Scientists in the USA and Germany have discovered that tiny amounts of clay are capable of allowing sections of the San Andreas Fault to move smoothly against each other without causing earthquakes.
The San Andreas Fault forms the tectonic boundary between the Pacific Plate and the North American Plate and has caused major earthquakes in the past such as the 1906 San Francisco quake. However, not all of the plate displays such violent activity. Some sections slide easily against each other in a process called creep.
Looking at rock samples brought up from a depth of two miles below the surface, researchers at University of Michigan and the Ernst-Moritz-Arndt Universität in Germany have found that fractured rock surfaces in the fault were covered with a thin layer of clay barely nanometers thick, which acts like grease on ball bearings. This allows the rocks to slide easily against each other without causing an earthquake.
What surprised the researchers was the relatively tiny amount of lubricant needed to allow this creep to occur. Professor van der Pluijm says, “What we can show is that you don’t really need a lot; it just needs to be in the right place. It’s a bit like real estate: location, location, location.”
Using argon dating, which is similar to carbon dating, the researchers also found that these clays formed relatively recently. They also found that the fault is in effect greasing itself. The clays grow in the fault zone and coat the fragmented rocks. When there is enough coating then it begins to affect the behaviour of the fault causing the creep.
This does raise the question, if the fault is greasing itself, why do earthquakes still occur?
The answer is time and the continuing development of the fault. The San Andreas Fault is actually a network of faults with new sections or ‘strands’ developing over time. “It takes some time for slick nanocoatings to develop in a new strand, the unlubricated, new strand ‘gets stuck’ for a time and then shifts in a violent spasm” says van der Pluijm.
Researchers also believe that these nanocoatings are occurring deeper than two miles and they may indeed be forming and affecting the fault behaviour at greater depths. Analyses of older, inactive sections of the fault even suggest that the coatings have had an effect on creep for millions of years.
This work has been published in the journal Geology and is part of the San Andreas Fault Observatory at Depth (SAFOD) project.