The aftermath of an earthquake: Aftershocks
After the big quake had shaken NZ two weeks ago, I thought that the danger was over as soon as the quake has finished. It luckily only killed two persons. As I thought that the danger was over my idea was that people could get back to normal and start to rebuild their homes now.
I was wrong.
I started to read the NZ herald on my phone during one of those really boring seminars in the afternoon. I read a lot about evacuations and dangers I didn’t really think about as I’ve never been to seismic active areas.
One example was the evacuation of the IRD headquarters in Wellington due to damage in a stairwell.
Another one was about 30 people being evacuated from Goose Bay because of a landslide which could come down to the area. The danger was caused by ongoing seismic activity in the aftermath of the quake.
Where does this ongoing seismic activity come from?
The answer: aftershocks.
The following question: How do they work? How many are there?
I conducted some research, checked wikipedia on the issue and found this graphic concerning the Kaikoura quake.
Almost 4000 aftershocks in less than a week after the initial quake… Although they were not that strong, as you can see in the graph — 1 out of 10 is above magnitude four — I couldn’t imagine the sheer number. Can you?
Nevertheless, why does this happen?
NZ is situated on the ring of fire. To get an idea what that means, the NZ Herald gives a first hint its article about the ring of fire: “No place on Earth is safe from the force of Mother Nature but countries on the “Ring of Fire” are certainly most vulnerable.”
The article furthermore explains the ring of fire: “The Ring of Fire is a horseshoe-shaped band of fault lines that circles the Pacific Basin and is prone to frequent earthquakes and volcanic eruptions. It is the most seismically active zone on the planet.”
So far, so good. NZ is shaken because of the tectonic plates and their movement at the fault lines. Nevertheless, that doesn’t explain the aftershocks… Luckily I found another article which presents the basic principle in a way everybody is able to understand… especially those of us who are not really concerned with physics and geology:
“Dig a hole at the beach, then watch as surrounding sand pours into it, each grain trickling down and consolidating its new position.” The big quake is like the guy that digs the hole. The aftershocks are the sand grains that reorder in order to fill the big whole. Following the logic behind that, the aftershocks take some time. The finish when every grain of sand has found his stable place. As a matter of fact, this idea can also be found in the graphic above: The number of aftershocks per day decreases with ongoing time. There you go, that’s how the aftershocks work. In fact the tectonic movement takes part in the first quake, while the aftershocks are an aftereffect of the movement but not directly caused by it.
Although aftershocks are really annoying, I found something quite beautiful to my mind: bubbles from the bottom of the ocean, caused by seismic activity... and a seal eating an octopus. I think that’s the best way to end this article, so I’ll just drop this vid here…
