NOTHING NEW. Decades ago, when I lived in Southern California (before moving into the Krembalin, according to the Zeepers here), Portuguese Bend was one of my ‘tourist spots’ when people visited me. Basically it was a land-glacier, where the state had to keep rebuilding the highway, due to it slipping down and breaking apart. In fact the state would be building a new, ‘upstream’ parallel highway path while people still drove the existing highway, as they knew that the existing highway’s days were numbered. Whether it was smart to spend money in that ‘sinkhole’ can be debated, but it was ALWAYS a huge problem.
We went up and down the coastal roads around San Francisco when we were there for five years in the mid-80’s . The dirt is simply ground up rocks churned by tens of thousands of years of fault movement.
Every hillside has small (300 - 1500 - 5000 sq foot landslide slumps.