Monday, November 22, 2010

The power of an earthquake

I came across a very interesting analysis of some of the surface effects of the earthquake in New Zealand last September. Writing in 'The Landslide Blog' on the American Geophysical Union Web site, Prof. Dave Petley reports:

The farmer at this site very kindly allowed us to enter his field ... where the array of cracks, and associated deformation, is astonishing. GNS have an aerial view of the field.

... below is the ground view:

. . .

A few more kilometres to the west and we are into the maximum displacement area. Here we see over 3 metres [almost 10 feet] of horizontal deformation and about 1.5 metre [almost 5 feet] vertically. This is the view from the hanging wall side – the drop down onto the footwall, and the lateral motion should be evident:

The picture below was taken on the footwall side looking back towards the fault – note the horizontal motion (the road has been patched up) and the vertical change. Remember that this was a straight road across a flat plain before the earthquake:

There's much more at the link, including more photographs. In a follow-up report, Prof. Petley provides photographs of the damage done to a railway line by the earthquake. Here's one of them.

That's quite a bend! I hope no train came along before it was spotted and the line closed . . . otherwise things might have got very interesting, very quickly! I find it fascinating how the horizontal and vertical shift occurred along the line of the fault beneath the earthquake zone, almost in a straight line across the countryside. Prof. Petley provides a map that shows how it ran.

If you, like me, are interested in how earthquakes move things around, and change the landscape, both reports are well worth reading.



Morris said...

Looking at that railway line reminded me of the quake we had here in Western Australia back in 1968.
It was one of the most powerful we've ever had here in Australia.

A link for info:

Doing a Google image search on 'Meckering Earthquake' will get you quite a few interesting pics.

Ritchie said...

In the third pic, it appears the fence line and hedgerow have both been displaced, but the electric poles have been reset.

Unknown said...

As a lifelong Christchurch resident I lived through the earthquake. It was a miracle that no one was killed in the quake, helped by it striking at 4.30 in the morning. There have been 10000-15000 house that will be demolished plus 2000+ commercial building that have been or will be demolished. I just saw, through work, a house today that has had one wing of the house separate from the rest of the house by around a foot and has dropped 6-8 inches, thankfully the elderly residents had their bedrooms at the other end of the house.

Anonymous said...

Near San Francisco, CA there's an Earthquake state park dedicated to the 1906 temblor. It has the displaced fence lines, shows hanging walls and other geology stuff, interpreted for the interested but not expert. The author of "Finding Faults in California," a geology road guide, highly recommends the park.

Janeen said...

I'm a geologist and this post was one of the highlights of my day! The first photo (the aerial view of the fault) appears to show textbook examples of shear and strain fracturing. Amazing!