LAUREL CANYON—Did you know that here in the Santa Monica Mountains we live on the Pacific Plate not the North American Plate?  It seemed odd to me that while we love living in the hills, we know almost nothing of their particulars, yet we each have at least six opinions and consider ourselves experts.

Wading through the scientific mumbo jumbo about the formation of the Santa Monica Mountains, scientists use terms such as geotechnical engineering, which is engineering that concerns itself with earth materials, and geomorphologic, which is the study of land formations. It seems that our Hollywood-Malibu fault underwent one surface-rupturing earthquake during the late Pleistocene to middle or late Holocene time. That means from 2.5 million years ago to 12,000 years ago.  The mountain tops are a result of the grinding of the two tectonic plates and subsequent erosion.  This explains the discovery of sea mollusk fossils in the hillsides.

The Santa Monica Mountains stretch almost 50 miles westward from Griffith Park to Point Mugu. It is part of the only east-west belt of mountains in California and one of only two in all of North America. Scientists call such a formation a Transverse Mountain Range. The orientation makes them breathtaking as the sun rises and sets over their shoulders. Nonetheless, many Angelinos would not be caught dead living here. Why the paranoia? Might this be because the North American and Pacific tectonic plates meet under the Santa Monica Mountains? The Channel Islands are considered an extension of our mountain range. Does this give credence to the speculation that we could all own oceanfront property given the right circumstances? No. It is reported that “the Earth will not open up. Nor will [we] fall into the sea, because the Hollywood-Malibu fault zone is only about 9.3 miles deep, which is roughly a quarter of the thickness of the continental crust. Furthermore, California is composed of continental crust, whose relatively low density keeps it riding high, like an iceberg above the ocean” squashing my hopes for beachfront property.

A Japan-type rupture of the entire Hollywood-Malibu fault by itself could produce a 6.6 earthquake.  Such an earthquake, because of its proximity to more densely urbanized areas, would wreck more havoc than the 1994 Northridge earthquake. However, scientists theorize such quakes could occur every 4,000 years.  If our Hollywood-Malibu fault contributes to other localized earthquakes then “the big one” is less likely.

After locals discuss earthquake possibilities, the next favorite topic is mudslides. We all have an opinion. In particular we want to know, “What’s up with the K-rails at the southern base of Laurel Canyon Boulevard?” Once again, science offers the answers.

A soil study conducted in 2006 of the Santa Monica Mountains National Recreation Area answered some of my questions. The study described the composition of our mountains in detail. Our soil is technically known as “Chumash,” which consists of topsoil from 4” to 10” in depth, with paralithic, fractures rock which is highly susceptible to slippage.  The study stated that hillsides which have more than a 34 percent slope have topsoil that is at the mercy of gravity for what they call slope slippage. We call them mudslides. I put Laurel Canyon at a 45 percent pitch. No wonder it’s such a mess. However, those areas with sedimentary rock, which is perpendicular to the slope, is less likely to suffer soil slippage—a handy tip for future homeowners. The report also confirmed what we all suspected: trees and bushes diminish the likelihood of mudslides. This puts the Fire Department’s demand for brush clearance at odds with the science. 

So we have to choose. What will it be? Mudslides, fires or earthquakes? Or are you sticking to the flatlands?

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