Tuesday, July 27, 2021

Some Otters for the Dog Days of Summer on a Special River

The Tuolumne River is at a low ebb these days. It has been flowing at a mere 97 cubic feet per second for what seems like months now, a level that barely keeps the river flowing at all. It's not a natural flow of course; an entire series of dams control the amount of water present in the river. In non-drought years, the river might be allowed to flow 3 to 5 times what it is doing right now.
But water is life, and the narrow thread of the river, while perhaps too warm for trout and salmon (there are reports of disastrous fish kills on rivers like the Klamath farther to the north) is still allowing animals and insects to survive this hottest part of the summer. I was out walking the river trail this morning as is my tradition watching for birds. I was delighted to see a crowd of Greater Yellowlegs and Least Sandpipers in the water with a watchful Osprey in the Cottonwood tree above. A pair of Black Phoebes were chasing flies just over the water. And then I saw a serpentine shape crest in the water.
I don't see River Otters (Lontra canadensis) all that often on the Tuolumne River. I know they are there, but they range widely up and down the river, and months may pass between sightings. They are usually pretty far off, but today I was just sitting as a family of four of them casually swam by. Enjoy the video!

The Tuolumne River is a majestic presence in Yosemite National Park, with a canyon that rivals the Grand Canyon in depth, and a valley that once rivaled Yosemite Valley at Hetch Hetchy before the dam was built. But don't discount the lower parts of the river where it flows through the Central Valley. It is this part of the river that supports the greatest diversity of animal and plant life. I've seen otters, bobcats, foxes, coyotes, raccoons, deer, and evidence of beaver in recent years, and nearly 150 species of birds have been reported on the two miles of river trail that I follow every day. It's a special place.

Saturday, July 24, 2021

The Other California: A Bit of the Rarest Ecosystem in SoCal at the North Etiwanda Preserve


This is ultimately a story about what may be the steepest mountain in the world (although I cannot yet confirm this). But the story involves a little bit of a journey down memory lane if you can bear with me a bit!

When I was a kid in Ontario, California in the 1960s, we possessed one of those wonderful things that kids don't have enough of today: a big backyard. There was room enough for a big lawn for ball games, large hedges and trees, and climbable walls around the lot. And enough bare ground that a kid could dig nice deep holes, looking for fossils or buried treasures. But what I found when digging those holes was a lot of rocks. Big rocks, cobbles really, of granite and gneiss and schist, although I didn't know those terms at the time. But I did wonder where the rocks came from.

Earth science wasn't much of a thing in my primary education in the 1960s, but I knew enough to think the somewhat rounded rocks came from a river. But there were no rivers to speak of in the Inland Empire east of Los Angeles. I got an education about that in 1969 when the biggest floods in nearly two generations hit the valley. Streets turned into rivers, and numerous houses and buildings were destroyed by mudflows coming out of the nearby San Gabriel Mountains. Nearby Day Canyon recorded an outflow corresponding to 33 inches of precipitation across its small drainage basin in 24 hours on February 25, a state record. 

The perspective of this photo may deceptive; all the road you see here is sloping downhill for the entire 14 miles

And then, in the 1970s, it was high school and the cross-country team. A favorite training route was to run up Euclid Avenue in Ontario and Upland (AKA State Route 83). It still is one of the prettiest city roads in the state, with a wide median planted in Pepper Trees and numerous architecturally distinctive homes dating from the early 1900s. It runs for 14 miles in a straight line from San Antonio Heights to the Chino Hills. 

On the easy days we needed only to run a four-mile out-and-back practice to Foothill Avenue, but when the coaches were bearing down, we needed to run all the way up to Baseline or further (6-8 miles). The thing is, the farther one ran up the hill, the steeper it got. Thus was my introduction to the geometry of alluvial fans. During the mudflows and flashfloods that produce the fans, the coarser debris drops out first, and finer-grained materials get carried further out into the plains below. A fan has a concave profile, becoming steepest at the top. 

Some days, the coaches would drive us up into the barrens at the top of the fan, and our runs included a series of breathtaking terraces (and I mean this in the literal sense, as we were breathless by the time we climbed them). I had no idea at the time why they were there. It seemed like alluvial fans should have a smooth profile, not a terraced one.

In the late 1970s I was in college, and my education about alluvial fans and earthquake faults became a bit more complete. These alluvial fans that I had exhausted myself on during cross-country practice were textbook examples of alluvial processes, and maps of them were indeed a part of my laboratory exercises. I also learned that the terraces were actually fault scarps, produced in the last few thousand years by titanic earthquakes that have been lifting the San Gabriel Mountains. They've been uplifted so rapidly that the mountain ridge that includes Ontario and Cucamonga Peaks may be one of the steepest mountain slopes in the world (I heard this statistic at a conference, but I have not been able to locate the reference). The mountains are so steep that mass wasting is a far more dominant form of erosion than river flow. And the mountains are indeed massive, rising 7,000 feet from their base to the highest peaks.

Thus it was that when I left the region in the middle 1980s, the cities below were growing, but the alluvial fans above them had defeated attempts at agricultural development (the lower slopes were ideal for vineyards and citrus orchards). The surfaces were ignored, or used for garbage dumping, shooting, and off-road vehicle travel. They were considered wastelands. Someone had had the bright idea of putting Chaffey College up there, miles away from the main population centers in the valley (it's visible in the lower left corner of the map above), but the college stood apart, surrounded by scrublands.

Sometimes, the lands that seem so barren do in fact have value, and the more they disappear, the more precious the remainder becomes. So maybe it is a good time to ask: what are alluvial fans good for anyway? Here are my thoughts in no particular order:

Artesian wells near San Bernardino in the early years of settlement. Source unknown, but found at you have water mail: artesian wells in San Bernardino, California

Alluvial fans are a vast sponge that could hardly be designed better to capture water and store it underground, safe from evaporation. The Inland Empire became an agricultural powerhouse in the last century on the basis of the citrus fruit industry. It was a desert climate that very rarely froze, and yet had a wealth of water underground. Sometimes at the distal end of fans, artesian springs produced fountains of water that could be easily utilized in the vineyards and orchards (artesian springs and wells are those that flow due to underlying pressure and don't have to be pumped to bring water to the surface).

Urbanization robs much of the fan surface of the ability to absorb water, given that pavement and buildings tend to shunt floodwaters into the concrete flood channels. They in turn are designed to carry water downstream without damaging buildings. If they have enough capacity, that is.
The south slope of Cucamonga Peak, Can anyone see a viable climbing route? I don't think chocks and pitons would work in the rotten rock, but I suppose you could anchor to the trees. That's how I climbed a similar (but shorter) canyon in my youth.

Alluvial fans are a buffer from huge mass wasting events. The mountains above the Inland Empire are, as pointed out previously, among the steepest mountains on the planet. In addition, the rocks that make up the steep cliffs are badly fractured and jointed from the intense faulting and pressure resulting from their uplift. I can't find many records of people climbing the mountain from the south other than up a ridge after a wildfire had cleared the brush. These slopes are exceedingly unstable, and landslides and slope failures are a constant hazard. 
The Blackhawk Slide on the north side of the San Bernardino Mountains. Credit: Kerry Sieh of the U.S. Geological Survey

It may be an extreme example, but the Blackhawk Slide on the north side of the nearby San Bernardino Mountains is a gigantic debris avalanche that traveled 5.6 miles across the surface of the alluvial fan about 17,400 years ago. It was probably set off by a large earthquake, and traveled on a cushion of compressed air. Such huge events are extremely rare, but not out of the realm of possibility.

Mudflows are also considered a form of mass wasting, and the upper parts of alluvial fans are the danger zone for the flows containing the largest boulders (which can be ten feet or more across in extreme instances). 
Mudflow that followed wildfires in the San Bernardino Mountains in 2004. Courtesy: U.S. Geological Survey

Wildland-urban interfaces are a rising concern as urbanization spreads into once wild landscapes. Among the greatest concerns are the incidences of wildfires spreading into cities because of their proximity to chaparral-covered slopes. I'm not speaking as an expert here, but it seems to me that alluvial fan surfaces are a more defensible surface than rugged slopes. Housing developments that butt up against the hillsides would seem to be in the greatest danger in our new normal of drought, rising temperatures and increasing wildfires.

Finally, alluvial fans are a unique and rapidly disappearing ecosystem. The alluvial fans hosted a wide variety of shrubs, grasses, and wildflowers, along with excellent habitat for all manner of mammals, birds, reptiles, and amphibians. Some of the Southern California species are found nowhere else in the world. This rare ecosystem has a name, the Riversidian Alluvial Fan Sage Scrub (RAFSS). It is limited to the alluvial fans along the southern exposures of the Transverse Ranges, including the San Gabriel, San Bernardino, and San Jacinto Mountains. Those fans are as much as 90% urbanized now. There is very little of the original landscape left.
Satellite image of the alluvial fans north of Interstate 210 at Upland and Rancho Cucamonga. The blue marker shows the location of the North Etiwanda Preserve
And so we come to the present day. I returned to the landscape of my youth on a trip last week, and the changes were astounding. I knew that urban development had been creeping up onto the fans, but I never had a good look at the magnitude of the changes. Housing developments and shopping centers have swept up like a tsunami onto the upper reaches of the alluvial fans. Chaffey College has been engulfed by the wave of development and is surrounded by housing tracts.

Despite my dismay at the magnitude of urban development, I found out that a significant portion of the RAFSS has been preserved as the North Etiwanda Preserve. The relatively recent extension of the 210 Pasadena Freeway into San Bernardino had destroyed a significant part of the RAFSS, and as mitigation, 762 acres were given to San Bernardino County for preservation in 1998. Other land acquisitions brought the size of the preserve to 1,176 acres (nearly two square miles). Even better, the lands preserved were contiguous with the slopes leading up Cucamonga Peak, providing an intact ecosystem connected to the nearby trees and chaparral of San Bernardino National Forest.

A three mile long trail winds through the preserve, with numerous interpretive signs detailing the geology, biology, archaeology and recent human history of the region. I didn't have time to walk the entire route, but I was able to see (to my tectonic delight) a perfectly pristine fault scarp running across the preserve. You can see the terrace in the pictures above and below. The last major earthquake probably happened one or two thousand years ago, and may have ranged as high as magnitude 7.5.
Cucamonga Peak is a southern California treasure, a fact not always appreciated by those who live on the alluvial fans below. It is a dangerous neighbor as well, with earthquakes, fires, and floods a serious concern. The North Etiwanda Preserve is a wonderful resource for learning about this unique landscape, and is one of the few places where one can get a sense of the landscape that existed before urbanization swallowed it up. 

On a final note, when I was at Chaffey College in 1976, the geology department got a call from a gravel quarry just east of where the preserve is today. They had found a bone of some sort. It turned out to be a fragment of a tusk from a Columbian Mammoth, one of the many megafauna species that wandered these alluvial fans during the last ice age (although no glaciers came close to this place). One could almost imagine the Columbian Mammoths, Dire Wolves, Sabertooth Cats, Giant Ground Sloths, Short-faced Bears, Horses, and Camels that once roamed across Southern California while strolling the trail at the preserve.
For more information about the natural history of the North Etiwanda Preserve, check out this website at the The San Bernardino County Museum (sbcounty.gov).

This post is part of The Other California blog series I've been writing since 2009.

Tuesday, July 20, 2021

The Other California: What to See When You've Run Out of Postcard Destinations (Reprise)

(published by Scope Enterprises, Inc)
My earliest blog project was an exploration of the geologically interesting places in California that don't always show up on the postcards (the earliest post is here). I've been working on this project off and on for a dozen years. If you've followed my blog at all, you know I have a type of Geologist Attention Deficit Disorder Syndrome (GADDS), that as soon as I start concentrating on one subject, something interesting pops up somewhere else, and I explore it for a few weeks, and then get distracted again.

Geologists divide California into eleven geomorphic provinces, areas that share unique geologic histories, rock types and topography that are distinct from the surrounding areas. I generally refer to the province when I am describing a particular feature or place. I am categorizing the posts that exist thus far in the same way:
The Other California: The Things it is Not: the first post describing what I am up to with this series: most people know about Yosemite, Sequoia, Death Valley and other famous places, but California has so much more...things and places that don't appear on the postcards
The Other California: Now This is a Postcard!: A brief overview of the geology of the state as it is represented on geologic maps and and introduction to the idea of provinces.
Come to California and You Could Die a Fiery Death! : A short introduction to volcanism in California

Although the original idea came with a post about California's prairie lands, I pretty much first started out by describing the geologic significance of some of our state symbols.

The Other California: Already Off on a Tangent: A review of the familiar symbols, the Golden Poppy, the California Quail, the California Grizzly Bear, and our state mineral, Gold!
The Other California: Geology and our Other State Symbols: A look at the state mineral, the state rock, and the state gemstone, the one hardly anyone has heard of.
The Other California: Geology and our State Symbols Part 1: a discussion of the evolutionary history of our state fish, the Golden Trout
The Other California: Geology and the other State Symbols II: the incredible journey of our two state trees, the Sequoia and Coast Redwood
The Other California: Geology and the other State Symbols III: a look at our state fossil, the saber-tooth cat Smilodon Californicus.
The Other California: Geology and the other State Symbols IV: California has some unique Gold Rush-era towns, but nothing is quite like Bodie, off in the high desert east of the Sierra Nevada.
The Other California: Geology and the other State Symbols V : The strange politics that led to the establishment of the official California Silver-Rush Ghost Town.
Say Hello to California's New State Dinosaur, Augustynolophus morrisi: The first dinosaur discovered in California was found in our county, Stanislaus.

A huge 400 mile long valley filled with thousands of feet of sediments deposited over 160 million years, and one of the most fertile agricultural regions in the world

The Prairie Lands: California has its own version of savannahs, both present and past.
The Prairies of the Past: An exploration of the most important Pleistocene fossil quarry in central California, the Fairmead Landfill
The Prairie Lands and a Transforming Fault: A journey through Carrizo Plains National Monument and the best exposures of the San Andreas fault to be found anywhere
Back on the Prairielands: A springtime return to the prairies, now green and full of life
Mammoths and Sabertooths rise from the Prairies Again!: The Madera Fossil Discovery Center was almost complete and expected to open in June. Here is a preliminary look
Sharktooth Hill: That's about it...thousands and thousands of shark teeth and a great many other species. 

The Sierra Nevada is the largest single mountain range in the United States, more than 400 miles long and averaging 50 miles wide. It also has the highest peak in the lower 48 states. Although composed mostly of granite, it also has large exposures of metamorphic and volcanic rocks that tell a remarkable story of traveling continents and terranes, as well as tales of violent eruptions.

A Gorge Deeper than the Grand Canyon: An exploration of little-known Pine Creek on the east side of the Sierra Nevada, and one of the most important mines in the country
The Other Yosemite at Hetch Hetchy: There was a counterpart to the Yosemite Valley, but we dammed it.
The Other Side of the Sierra, down the West Walker River: The West Walker River flows down the east side of the Sierra, and hosted the longest glaciers to be found on that side of the mountains.
An Enigmatic Gorge, the West Walker River Canyon: A strange entombed forest, and a deep gorge with a violent geologic temper.
The Day of the Fiddlenecks (A Trip Through the Mother Lode): A brief foray for wildflowers on Highway 132 in California's Mother Lode
There's an Endemic in those Red Hills! Life and evolution on one of California's unique environments, the serpentine soils. Exploring the Red Hills Area of Critical Environmental Concern
The California State Mineral Exhibit-This is art, darnit! One of the best ways to see the incredible mineral wealth of California is to explore the state mineral exhibit in Mariposa at the south end of the Mother Lode. Because of the morons in the state legislature, it is about to shutter its doors
It's a Real Grind...Chaw'se State Historical Park: A look at more grinding mortars than you'll ever see anywhere else, the Miwok culture, and some interesting metamorphic rocks
The Other California Goes Underground: Hella Hot Helictites at Black Chasm Cave: Never heard of helictites? That's because they are the first cave features to be destroyed. But we have a world class collection of them in the Sierra foothills
What do you do with a Used Forest?: The Sierra Nevada between Yosemite and Kings Canyon National Parks is terra incognita for most Sierra travelers. The region has been logged, mined, and grazed...and is still spectacular. We take an excursion on the Sierra Vista Scenic Byway
Why Worry About Yellowstone? We've got our own "supervolcanoes" in California. Some are active. Some have been extinct for tens of millions of years. At the Minarets we can explore one from the inside out

The Cascadia Subduction Zone is a place of geological violence: The crust underlying the Pacific Ocean is sinking beneath the western edge of the North American Continent, producing earthquakes, mountain-building, and volcanism. Two of California's most familiar volcanoes formed here, and the largest volcano in the state sits astride the boundary with the Modoc Plateau

Exploring California's Biggest Volcano: An introduction to the Medicine Lake Highland, one of California's most active volcanoes.
Geologists Who Live on Glass Mountains Shouldn't....: Looking at volcanic glass, obsidian and pumice, on the Medicine Lake Highland, with a view towards the Modoc Plateau, too.
A Land of Fire and Ice (but mostly ice): California's largest glaciers
Five For the Price of One: California's most prominent volcano is really five volcanoes, with a violent past.
A Monday Mystery: A river that comes out of nowhere, and a gratuitous picture of a deer family
A Mystery Solved, and One of California's Prettiest Little Waterfalls: McArthur Burney Falls is California's second oldest park, and is one unusual waterfall.
Lassen Peak, A Volcanic Afterthought: A well-known volcano that sits on the remains of a much older, much larger volcano, Mt. Tehama
Getting all excited about natural disasters: an eyewitness account of some of the events surrounding the 1914-15 eruption of Lassen Peak.

The Modoc Plateau is a high flat region underlain by thick flows of basalt lavas in the remote northeast corner of the state. It is one of the least-known areas of California, but has some nice geological surprises.

California's Biggest Volcano: here is the first surprise; the biggest volcano is not named Shasta or Lassen!
Waiter, There's A'a in My Pahoehoe!: A comparison of basalt lava flows at Lava Beds National Monument
The Volcano Underground: the formation of lava tubes (via a short excursion in Hawaii) and Lava Beds National Monument
Exploring the Volcano Underground: Walking and crawling through the most extensive lava tube system in the continental United States
Whispers From the Past: Huge explosions from 270,000 years ago, and the largest petroglyph panel in the United States
Cries From the Past: A tale of rebellion, resilience and betrayal; the Modoc Indian War of 1872-73. And why armies should study geology before fighting wars.

In northwest California, a series of mountain ridges reveal stories of continental and oceanic fragments that were crushed into the western edge of North America. The rocks bear a close resemblence to the Sierra Nevada, although they are offset more than sixty miles.

Flotsam and Jetsam: An introduction to the Klamath Mountains as a series of accreted terranes, tracts of crust and old ocean floor that traveled hundreds or thousands of miles.
Havin' fun with Sasquatch: a discussion of the legendary and mythological creature that supposedly dwells in the Klamaths
I've seen these mountains before! The Big Ripoff: Viewed on a geologic map, the Klamath Mountains look like a continuation of the Sierra Nevada, but lie sixty miles farther west.
A Journey to the Center of the Earth (sort of): A review of the some of the rocks from the deep crust and mantle that are found in the terranes of the Klamath Mountains
Taking Stock of Castle Crags: One of the most imposing sights (besides Shasta) to be seen on a journey north on Interstate 5, the Castle Crags are towers and domes of granite, surrounded by more easily eroded metamorphic rocks
The series of mountain ranges that roughly parallel California's coastline are one of most diverse areas of the state from a geologic standpoint. There are thick sequences of sedimentary rocks including the Great Valley Group and elements of the Franciscan Complex, and there are plutonic and metamorphic rocks of the Salinian Block (and parts of the Franciscan). There are even volcanic rocks and potentially active volcanoes.

I Need This Like I Need a Hole in the Head: Scenic Bodega Head at Bodega Bay was the nearly the site of one of the most mind-bogglingly stupid energy developments ever conceived by the minds of engineers
Baymouth Bars - It's Five O'Clock Somewhere? Along the incredibly rugged north coast amid the violent surf there are long, perfectly straight sand bars that seem to defy explanation. They're explained here Humboldt Lagoons State Park
A Mystery Photo For a Saturday: A look at San Francisco from a unique angle, Monte del Diablo
The Thicket of the Devil (the mystery photo revealed): An introduction to a place with an incredible view, Mt. Diablo. How it got its name and why every landowner in Central California should care
Limekiln State Park Part 1, Part 2, and Part 3: Limekiln is a beautiful gem of a state park on the Big Sur Coastline. Unfortunately we have morons in the state legislature and this park is closing. See what is being taken from us (PS: It was eventually saved)
Catch it While You Can- The Devil's Slide: You can no longer drive over one of the most notorious rockslides on Highway One. But you can walk it! 

The crust of the earth east of the Sierra Nevada and Peninsular Ranges is being torn apart by horizontal extensional forces that have produced an alien landscape of deep fault valleys (grabens) and high mountain fault blocks (horsts)

The West Walker River and the Antelope Valley: A deep fault valley becomes an important ranching and farming region
Dammed if we do Dam, Dammed if we don't Dam: An unusual reservoir without a dam, Topaz Lake stores Walker River water, but has great fishing too. Why is it there?
Damned if we do Dam, Damned if we don't Dam: A slightly modified name, and a much bigger issue - What are we going to do about Walker Lake? It's dying. It's also in Nevada, but it is a California river.

Often ignored or tolerated by tourists on their way to Las Vegas, the Mojave Desert is one of the most geologically diverse regions of California

The Calico Mountains: An exploration of a unique mountain range, well beyond the confines of the tourist trap ghost town.
Prairies past and Some Great Folds: A closer look at a parking lot in a tourist trap
A Wandering Volcano and a Floral Outburst: The Antelope Valley has a springtime show of wildflowers that is simply audacious, and the underlying rocks include half of a volcano. The other half is nearly 200 miles away...
Caught in the Vise (the Western Mojave Desert): A discussion of complex fault relationships in the western Mojave and another chance to show some wildflower shots.


A diverse series of mountain ranges that run against the grain, trending east-west instead of north-south. These include some of the highest mountains in southern California

The Other California: A Friday Fun Foto: A first glance at San Gorgonio Peak, the highest mountain in southern California, and the southwesternmost glaciated peak in the United States
Scarps to the Left of Me, Sag Ponds to the right, Here I am, Stuck in the Middle with You!: The San Andreas fault cuts across many of California's province. In this post we look at some fault features at the top of the Grapevine in the Transverse Ranges
A Monday Mystery Photo: A quick introduction to the Cajon Pass country where the San Andreas fault splits the San Gabriel Mountains from the San Bernardino Mountains
Cajon Pass and No Strange Sci-Fi Creatures: Cajon Pass, the major freeway access route into the Los Angeles basin, is filled with strange looking sedimentary rocks tilted this way and that. But it's not where Captain Kirk fought the Gorn...
The Mountains of My Youth: The eastern San Gabriel Mountains aren't all that familiar to people from outside the state, but they are spectacular and they were the mountains where I grew up. We explore an extraordinary gorge, San Antonio Canyon
Hemming and Hawing on the Hogback: The San Gabriel Mountains are the steepest mountains in the world. Often the only flat spots are on dangerous stream floodplains and on top of landslides. Several examples from San Antonio Canyon include the Hogback and Cow Canyon Saddle
A Canyon as Deep as the Grand, and a Road For No Reason: The Glendora Ridge Road offers some of the greatest panoramas of any road in southern California, and there doesn't seem to be a reason for it being there. I suspect I know what the reason is
The Forbidden Valley: An introduction to the San Dimas Experimental Forest
A Minor Challenge: A quiz to introduce the unusual geology of the Santa Clarita Valley
Dreams of Avarice and the First Gold Rush: You thought the gold rush started in the Sierra Mother Lode? There was a rush six years earlier, but the Mexican miners kept their secrets better (and there wasn't very much gold, either)
The Oldest Rocks (Well, maybe...): The San Gabriel Mountains have very old rocks, maybe the oldest in the state. But it depends on how you define "oldest". A short introduction to radiometric (isotopic) age dating
A Bit of the Rarest Ecosystem in SoCal at the North Etiwanda Preserve This is what happens when the schist hits the fan. Literally, there is schist, and there is a fan. Also, learn about the Riversidian Alluvial Fan Sage Scrub (RAFSS) ecosystem...
It's Not The The Tar Tar Pits, and Saving the Coast a Bit at a Time: The story of the Carpinteria Bluffs Nature Preserve and the "other" tar pits.

A granitic mountain block vaguely similar to the Sierra Nevada, but also very different. The "peninsula" refers to Baja California, which makes up the bulk of the province

The Other California: Another Friday Fun Foto: A brief introduction to San Jacinto Peak, the highest mountain in the Peninsular Ranges, and one of the most prominent mountains in the state, with a 10,000 foot slope in one area.
A Mystery Photo for the Day: A view of a rock that looks like it belongs somewhere in the Sierra Nevada, but that is not where it is...
When is a Peninsular Range Not a Peninsula? Baja California is a peninsula, but the rocks continue into Alta California. This post explores the village of Idylwild next to the highest part of the province at San Jacinto Peak.
The Deepest Pass in North America, and Finally Taking THE Tramway: Well, sure, the Palm Springs Aerial Tramway is on a lot of postcards, but how many of them tell you the geological story?

I clearly have lots of ground to cover, and will update this page as necessary.

Thursday, July 8, 2021

My Thesis Area is Misbehaving: 6.0 Earthquake Near Walker, California


That was exciting. I live in the Central Valley of California. The valley is famously boring for a number of reasons, and one of the good ones is because we rarely feel earthquakes here. But that wasn't the case today. I was at my computer station and the desk started vibrating and I had to look and see if my wife was shaking it. She wasn't and she was not looking happy. 

It took a while for the USGS to untangle the many wave signatures in the seismic network (there was a brief report of a 4.8 quake just 20 miles from us), but as things have settled out, it looks like the quake took place smack dab in the middle of my masters thesis area in the eastern Sierra Nevada at the small town of Walker, California. The most recent estimate of the magnitude is 5.9 (upgraded to 6.0). The quake has the signature of a normal fault, the kind of faulting to be expected in a crustal region that is being stretched apart. 

Walker is the village in the picture above at the south end of the Antelope Valley, which exists because of faulting. The valley has sunk as the mountains on the right side of the photograph rose along the fault indicated by the solid black line. The epicenter of the quake would be just out of the picture on the lower right side (the picture is looking south).

Antelope Valley sits astride the boundary of the Sierra Nevada and Basin and Range provinces, where the solid block of the Sierra is being sliced up into a series of fault-bounded grabens. The picture below shows the upper (southern) end of the valley from Monitor Pass, with the Sweetwater Mountains in the distance. 

The next picture is from the hill behind Walker looking north. The break in slope on the left is where one would look for evidence of recent earthquakes, but slopewash has covered the fault terraces (scarps) in most places except for the streams and alluvial fans that cross the fault trace. That was what I was searching for when I was doing my masters thesis many years ago in this valley. I was very pleased when I found some.

The person who did the original mapping in the 1950's was working primarily on the rock exposures, and wasn't really looking for recently active faults. By the 1980's a number of people were looking a lot harder, trying to determine the seismic hazard for the region. Fresh alluvial fans provide a possibility of dating the occurrence and size of the last earthquake to cause ground rupture in an area.

The picture below is the Mill Creek fan, at the extreme south end of Antelope Valley. Under normal circumstances, an alluvial fan should be a smooth, gently sloping surface. Here at Mill Creek, the surface steps down to the left, forming a terrace. Immediately after the earthquake this terrace may have been essentially vertical (examples of scarps are shown on this post - Slinkard Valley lies immediately west of Antelope, and the post has a nice cut-away showing the arrangement of the fault blocks).

Scarps like these show that the last major earthquake took place in the recent geologic past, very likely less than 10,000 years ago, and maybe as recently as 3,000 years ago . The length of the fault and the size of the scarps are characteristic of quakes in the range of magnitude 6.5-7.0. A magnitude 5.8 event, the Double Springs Flat earthquake, shook the extreme north end of the Antelope Valley fault system in 1994. Today's quake one-upped that event, but do not be surprised if the magnitude is revised upward or downward (NOTE: the quake was revised upward to 6.0). It takes awhile to fully analyze the seismometer records. It is not inconceivable that some small cracks may appear along some of these older scarps.

I'm listening to reports of a rockfall off the cliffs above Meadowcliff Lodge. That would be very close to the epicenter.

I will revise this post as more information comes in. 

Postscript: I finally got to my office at Modesto Junior College to download the seismogram of the quake, and here it is. The shaking was off-scale for nearly two minutes.

The second shows a compressed version of the quake, along with some of the larger aftershocks.