San Pedro landslide - a slow build up to a dramatic event

San Pedro is on the Palos Verdes peninsula just south of Los Angeles - and in Sunday 20th November 2011 a slow-moving landslide which had been moving at a few 10s of mms per day for several months suddenly failed dramatically in a few minutes. A 200m stretch of the main Paseo del Mar road slid away from the clifftops, dropping some 30m.  It's a beautiful hilly area, with upmarket housing and fantastic coastal views.  It's also a hotspot for landslides, as evidenced by the excellent California Geological Survey landslide map produced in 2007 which you can access at:
ftp://ftp.consrv.ca.gov/pub/dmg/pubs/lsim/LSIM_PalosVerdes.pdf
Be patient as it is a large file and takes some time to download.  if you read the text on the right hand side of the map you'll find out more about why the area is prone to landslides.  The essential reasons are:

1. The peninsula is underpinned by Miocene rocks, including shale.  Many of the landslides are located on the easily erodible shale.
2. The peninsula is crossed by the Palos Verdes fault and folded into a complex anticlinal structure.

Human activities have also been blamed for the high occurrence of landslides here. Not only building extensively on the clifftops, but also mis-managing drainage systems and, one factor quoted in the local news as being responsible for the most recent event - cold war underground missile silos are allegedly found in the area.  On Sunday 20th November 2011 it was a horrendously rainy day in Los Angeles - over 20mm of rain fell in a few hours.  

You can explore the coast on Google Maps below - the site of the 20th November 2011 landslide is at the centre of the view - but if you head N and W up the coast you can see vast numbers of houses and impermeable concrete structures along the clifftops.  Surely asking for trouble?


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Think about how would describe the causes of this landslide, using Mike Crozier's predisposing, triggering and maintaining factors framework. Was the rainstorm on November 20th the trigger? Or not?