How an ‘ancient landslide’ keeps threatening a railroad, homes in San Clemente
The historic landslides that periodically threaten the towns of San Clemente and San Luis Obispo are not from the occasional quake or hurricane; they are caused by ancient debris movements from the Pacific Ocean.
The historic landslides that periodically threaten the towns of San Clemente and San Luis Obispo are not from the occasional quake or hurricane; they are caused by ancient debris movements from the Pacific Ocean.
The landslide that struck at 8:30 a.m. Wednesday did not occur as the result of a “volcanic” event, a meteorological condition described in science fiction as an imminent earthquake that erupts in the atmosphere. This is why volcanologists prefer the term “precipitation event” or “precipitation flood.”
Instead, the historic landslide that occurred on a residential street in San Clemente on Nov. 9 was caused by the shifting sands of the California Central Valley, which carry ancient landslide deposits across the Pacific Ocean from the inland ocean to the ocean itself. In fact, this movement is the primary reason for the landslide being found here rather than in land that was not on the coast.
What this means is that landslides are not a regular part of California’s geological past. The California Coast Ranges were all part of the Pacific Ocean until the end of the last ice age. And while California has been on the ocean for millions of years, our Pacific coastline had been settled by human beings since before there were people here.
So human activity, from fishing to agriculture to fishing to the arrival of the first Europeans, has been a factor in the past movement of some ancient landslide deposits.
And those deposits have taken over the state for the past 1,500 years. The oldest one yet seen was found in 1976 on the coast south of Monterey, which contains some of the oldest stone artifacts in North America and which was the largest deposit of rocks on