March 2, 2011

Aftershocks in Christchurch

Posted by Austin Elliott

No doubt most people’s most recent earthquake-related thoughts pertain to last week’s temblor in New Zealand. Of course as we’ve all heard this was a particularly unfortunate event because of the location and modest depth of its source: merely ~5km below the streets and only a few measly kilometers from the city’s dense and populous center. We’ve all also heard it described as an aftershock of a bigger quake last September, a quake which used to be “the big Christchurch earthquake”. Many people may be surprised to hear such a significant event deemed an aftershock, so let’s explore aftershocks for a bit.

There’s a more technical spin-off discussion here, which Chris Rowan has done a marvelous job covering in a post on Highly Allochthonous. He examines the difference between an “aftershock” and a “triggered earthquake”. The gist of his distinction between the two is that aftershocks are earthquakes that occur adjacent to a large rupture due entirely to the immense change in stress accompanying the rupture; triggered quakes are those that were bound to happen in the near future and only required a small change in stress (provided by a large rupture nearby–or perhaps distant) to let loose. Triggered earthquakes are a fascinating subject, but for now let’s just talk about aftershocks and related earthquake sequences as a whole.

Firstly, have a look at this interactive map of the Christchurch earthquake sequence, put together by Paul Nicolls at New Zealand’s University of Canterbury. Atop a Google Maps applet the series of earthquakes appear through time, colored by depth and sized by magnitude–a fascinating way to visualize how this sequence of seismic events unfolded. Notice the conspicuous bursts of activity following the largest events.

One of the most unsettling aspects of major earthquakes is that they are followed by literally thousands of subsequent tremors. People who have been traumatized by the first violent temblor are virtually constantly beleaguered by the startling jolts of subsequent tremors. You begin to get a sense of the incessant rumblings in this couple’s video of the quake damage at their house: there are aftershocks at 0:43, 1:43, 3:08, and 4:53 in the clip. Don’t mind the swearing, it’s natural.


After September’s quake, the ongoing sequence of aftershocks became a fact of life in and around Christchurch. The following three videos show some of the bigger aftershocks preceding the 6.3. As far as New Zealander’s knew, they were dying down.



(and my personal favorite)


There are plenty more where those came from.

In the immediate aftermath of a large quake, this constant shuddering can understandably be much more alarming. Other than rattling already frayed nerves and doing further harm to damaged buildings [minute 2:55], the big problem–as illustrated in Christchurch–is that location is important. If the 6.3 had started where the 7.0 did, it would indeed have been less trouble than the original quake. Instead it occurred at the near end of the September rupture, supplying a much sharper dose of strong shaking to the city than they had been subjected to by the somewhat removed mainshock.

When a large earthquake occurs (what makes an earthquake turn into a large one and not just stop at a tiny M3 is a frustratingly unknown part of the earthquake process, and it drives lots of debate as well as lots of research) it drastically alters the stress in the crust around it (see pretty pictures at the bottom), requiring major adjustments of the surrounding rock. Imagine tightly crumpling a plastic grocery bag and leaving it on the floor for it to slowly crackle and unfurl until it’s sufficiently unconstricted to settle calmly. That’s more or less what goes on after the huge readjustment of the crust in an earthquake. Each of those little crackles is a new earthquake–an aftershock–generally smaller than the original. In fact, after decades of recording earthquakes, we’ve found that aftershocks follow very regular patterns. Over time the likelihood of having an aftershock of a certain magnitude drops logarithmically. That doesn’t mean that over time they become smaller, it means that it rapidly becomes less likely that you’ll have a biggish one. It still may happen. No wonder people’s nerves are on edge; they should be! In many parts of the world earthquakes are an inevitable reality. Earthquakes beget other earthquakes. Large seismic events initiate a cascade of yet unpredictable seismic shifts, and it pays to be mentally and physically prepared.