Back in September I posted a series of images that I took of the of the surface fault rupture for the 4th September 2010 Mw = 7.1 Canterbury earthquake in New Zealand. Included was this one, taken of a railway line that crossed the fault rupture at the eastern end of the fault near to Rolleston:

Thanks to Malcolm Teasdale of Kiwirail for sending these two images of the state of the track at this site immediately after the earthquake (posted with permission):

The second image is particularly interesting. Note how the rails show high levels of deformation whilst the surrounding ground shows comparatively little. Notice also how the big kink on the left side has pushed the ballast aside and into the track side ditch This has happened on the right side too, but to a lesser extent. The right side bend nearest the camera has pushed the ballast towards the camera.

My initial hypothesis here (I am no expert on railway track deformation) in order to stimulate discussion is that the buckling may be the result of compressional deformation across a broad zone. The compression on the very strong railway line was accommodated when a weak point was found, leading to a comparatively rapid deformation to form the main buckle on the left. This then concentrated stress on both sides of the buckle, allowing the other (right side) bends to form. Can anyone who knows more on this topic comment further?