Buried deep in Earth’s crust, sometimes beneath large cities like Los Angeles, are geologic faults that show no evidence at the surface. Although invisible to humans walking around above, these aptly named “blind faults” can cause significant seismic damage. Studying the potential hazard they pose could help prevent damage and human injury from earthquakes when they slip, but the faults are also challenging to study because they are so inaccessible.
In a recent study, Johnson created a model to study how folds in layered rock grow above a reverse blind fault where rock is squeezing together. The study is the first to use a model to investigate how two processes—faulting and folding through slip between layers of rock—interact to affect the folding growth and shape. The author was particularly interested in how these physics play out in anticline formations.
An anticline is a folded rock formation that forms when stratified rock is bent so that the layers slope downward from the bend’s crest in an inverted U shape. Previous studies have shown that anticlines with multiple layers can grow to more than twice the amplitude of a fold that has no layering. This new study is an extension of this research but combines an additional factor called flexural-slip folding into the model. Flexural-slip folding occurs when thin layers of rock slip past each other while the layers are folded and deformed.
The author simulated the growth of anticlines over a reverse fault. He ran simulations with different sizes and numbers of rock layers in the folds, low and high friction strength between the layers, and two types of fault geometries.
The model revealed that the fault geometry will affect the shape of the fold. For example, straight ramp faults that slope upward will produce symmetric folds. But a listric fault, which is curved up like the side of a bowl, will make the fold asymmetrical. In either case, as the anticline folds over time, it will, in turn, deform the fault geometry so that both the fold and the fault are shaping each other.
The results of the simulations also confirm that folding is amplified if the rock above the fault has thinner layers. Likewise, the more layers there are on top of the fault, the more folding there will be. The folding rate can be affected by how much frictional strength there is between the layers, or the amount of resistance they have while sliding past each other: A fold with low frictional strength can grow 4–5 times faster than a similar fold without layers.
Finally, the author used his model to help identify what could be causing the earthquakes from the fault-cored anticlines. The results indicate that as the fold grows, the place where an earthquake originates evolves. Early in the folding, earthquakes occur on the slipping fault. But as time goes on, the quakes are increasingly caused by the slipping rock layers above and around the fault as the folds buckle under pressure. This means that not all earthquakes over a blind reverse fault are caused by the fault itself, but rather by the folding around it.
This research could help scientists better understand how blind reverse faults shape Earth’s crust and ultimately lead to better hazard predictions for heavily populated areas above these faults. (Journal of Geophysical Research: Solid Earth, https://doi.org/10.1002/2017JB014867, 2018)
—Alexandra Branscombe, Freelance Writer