Sixteen people were evacuated from twelve properties in the Rolling Hills Estate on the Palos Verdes Peninsula in Los Angeles County, California, after the properties began to slide into a canyon on Saturday 8 July 2023. The evacuations were carried out after fire officials called to investigate a leaking pipe realised the hillslope was moving, with people being given just 20 minutes to grab personal items and leave. Following the evacuations many of the properties, which are valued at upwards of $1 million each, have slowly cracked and slid into the canyon.
Although the Rolling Hills Estate was built in the 1970s and has not previously shown any stability problems, the Palos Verdes Peninsula is notorious for landslips, with Rolling Hills being very close to Portuguese Bend, an ongoing earthflow, rotational and translational slide, an ancient movement which was reactivated in 1956 and by 1961 had damaged or destroyed over 150 homes. This movement still requires an expenditure about $150 000 in repairs and mediation measures each year.
The geology of the Palos Verdes Peninsula is comprised largely of weakly consolidated Miocene sediments uplifted to form the peninsula, and overlain by Quaternary alluvial deposits. The Miocene sediments are not flat, but uplifted in a series of synclines and anticlines, giving the beds a 'WWW' structure. The problem is made worse by layers of bentonite (a clay formed from volcanic ash) within the Miocene deposits. Bentonites both expand and become very slippery when they become wet. forming a natural slide plain along which landslips can move.
This year southern California suffered an exceptional series of winter storms, brining high levels of rainfall to normally arid areas, causing widespread flooding and many landslide events. Although the worst of the flooding has now passed, in many areas the excess water will have penetrated deep into the ground, potentially activating earth movements in areas which have until now seemed stable.
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