It’s an odd thing to admit, but I’m obsessed with faults. Irrespective of their shape or size, I find the deformation that manifests the movement of our continental plates fascinating (though I’d prefer them if they didn’t result in a humanitarian crisis). The San Andreas Fault is perhaps one of the most famous faults in the world. Stretching over 800 km along the boundary between the Pacific and the American Plates, through California and major cities such as San Francisco, it has a very real and ominous interaction with humankind (Figure 1). In fact, it owes its fame in part to the 1906 San Francisco earthquake which struck at 5.20 am on April 18th. At an estimated 7.8 magnitude on the moment magnitude scale (MMS) it destroyed about 80 % of the city and killed up to 3000 people1.
Now let’s just take a step back for a minute….
The San Andreas is a classic example of a right-lateral strike-slip fault (Figure 2). Strike slips are those that slide past each other without any vertical displacement. Imagine you were facing another person and each of you stepped to the right – you are on either side of a ‘right lateral’ strike-slip fault. Such movement means the tsunami or megatsunami that occurred in the ‘San Andreas’ movie is not a realistic response to this type of earthquake – but let’s not get onto the inaccuracies of that film…
Since its beginning, roughly 15-20 million years ago, the San Andreas is believed to have accumulated 350 miles of movement, with some crustal blocks having moved through 20 degrees of latitude2. To put this into context, that’s like moving Oxford down to the same latitude as Marrakech.
The San Andreas isn’t the only fault that accommodates the movement of the two plates; it’s actually part of a ‘fault zone’ – an accumulation of parallel faults which share the movement. The Pacific Plate moves with respect to the North American Plate at around 2.5 inches per year – as fast as your fingernails grow. In comparison, the average slip rate on the San Andreas Fault is 1.5 inches per year, meaning one inch a year is accommodated by alternative faults.
So what’s so special then?
There are a couple of reasons why this fault is a personal favorite, the first being that it breaks the Earth’s surface (Figure 3). The top 3-5 km of the Earth tend to be a ‘strength hardening zone’. This sounds fancy but, simply put, means that an earthquake is unable to nucleate (form) within this zone, and also that faults that propagate from their nucleation at greater depths are dampened as they move towards the surface3. In short, it means that although earthquake effects can be violently felt at the surface, they don’t always break through – but, on the San Andreas they do.
Not only can this fault be seen at the surface, but samples have been obtained which enable a glimpse of its deep structure. The San Andreas Fault Observatory at Depth (SAFOD) undertook a drilling project to access rocks at depth from within the fault zone. These samples were then deformed experimentally to better understand the fault’s strength4. Which leads me onto my second favorite thing about the San Andreas – it’s exceptionally weak. The weakness of this fault is somewhat of an oxymoron, as faults are required to be strong to enable stresses to build up and large earthquakes to occur. A weak fault will creep instead, with the fault planes very slowly sliding past each other without the occurrence of earthquakes. One explanation for creep on the San Andreas is the presence of a clay (smectite) within areas of the fault zone4. Such clays have a low frictional coefficient making them very weak and able to fail with only a small amount of force.
It’s understandable that not everyone, particularly those who live in vulnerable areas, will love faults in the same way I do. I also appreciate that writing this from a cosy room in Oxford, England, means I’m able to take a more objective view of them. However, even if you don’t love, I hope you now see that the San Andreas is not the ‘monster’ it’s depicted to be by Hollywood (Figure 4).
General information on the San Andreas Fault:
On measuring the size of an earthquake:
- 1906 San Francisco earthquake. Available at: https://en.wikipedia.org/wiki/1906_San_Francisco_earthquake.
- Schulz, S. S. & Wallace, E. R. The San Andreas Fault. Available at: https://pubs.usgs.gov/gip/earthq3/safaultgip.html.
- Scholz, C. H. Earthquakes and friction laws. Nature 391, 37–42 (1998).
- Carpenter, B. M., Marone, C. & Saffer, D. M. Weakness of the San Andreas Fault revealed by samples from the active fault zone. Nat. Geosci. 4, 251–254 (2011).
“I am a first year PhD student in the Earth Sciences Department researching the partition of stress in multi-mineral rocks. Other than my PhD topic, my interests lie in large scale structural geology, particularly in large magnitude earthquakes and their corresponding faults.”