3 November 2010

## Updates – the Canterbury earthquake railway line; Hurricane Tomas and Haiti; and Manchhar lake in Pakistan

Posted by Dave Petley

This is a general update email on a range of recent posts:

1. The railway line affected by the Canterbury Earthquake in New Zealand

Yesterday I posted the image to the left of the deformation to a railway line were it crossed the surface expression of the fault that was responsible for the Canterbury Earthquake in New Zealand.  A number of people contributed to the discussion (thanks to you all) – the general consensus was that it is indeed a compressional feature associated with a broad zone of deformation rather than a distinct shear plane.  I was emailed by Steve Hill, an engineer, who offered the following explanation (reproduced here):

First, some points about the railway line:

– It is VERY rigid in the vertical axis, not so much in the horizontal. This is so that it can carry load and distribute it over a large number of crossties, yet be bent around a curve without major bending machinery (basically, it can be bent around even tight radius turns with little more than large levers).

– The crossties pin the rails a fixed and uniform distance apart

– In the picture, there is no vertical deformation (see point one) or any rotation of the track assembly.

– This deformation occurred along a VERY straight line of track

– You will note that none of the fasteners that join the rail to the cross-ties have failed.

– If you look at the first picture (the one with the motive power in it) and compare it to the second, you will note that the entire failure is VERY symmetrical.

From here it’s simple geometry: the failure is symmetrical, and failed in both directions at the same time, because geometry requires it. The rails failed in their weakest axis, and since, over the failure zone they are of equal length, geometry dictates that as long as they maintain their uniform separation, the rails MUST maintain the same distance (length) over the failure zone. If one labels the ends of the failure as (arbitrarily) Points A and B, then the two rails, pinned an unchanging distance apart between those two points, MUST maintain their length between those two points. Since that length is equal, they must spend equal amounts of length on the inside and outside of any common radii they share – thus the symmetrically of the failure. This can be seen when one extends an imaginary line from the vanishing point of the photo, down the center of the track assembly. You will note that the rails are displace (roughly) equally on either side of that imaginary line, again, because geometry dictates that they must. Another bit of evidence of this is that if one looks closely, one will note that NONE of the ties have rotated – they have all displaced laterally.