24 November 2011
Approximate location is 33.715100°, -118.312185°. We have had some heavy rains over the weekend so a nice sized chunk of rock hastened its slip into the ocean last night. Here’s a video clip http://www.nbclosangeles.com/news/local/San-Pedro-Landslide-Cliff-134238418.html. I will look for more footage through the day. The local residents are notably upset. I wish there was a way to let them know that:
1) For the very long term, yes portions of the area could potentially be unstable.
2) No, they will not become an island like Catalina (located 35 km south of this region); and
3) Although Facebook is a wonderful medium for getting messages out quickly, one must take it all with a grain of salt and look a bit further when possible.
This area consists of a series of marine terraces unconformably sitting on Miocene sedimentary rock (Monterey Formation). The bedrock has been uplifted into an anticline and is now out-of-slope towards the ocean. Ancient landsliding occurred here during the Late Pleistocene and naturally stabilized. Local relative sea levels have also changed, partly due to Pleistocene-Holocene climate shift and partly due to tectonic uplift. As those sea levels have increased through the last several thousand years, the existing landslide toes slowly get eroded.
Every once in a while, another block slips along and falls into the sea. Last week’s sloughing was a part of that. To answer the newsperson’s question in the video clip, I am sure seasonal rains slightly help accelerate the process.
Here’s some more interesting information on this area. It has had documented movement since the late 1920’s. Most notably the Point Fermin landslide located 600m to the southwest. Here is a nice little virtual tour from Cal State Long Beach (one of the local universities) http://geology.cnsm.ad.csulb.edu/VIRTUAL_FIELD/Palos_Verdes/fermin.htm
Meanwhile, as Jim Heddingon pointed out in a comment, the Daily Mail has probably the best gallery of images of the landslide that I have seen to date, including this one:
At some point in the next few months that large displaced block is going to collapse into the sea, which should be fairly spectacular.