Whose fault is it? Is it always Fair to Blame the San Andreas?

Before 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).

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?

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.

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.

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.

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).

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.

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. 

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!

Source: https://scedc.caltech.edu/recent/index.html

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!

Explore the Pacific Northwest and Northern Rocky Mountains with Geotripper! June 18-July 2, 2026

Grand Tetons National Park, Wyoming

Be forewarned. This post is a TRAP! It is designed to draw you in, weaken your defenses, and cause you to do something different than everyday life. Warning given...

Have you ever dreamed of hitting the open road and finally seeing those places you've dreamed about, but haven't acted on that dream yet? What if you found out about an excursion that doesn't just tour, but allows you to learn the geology and history of those wild places? A tour on which you can even earn college credit? AND, a tour that is affordable? Maybe this is the one...

Mt. Shasta, a Cascade volcano in northern California

From June 18-July 2, 2026, the geology and anthropology departments of Modesto Junior College will be conducting a field studies course (Geology 192/Anthropology 192) in the Pacific Northwest and Northern Rocky Mountains. It will be a six semester unit course designed for our community college clientele: first year geology/anthropology majors, potential majors, as well as community members (especially teachers) and life-long learners interested in geology, archaeology, and natural history. We will develop the necessary geological/anthropology background prior to and in the early days of the trip, so people of all backgrounds are encouraged to attend. The total cost is $850 which will include all food, camp fees, entrance fees, transportation costs for the trip. The tuition cost for the six units of semester credit will be around $380 (out of state tuition is higher). The only additional costs should be for showers, laundry, books and other souvenirs, and junk food (we provide healthy food for the most part; if you want Twinkies you are on your own!). We will be camping each night, and the school provides the transportation (vans). The excellent meals are planned by our professional volunteer staff, and cooked by the participants under their watchful eyes.

Lava Tube in Lava Beds National Monument

What will you see and experience? On the 18th we'll leave MJC and drive north through the Great Valley of California and arrive at the south end of the Cascades Range. The huge edifice of Mt. Shasta looms over the north state at 14,163 feet, and still is potentially active. It last erupted in 1786. Depending on snow conditions, we'll climb to the 8,000 feet level at the old ski bowl and have a close look at the rock and ash deposits. We'll continue north and end the day at Lava Beds National Monument near the Oregon border. There will be chance to explore some lava tubes while we are there.

The view from Smith Rock State Park in Oregon

We drive through Oregon the next day, with possible stops at Crater Lake National Park and Newberry Crater (depending, once again, on snow conditions). Camp will be at Tumalo State Park. The following day we will explore Smith Rock State Park (above), Mt. Hood, and the Columbia River Gorge (if there is time we will climb Beacon Rock in the gorge). The third camp will be at Seaquest State Park at the foot of Mt. St. Helens in southern Washington.

Mt. St. Helens in Washington. It erupted in 1980 and 2004

The following day will be devoted to the exploration of Mt. St. Helens (weather allowing!). We'll then move on to the Pacific Coast and the temperate rainforests of Olympic National Park. We'll visit some tribal lands near the westernmost point of the lower 48 states and explore Hurricane Ridge at Olympic's north side. 

We'll then descend the eastern flank of the Cascades (including a close look at Mt. Rainier) and drive onto the Columbia River Plateau, a vast basalt plain that covers much of eastern Washington and Oregon. Camp will be at Sportsman's State Park in Yakima.

Dry Falls State Park in Washington. The floodwaters covered this entire landscape to a depth of 300 feet during the Spokane floods.

The next day we will view the evidence for vast floods that swept across the plateau during the Pleistocene ice ages. The discovery of these floods by J Harlan Bretz in the 1920s and the long road to acceptance of the hypothesis by the geological community is one of the great stories in the history of geology as a science. We'll have a look at the Channeled Scablands, Soap Lake, and Dry Falls as we travel east through Washington. We'll spend the night at Riverside State Park in Spokane, Washington.

We'll head through the copper mining districts of Idaho and into Montana where we'll see more evidence of the ice age floods. We'll drive east to Lewis and Clark Caverns and stay near Bozeman, Montana. We'll check out the Museum of the Rockies and move on to explore Yellowstone, America's oldest national park. We'll spend two days exploring this incredible park.

Yellowstone Falls in the Grand Canyon of the Yellowstone River

There is the Grand Canyon of Yellowstone, and a menagerie of incredible animals, including elk, bison, moose, bighorn sheep, and if we get really lucky, wolves.

Wolf near Norris Geyser Basin, Yellowstone National Park

Then there are geothermal features for which Yellowstone is so famous. Grand Prismatic Spring, for instance, and 70% of the world's geysers (there's lots more besides just Old Faithful!).

Grand Prismatic Spring, Yellowstone National Park

We'll then head south and spend two days at Grand Tetons National Park with time for some spectacular hikes. Then we start the road home with a drive through northern Nevada.

Big Geyser (not Old Faithful!) in Lower Basin, Yellowstone National Park

It's hard to describe the wonders that exist across the Pacific Northwest and Northern Rocky Mountains without getting an overwhelming urge to get up and leave right away. If you are in the Modesto region, we are having our first information meeting on Monday, March 23 in the Science Community Center on the west campus of the Modesto Junior College at 7 PM in SCC 325 (attendance is not mandatory to go on the trip). We hope you will join us!

Travels in Cascadia: Victoria B.C., the City of Mutton Rocks

The next stop in our recent travels through Cascadia was the city of Victoria on Vancouver Island in British Columbia. We got there by way of a ferry from Port Angeles, Washington, as described in the last post. Victoria is an attractive city, one of the most temperate in Canada, given its location on the Pacific shoreline (the adjacent ocean moderates the seasonal temperature extremes). It is also unique in another respect. The city is partially constructed on bedrock (the solid rock that underlies surface soils and sediments), and the land was under a vast 1,000 meter thick ice sheet only 14,000 years ago.

This geologically unique combination means that Victoria is sort of a city of mutton rocks.

I imagine that sentence needs explanation...

The term roche moutonnée describes an asymmetrical glacially scoured rock outcrop that has a smooth slope on the side facing the flowing ice, and a steep cliff on the side where the glacier pulled away from the outcrop ("stoss and lee structure" is a related term). The scale can range from a few meters to many hundreds. They are common features in regions of bedrock that have been scoured by massive continental ice sheets, such as happened in Victoria. One of the tallest hills within the city, Mt. Tolmie (below), is an excellent example. In the picture one can see the gently sloping forested flank on the left side of the hill, and the steeper plucked side to the right.

Mt. Tolmie, a roche moutonnée in Victoria

The problem with roche moutonnée as a geological term is that we geologists can only barely agree on its meaning. It's derived from French, and the "roche" part isn't a problem. It means "rock". But "moutonnée" is the tricky aspect. It can be translated loosely as "sheep" (think "mutton""), but not exactly (French: "mouton"). Moutonnée (with the extra e's) translates to "frizzy", and is taken as a reference to sheep's wool. The term originated in the 1700s with a naturalist named Horace-Bénédict de Saussure (it would be decades before the term "geologist" was coined) who noted that the rocks looked like a type of wig apparently well-known at the time whose locks were held in place with mutton grease. Except that there seem to be few or no references to wigs that were actually called that (the closest version was a tête de mouton).

So we teachers are left with trying to define the term as meaning "rock sheep" based on the nebulous idea that the rocks look like sheep grazing in meadows. Which they really don't. But it's still easier than trying to describe obscure French wigs from the 1700s and mutton grease.

There are consequences to building on such rocky landscapes. There are plenty of large patches of glacial till that are easy to plane off with a bulldozer for building construction, but when the rock crops out, allowances have to be made. Perhaps it might involve blasting to construct a flat foundation. The rock is pretty tough stuff, gabbro, diorite, and greenstone of the Wrangellia terrane, dating back to the era of dinosaurs.

There are problems in this kind of situation. When the Empress Hotel (see the picture above) was constructed in 1904-1905 it was placed partly on solid rock and partly on mud-rich sediments. Complications quickly ensued. The south part of the building subsided several centimeters within the first year and ultimately sank about a meter. It's a real headache for those who must maintain the building.

Still, Victoria is a truly beautiful city, and the city has a unique personality. The city has miles of coastline, and there are dozens of scenic shoreline parks. We pulled up at Cattle Point to look at smaller roches moutonnée along with glacial scour marks and striations. Strangely enough, there was a piano sitting on the bluff. It was brightly painted, there was a bench, and the piano was in tune! I wondered what was going to happen to it if it rained, but then noticed a waterproof cover. We later found that there are five such pianos in the parks, and we were actually treated to a tune by a passing jogger, followed by a song by one of our students!

If you ever have a chance to visit Victoria, enjoy the city, but be sure to follow the shoreline drive to see the glacial heritage of the landscape (as well as seeing some dramatic coastal scenery). But the other thing you should do is to hike or drive to the summit of the mutton rocks of the city. Mt. Tolmie is a good choice within the city limits, or you can go just north of the city to the much higher summit of Mt. Douglas. It's a short hike from the parking lot to a summit with a 360 degree view of the region. The Olympic Mountains are visible across the Strait of Juan de Fuca to the south beyond the city, and the many islands of the Strait of Georgia and Saanich Inlet are visible to the north and east. Glacial polish, striations and grooves are present underfoot.

On the day we were there, I saw some unusual looking clouds far to the southeast. I took a few highly zoomed shots and forget about them until weeks later. I started working with the contrast and exposure of the picture and realized I had captured an image of Mt. Rainier across the Puget Sound. The volcano is more than 130 miles away (below)!

I had three main resources for the geology in and around the city:
The Geology of Southern Vancouver Island by Chris Yorath
Roadside Geology of Southern British Columbia by Bill Mathews and Jim Monger
Geology of British Columbia, A Journey Through Time by Sydney Cannings, JoAnne Nelson, and Richard Cannings.

Good-bye to 2018: 12 Months of the Joy of Geology

Mesa Arch in Canyonlands National Park

It's been a year. There are lots of ways of looking at the events of 2018 and so many of them are tragic and unjust. But there is always hope as well. There are still the beautiful places of the Earth, and always a chance to seek them out, no matter where you are. That's the joy of being a geologist and a teacher. Or being anyone with a curiosity about our planet. What follows are twelve months of exploring our planet from my base in California's Great Valley.

Merced National Wildlife Refuge, south of the city of Merced.

Where does one go in January? So much of the countryside is covered in snow, but California is kind of special in that regard. We get to visit the snow if we want and then we can go home to our valleys and coastlines. The migrant birds know all about this. Millions of geese and other species spend their summers in the Arctic, but when the snows come they fly south and spend the winter in a series of wetlands and prairies that have been preserved in places like the Great Valley. Our destination in January was the Merced National Wildlife Refuge. There were thousands of Sandhill Cranes, Snow Geese, Ross's Geese, and multitudes of ducks. It is an overwhelming experience to stand at the edge of the wetlands and imagine this valley hundreds of years ago before agricultural development took over 95% of the landscape.

Zabriskie Point and Manly Beacon in Death Valley National Park

The most satisfying aspect of a teaching position in geology is the chance I have to share the incredible planet with my students in my classroom of course, but also in the field. In February our intrepid crew headed out to Death Valley National Park in the Basin and Range Province of eastern California. The valley is a vast fault graben with 11,000 foot peaks next to valley floors below sea level. It is the hottest place in the world and the driest place in North America, yet 20,000 years ago it contained a 100-mile long freshwater lake and grassy savannas with horses, camels, and mammoths. Some of the fish who inhabited the lake still survive today in isolated spring-fed ponds.

Natural Bridge in Death Valley National Park

Death Valley is recognized as the premier geology park in the national park system, containing rocks as old as 1.7 billion years, and including layers and intrusions from every period in Earth history ever since.

We survived a phenomenal windstorm on the last night of our trip. I hate to say I littered in a national park, but somewhere out on the dunes there is a tarp that blew away from under my tent in the night.

The Ghirardelli Store in Hornitos within the Sierra Nevada Mother Lode

Our field studies in March were closer to home. We journeyed through the southern Mother Lode of the Sierra Nevada. We saw ghost towns, old mines, and the quartz veins that were the source of the ores of the California Gold Rush in 1848. The ruins in the picture above are the Ghirardelli Store in Hornitos. Before he made chocolate in San Francisco, the man sold supplies to the miners in the Mother Lode.

Yosemite Falls in Yosemite Valley

In April, Yosemite National Park took center stage, as we took two field studies trips there. One of them was the morning after an epic flood that closed the park for a day or two. When we got there, the Merced River was still at flood stage and the waterfalls were booming.

Mt. Rainier in Washington

In May, family matters found us on a plane out of Seattle, Washington. We left for home in the early evening while Mt. Rainier was capturing the last rays of the sun. As you may have read in the last post, Rainier (14,411 feet) is the tallest volcano in the Cascade Range, and has more glacial ice than any mountain in the Pacific Northwest. The potential of eruptions happening under the ice makes Rainier one of the most dangerous volcanoes on the planet. Lahars (volcanic mudflows) could overwhelm some of the cities of the Puget Sound.

Horseshoe Bend on the Colorado downstream of Lake Powell and Glen Canyon Dam

One of the greatest adventures one could ever experience is an in-depth exploration of the Colorado Plateau. It is a showcase of geology with a treasure trove of national parks and monuments. In June, our intrepid crew headed out to plateau country where we explored Grand Canyon, Zion, Bryce Canyon, Black Canyon of the Gunnison, Mesa Verde, Arches, Canyonlands, Capitol Reef and Great Basin National Parks. The region is so extraordinary that a place like Horseshoe Bend on the Colorado River isn't even in a national park or monument.

House on Fire Ruin in Bear's Ears National Monument

One of the great crimes of the present administration in Washington D.C. was the attack upon the Bear's Ears National Monument. Even though the law didn't allow it, the park was reduced in size to 15% of what it had been before. And for the sole purpose of money, i.e. oil drilling and uranium mining. I hope the courts do the right thing this year.

Hurricane Ridge in Olympic National Park

In July Mrs. Geotripper and I escaped the heat and dust of the Great Valley and headed north along the coast, eventually reaching Olympic National Park in Washington. Hurricane Ridge is one of the most stunning viewpoints in America. The views take in the alpine peaks glaciers of Mt. Olympus, the temperate rainforests of the valleys, and the coastlines around Port Angeles. The mountains are composed of oceanic sediments and seafloor crust pushed upwards by the motions of the Cascadia Subduction Zone.

We stayed close to home in August, but almost every morning found me walking the Parkway Trail along the Tuolumne River where it flows from the Sierra Nevada foothills onto the floor of the Great Valley. Birders (including me) discovered 115 species on this trail over the course of the year.

Valentine Cave in Lava Beds National Monument

The resumption of classes in August and September found me and my students on the road again. We explored the Cascades Volcanoes, including Mt. Shasta, Lassen Peak, Medicine Lake Highland and Lava Beds National Monument. Lava Beds has miles of lava tubes, one of the greatest concentrations in the world.

In October we went underground again with an exploration of Black Chasm Cavern. The cave has been carefully managed so that visitors can see cave decorations that are absolutely pristine. The cave has thousands of fragile speleothems called helictites. I call them stalactites on acid...

November was the lost month. The horrific fire in Paradise caused air quality to suffer throughout the state and the school shut down for a week and a half. It's the first time this has happened, and our scheduled field trip to Pinnacles National Park was postponed to the beginning of December. We went underground again, in yet another kind of cave. It wasn't limestone or marble, and it was a lava tube even though the Pinnacles are volcanic. It's a talus cave, one caused by giant boulders falling into and covering a narrow canyon. Hikers are in near total darkness for upwards of a quarter mile.

December arrived and along with it came the holidays. We have family all along the coast from California to Washington, so Christmas for us was a very long road trip. On our last night we stayed near Mt. Shasta in Northern California. The sunset provided beautiful lenticular clouds around the summit of the second and third highest volcanoes in the Cascades (you'll have to check out the last post to learn about that one).

And that was the story of my year and that of my students and Mrs. Geotripper. We saw a lot of incredible things, and my whole purpose in describing these places is to encourage you to explore them for yourselves. I know that this is impossible for many, which is quite literally why I write this blog. But if you can make time and get away, seek out the wild places where you live. If your local college offers field studies courses, consider getting back into school for some personal enrichment. I guarantee you won't regret it.

If you live in the vicinity of Modesto, the Modesto Junior College Geology Department will be offering a great line-up of field studies courses. We'll explore Death Valley again in February, the Southern Mother Lode in March, and Yosemite Valley in April. In September we'll head out to the Eastern Sierra Nevada and Owens Valley. In October we'll travel through the Mother Lode, and in November we'll be back to Pinnacles National Park.

But our premier trip with be on June 26-July 10 when we head to British Columbia and the Pacific Northwest. We'll be exploring Olympic and North Cascades National Parks, Vancouver Island, and the Coastal Mountain Ranges around Whistler and Pemberton. It will be a memorable experience. If you are interested, keep an eye on this blog, follow our Facebook page at https://www.facebook.com/groups/1920712791360611/, or the class web page (soon to be updated) at http://hayesg.faculty.mjc.edu/GeologyPacificNorthwest.html.

See you on the road!