, there's not a lot going on that would show at the surface, unlike the San Andreas, which is bleeding obvious.

New Madrid is a slow, sleepy fault, despite the excitement it caused over the winter of Compared to New Madrid, the San Andreas fault is a speed demon, and it shows. There are other differences, of course — one's a transform fault where two plates are scooting past each other, the other's more of a rift type thing where North America started splitting apart, then decided to stay together — but the main thing is speed.

According to the study, San Andreas locks and loads within a decade or so, leaving the aftershocks in the dust and nervous Californians waiting for the Big One. New Madrid's still squirming around trying to get comfortable after a fairly dramatic disruption. And every time it twitches noticeably, folks in the Midwest get twitchy themselves. The river did, after all, run backwards the last time this thing went crack.

Bound to worry folks a bit. But according to Stein and Liu, there's nothing much to worry about — at least, not where New Madrid's concerned.

You're just in for hundreds of years of aftershocks, since the fault moves more than times slower than the San Andreas. This is good news. And the data are beautiful :. And this study points to the fact that the small isn't always a foreshadow of the big :. Sounds like a very good idea to me. Anything we can do to increase the chances of successful earthquake prediction could help save a lot of lives.

And it allows us to rest easier when we find out that those little temblors are just past earthquakes saying "So long, and thanks for all the fish.

The views expressed are those of the author s and are not necessarily those of Scientific American. A confirmed adorer of the good science of rock-breaking, Dana Hunter explores geology with an emphasis on volcanic processes, geology news, and the intersection of science and society. Already a subscriber? Sign in. Thanks for reading Scientific American.