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Orgo-Life the new way to the future Advertising by AdpathwayBefore I go much further, I want to stress how serious earthquakes are. It's not really a laughing matter if you live as I do in earthquake country. Millions of people have died from earthquakes in the last century or two, and they are still recovering bodies in Venezuela from the quake last June (6,500 dead so far with tens of thousands still missing).
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| San Francisco after the 1906 earthquake |
Still, it is important to be reminded about the threat that we in California live under. Quakes happen regularly in this state, and sometimes they are catastrophic. It's best to know the threat and to be ready for the day when the BIG ONES hit.
And we all know who to blame when that day comes, right? It's the San Andreas's fault...
Well, let's see if that is true...how well do we know our most famous fault?
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| Trace of the San Andreas fault on the San Francisco Peninsula. San Andreas Lake is in the foreground. |
The fault was discovered and named in 1895 by Andrew Lawson, a professor at UC Berkeley. He was mapping the geology of part of the San Francisco Peninsula, and he named what he thought to be a minor fault after the nearby San Andreas Lake. In 1906 the catastrophic San Francisco earthquake struck and it was immediately apparent that the fault was far more important, as the surface rupture extended more than 200 miles. Subsequent mapping revealed a total length of some 600 miles.
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| Source: US Geological Survey |
The fault is a transform plate boundary, dividing the Pacific and North American plates. The two plates are moving northwest, but the Pacific Plate is moving faster, causing relative motion that is right lateral.
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| Tectonic plates of the world. Source: US Geological Survey |
Right lateral means that the side opposite the fault from the observer has moved to the observer's right, as seen below in a curb offset by lateral motion on the Calaveras fault in Hollister, California. The Calaveras parallels the San Andreas in Central California, and is considered part of the same system of plate motions.
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| Curb in Hollister, CA offset by right lateral motion on the Calaveras fault. |
This seems an odd thing to point out, but the fault is segmented, and that's a good thing. It means that sections of the fault behave independently, with some portions moving via occasional earthquakes, while others move more or less continuously (fault creep, as pictured above). This segmentation means that stress can't build up along the entire length of the fault so that possible earthquakes have an upper limit of magnitude (magnitude 8 instead of 9 or 9.5; the difference is NOT trivial).
The fault began forming about 29 million years ago, and has moved more than 200 miles (during countless earthquakes of 15-20 feet per earthquake). This is best illustrated by the Pinnacles Volcano (in Pinnacles National Park in the Coast Ranges south of Hollister), which formed 22 million years ago, and was split by the fault and shifted 195 miles (the other half is in the Palmdale region).
Lots of people talk about the San Andreas fault, but relatively few people have actually seen it (or didn't recognize it when they did). What does the San Andreas look like? As it turns out, it's not always readily visible. Erosion tends to quickly erase evidence of fault motions, but they are preserved in many places if you know where to look. One of the most famous localities is Wallace Creek at Carrizo Plains National Monument west of Bakersfield. At that spot the San Andreas has offset the creek by 400 feet during perhaps two dozen earthquakes over the last 4,000-5,000 years (below).
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| Wallace Creek in Carrizo Plains National Monument |
If the offset isn't clear from the ground view, check this aerial photograph of the same spot...
The fault is much easier to see where it is creeping continuously. The fault crosses Highway 25 near Pinnacles National Park, and I've documenting the changes there over the last quarter-century. The photo below shows the cracks in the highway as of October 2026.
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| San Andreas fault crosses Highway 25 north of Pinnacles National Park |
The best way to see the fault is from above. I was lucky a few years ago to have a flight take me over Cajon Pass in Southern California where the San Andreas crosses the San Gabriel Mountains at Lone Pine Canyon (below). Lone Pine Canyon is visible briefly from Interstate 15 as you drive north.
People say things about the San Andreas fault. They're always finding fault, so to speak. Are these things true? Let's see...
- California will fall into the sea
- The San Andreas Fault causes all California earthquakes
- California has more earthquakes, and bigger earthquakes than anywhere else
- We are waiting for the “BIG ONE”
Let's check them out.
California will fall into the sea
Well, that is TRUE in a manner of speaking. To be sure, it doesn't happen Hollywood-style with Los Angeles or San Francisco sinking beneath the waves in a catastrophic quake complete with falling buildings and giant tsunamis.
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| Attribution: Unknown. These things show up after many disasters, and it's a nuisance. |
But sections of California have been slowly submerged over millions of years, especially in the Borderlands off the coast of Southern California. And Baja California is moving north along the San Andreas, eventually to become an island, heading ultimately to the Aleutian Trench off of Alaska.
So, for this one we must place the blame on the San Andreas...but we can blame Hollywood too, for incredible exaggeration on such matters
The San Andreas Fault causes all California earthquakes:
This one is easily dismissed as untrue. Just take a look at any site showing the locations of earthquake epicenters. The SCEDC site is especially good, Look below to see a week's worth of earthquakes in California and Nevada: it is immediately clear that earthquakes happen all over the region, and that dozens of other fault zones produce earthquakes.
Lots of blame to spread around for the earthquakes in California, not just the San Andreas!
California has more earthquakes, and bigger earthquakes than anywhere else:
Let's roll the numbers. I'm not including the magnitude 9 quake of 1700, as it was most likely far too the north of California. Here they are:
- Number of (near) magnitude 8 quakes: 3 (1857, 1872 and 1906)
- Number of magnitude 7 quakes: 12-15
- Number of magnitude 6 quakes: 140+
As to whether these are the biggest in the world, it's not even close. Check out the list of the 10 largest quakes (actually 17 when including honorable mentions) in recorded history from the USGS:
Except for the bit of the Cascadia Subduction Zone in the far north part of the state, California simply doesn't have the conditions that can allow the incredible stresses required to produce magnitude 9 quakes.
We need to have a quick word about the nature of the magnitude scale. It is exponential, not linear. When calculating the energy of earthquakes, an increase of one magnitude represents an increase of about 32x the energy. Put another way, a magnitude 9 quake represents the energy release of 32 magnitude 8 quakes, and over a thousand magnitude 7 quakes (32x32).
Put another way, the Great Tohoku quake of 2011 in Japan at magnitude 9.1 released more energy in one event than all the recorded earthquakes in California's history!
So, no blame for the San Andreas when it hasn't done what it is blamed for!
We are waiting for the “BIG ONE”
The Big One. The one no one wants to think about. The Big Show.
The problem is, and you won't like it, that the statement needs to be refined a bit.
We are waiting for the Big Ones!
As noted previously, the San Andreas fault is segmented, and different parts have different seismic histories. There was the 1906 event in San Francisco, and the 1857 in Southern California. There is a segment in the Coachella Valley that has been lock up and building stresses for some 300 years. And there are other major faults capable of mayhem. The 1872 quake in Lone Pine wasn't on the San Andreas fault, and neither were many other magnitude 7 events.
Seismologists cannot predict individual quakes with any kind of reliability, but they deal quite well with probability. Working groups have produced maps that reflect the possibility of major quakes on any number of California fault lines and have produced probability maps like that above. It is a bit dated (2008), but there have already been 3 or 4 magnitude 6.7 and greater quakes, showing that the long-term predictions are valid. The quakes are going to happen. We just don't know precisely when.
So, no blame on the San Andreas, at least not yet this century!
This one is subject to change...
This blog post was written to coincide with the Great California Shakeout to be held on Thursday Oct. 15 at 10:15 am.
I will also be presenting a public lecture as part of Modesto Junior College's MAPS Speaker series on Friday night, October 16 is Sierra Hall, Room 132 on our west campus at 7:30 pm. I'll be speaking about, of all things, the San Andreas fault!
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