Finding Faults and Bottomless Lakes in Southern California

My previous post on the geologic hazards of living at the eastern edge of the San Gabriel Mountains resulted in a fair number of comments, essentially all correct, pointing out that the local inhabitants are threatened by earthquakes, landslides, mudflows, floods, fires, and...pet-munching critters. Some also pointed out that every locality faces natural hazards, which is very true, but not all places are equal! One of the serious hazards of living in California is the ever-present threat of major earthquakes, due to the proximity of numerous active fault systems. But ask a Californian (or anyone else for that matter) the name of a fault in our state, and the answer is invariably "the San Andreas". Ask for the name of another, though, and most people blank. The next question one might ask is "how close do you live to the San Andreas, or any other fault?" Unfortunately, that question may result in a blank stare, too.

So, today's post is about finding fault in California. If one thinks he or she has never seen the San Andreas or any other fault, they might find that they actually have, but just didn't know it. The San Andreas in particular is spectacularly exposed along several major freeways leading out of the Los Angeles Basin, along Interstate 5 at Gorman, Highway 14 at Palmdale, and on Interstate 15 over Cajon Pass. That's where we are exploring in today's adventure. A few miles north of the junction of I-15 and I-215, the Interstate climbs a hill, and an exceedingly straight valley appears to the northwest (the picture above). It is called Lone Pine Valley, and it is one of the better exposures of the San Andreas. One can leave the highway at Kenwood Avenue or Cleghorn Road and follow the original Route 66 to see some interesting fault features, including an exposure of the rocks in the fault zone at the Blue Cut.

But first we have to find the Bottomless Lake! It is not clear to me how the whole thing started, but a short Google search reveals discussions of the mysterious bottomless lake in Cajon Pass, including this delightful little excerpt:

"I was there swimming not to long ago and saw a man catch a fish that looked like a bass but the eyes were huge and almost popping out! He was gonna take it home to find out what it was but released it fearing it might die and it turn out to be a pre historic fish. I just believe that its a bass that was way deep down under and its eyes popped out as it was surfaced. But what has my attention is the rumors of lost lake having no bottom! Any info?

Anonymous (not verified) on Mon, 09/06/2010 - 9:32pm.
THATZ TRUE LOST LAKE HAVE NO BOTTOM THEY CHECK ALREADY"

I always find myself wondering who the mysterious "they" are...

Anyhow, Lost Lake is a sag pond along the fault trace, which is fed by springs (groundwater is often forced to the surface along fault lines). I am guessing that the "bottomless" rumor came along when the association of the lake with the fault became locally known... a line of thinking that might go... "faults go to the center of the earth, or something like that, so a lake on the fault won't have a bottom either". Of course one can see the bottom of the lake in the GoogleEarth image, and the crime report I read noted that divers looking for the body mentioned that the the lake is barely 20 feet deep.

The lake has been a trash heap and hangout for undesirable people over the years, but it looked on my visit like the Forest Service was trying to improve the situation. There was not as much trash as I remembered from the past, and signs noted that a recreation fee was required for parking. The cattails were healthy too.
A short walk up the hill provides a view of the fault scarp and the Blue Cut, an exposure of the mangled and broken rock within the fault zone. In the picture below, the San Andreas cuts through the left flank of the hill in the distance and crosses the freeway in the left margin of the picture. Lost Lake is the small pond in the center of the picture.
The GoogleEarth image below shows the fault relations at Lost Lake, including a very nice example of an offset stream. The San Andreas is a right lateral fault, meaning that an object across the fault from the observer is moving to the right (note the arrows in the picture at the bottom). Terraces, scarps and channels at Lost Lake have been extensively studied to construct a chronology of past earthquakes at this site, and there have been some big ones (an older reference is included below). Two historical events involved nearby portions of the fault, the 1857 Ft. Tejon earthquake (with a magnitude in the vicinity of 8), and possibly in 1812, an earthquake that caused serious damage as far away as Mission San Juan Capistrano. The average recurrence interval may be as little as 100 years.

And all of this can be seen from a quick detour off the freeway! It's a great place to learn about faults.

Weldon, II, R.J., and Sieh, K.E., 1985, Holocene rate of slip and tentative recurrence interval for largfe earthquakes on the San Andreas fault, Cajon Pass, southern California: Geological Society of America Bulletin, v. 96, pp 793-812

5.4 Earthquake in Southern California: Not an aftershock this time

A magnitude 5.4 quake has struck in southern California, but in this instance, I don't think this is an aftershock to the Easter Sunday El Mayor-Cucapah quake earlier this year. The quake took place on a different fault system, probably the Coyote Creek or San Jacinto fault. The constant aftershock activity from the El Mayor quake may very well have had a role in destabilizing the fault that moved today. As always, moderate quakes like this serve as a reminder that California is earthquake country, and of all natural disasters, these are the most unexpected. We can't predict them, so we have to be ready for them, and understand the faults and their history the best way we can. I am repeating a description of the San Jacinto fault that I wrote in February after an earthquake swarm in the Redlands area along the same fault system....

From February 19, 2010:

"A lot of people, when they hear of quakes in California, think San Andreas Fault. Many California residents who have lived here all their lives cannot think of the name of another fault in the state, but our landscape is literally crisscrossed by active faults (look at the map above; most of the brown lines are active faults). Most earthquakes in CA happen on faults other than the San Andreas, and this week's swarm is no exception. It appears to be taking place on one what is arguably the most active fault in the state, the San Jacinto fault.

The San Jacinto fault is certainly part of the San Andreas system. It splits off from the San Andreas at the east end of the San Gabriel Mountains, and runs roughly parallel to the San Andreas for 140 miles south into the Imperial Valley. It has the same type of motion, right lateral (features on the opposite side of the fault have been shifted to the observer's right). Since it began moving a few million years ago, something like 15 miles of lateral motion has taken place.

The San Andreas fault is justly famous for several devastating earthquakes, including the 1906 San Francisco event that killed 3,000 people, and the 1989 Loma Prieta quake (the World Series quake). It also produced a magnitude 8 event in southern California in 1857. But most of the time the fault is quiet (in a menacing way; it is storing up stress). Compare this with the history of the San Jacinto system (courtesy of Wikipedia):

1890 - Magnitude 6.5 that occurred in the "San Jacinto or Elsinore Fault region".

1892 - Another magnitude 6.5 occurred in the same region as the 1890 earthquake.

1899 San Jacinto Earthquake - Magnitude 6.4 earthquake destroys San Jacinto and Hemet.

1918 San Jacinto Earthquake - Magnitude 6.9 earthquake strikes the same area that was damaged by an earthquake 19 years earlier, with an epicenter roughly 10 mi NW of the previous earthquake.

1923 North San Jacinto Fault Earthquake - Magnitude 6.3 earthquake damaged the San Bernardino and Redlands area. Last time the fault, which runs under the I-215/I-10 interchange, ruptured in this area.

1937 Terwilliger Valley Earthquake - Magnitude 6.0

1942 Fish Creek Mountains Earthquake - Magnitude 6.3

1954 Arroyo Salada Earthquake - Magnitude 6.2

1968 Borrego Mountain Earthquake - Magnitude 6.5

1987 Superstition Hills Earthquake - Magnitude 6.6 (Note: some consider it to have occurred on a fault completely unrelated to the San Jacinto Fault Zone)

These quakes are considerably smaller than the 1906 event, by a factor of 32 or more (it takes the energy of 32 magnitude six quakes to equal the energy of a single magnitude 7 quake; a magnitude 8 quake is 32 times more powerful than a magnitude 7 quake and more than a thousand times more powerful than a six). But they clearly happen more often . The message is clear: to live in California, we must be prepared not just for the BIG, HUGE ONE, but also lots of lesser BIG ONES."

The picture above is a linear valley along the San Jacinto fault near the mountain town of Idylwild in the San Jacinto Mountains. I was there four days ago. As usual I missed the quake.

Big Waves in Southern California: the Backwash...

A follow-up to my previous post about the huge waves that struck the Southern California coast last week. I was privileged to see some of the intense wave action at Balboa and Newport, and it was a sight. Some of the waves looked like they were reaching the height of the pier, and surfers were delighted. Unfortunately, conditions were very bad for casual swimmers with radio reports of at least 200 rescues, and one person was killed when he got caught between the waves and the boulders of a jetty. Where did these unusually large waves come from?

Waves result from the application of energy to the water in some way. On rare occasions the energy is supplied by an earthquake or volcanic eruption, and disasterous tsunamis (incorrectly called tidal waves) are the result (update: you can read more about tsunamis in this new post).

Most of the time waves are generated by wind blowing across the surface of the water. We are all familiar with ripples resulting from a light breeze, and it may be hard to imagine wind producing 20 foot monster surf, but when the wind is blowing hard, and over a distance of many miles, that is exactly what happens. We know from movies and "Deadliest Catch" (and for some, from personal experience) that waves in the middle of storms are violent, large and unpredictable. The waves are capable of swamping and sinking boats and ships. But what about when the storm is past? Shouldn't the sea calm down, and the waves disappear?

Sort of. They disappear because the storm is dissipating, but waves are a form of energy transfer, and the energy is not stored. It has to move. The longer the wind blows and pummels the surface of the sea, the more the energy builds up and it starts to emanate outwards, sort of like ripples in a pond (well, exactly like that). The thing is, waves are not exactly physical objects. The swells are moving, but the water is staying in place, rising and falling in a circular pattern.

Winter storms generally produce the biggest waves along the California coastline. When I lived in Santa Barbara, the summer swimming was usually no more turbulent than a swimming pool full of kids. But when the big storms started to blow out of the Bay of Alaska, the surf got intense. I lived a third of a mile from the seacliffs, and some nights I could feel the force of the waves impacted the rocks.

So, why doesn't anyone surf in the open sea? That would be because the waves in the open sea are in the form of swells, which don't break or roll, they just simply pass by. The turbulence of the water extends to the depth of about half the wavelength (the distance between the crests of the swells). They are the cause of sea-sickness, though. It isn't until the waves hit the shallow coastlines that things change, and that is the whole dynamic of what happened in California this week.

When swells encounter shallow water the energy is compressed into a smaller space. Friction slows the forward movement of the base of the swell, but the top of the swell continues forward unimpeded and begins to stack up, very steeply on the side of the wave facing the beach. The water finally spills over, forming the breaker, and sending a pulse of water upwards onto the beach itself. What's different now is that the water is physically moving forward with great force, and the energy is expended against the cliffs, the sand, or the bodies of the surfers who have wiped out. The precise amount of energy is determined by many things, including the shape of the coastline, the orientation of the coastline, the arrangement of offshore islands, and the distribution of shoals beneath the surface.

So it was that a typhoon in the south Pacific Ocean around Tahiti generated swells two weeks ago that traveled thousands of miles across the ocean basin in a predictable pattern and speed. Their arrival in southern California was foretold and warnings went out to safety authorities. The red flags went up on the beaches, the surfers appeared out of nowhere, and the giant waves started to pound certain south-facing beaches. For a few people it was a nightmare and a tragedy, but for most it was a great spectacle (and one geo-blogger put up a couple of pics of the waves and saw his readership quadruple this week...thanks for the visits!).