Southern San Andreas fault waiting to explode

Something sure to put So Calif TUGgers at ease: Southern San Andreas fault waiting to explode

The southern end of the San Andreas fault near Los Angeles, which has been still for more than two centuries, is under immense stress and could produce a massive earthquake at any moment, a scientist said on Wednesday.

Yuri Fialko, of the Scripps Institution of Oceanography at La Jolla, California, said that given average annual movement rates in other areas of the fault, there could be enough pent-up energy in the southern end to trigger a cataclysmic jolt of up to 10 meters (32 ft).

"The observed strain rates confirm that the southern section of the San Andreas fault may be approaching the end of the interseismic phase of the earthquake cycle," he wrote in the science journal Nature.

A sudden lateral movement of 7 to 10 meters would be among the largest ever recorded.

According to the U.S. Geological Survey (USGS), the earthquake that destroyed San Francisco in 1906 was produced by a sudden movement of the northern end of the fault of up to 21 ft.

Fialko said there had been no recorded movement at the southern end of the fault -- the 800-mile long geological meeting point of the Pacific and the North American tectonic plates -- since the dawn of European settlement in the area.

He said this lack of movement for 250 years correlated with the predicted gaps between major earthquakes at the southern end of the fault of between 200 and 300 years.

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Back in my days working for Calif government, I was involved in disaster planning for water utilities. For awhile I was a member of a committee of the American Society of Civil Engineers on earthquake engineering for lifeline utilities. I first got involved in that topic when I was working in San Bernardino and crossed the San Andreas Fault every day on my way to work.

The information in the story is really nothing new. It's been clear for years that the southern leg of the San Andreas Fault south of Carrizo plain (in the Coast Ranges west of Bekersfield) is the most dangerous section of the fault.

For those familiar with So Calif geography, the problem is easy to see. The San Andreas fault separates the Pacific Plate (everything west of the San Andreas fault) from the North American Plate (everything east of the San Andreas fault). The Pacific Plate is mostly sliding past the North American plate, moving to the north-northwest relative to the North American plate.

Most of the movement on the San Andreas is called "strike-slip" movment, where the two plates slide past each other, without one of the plates riding over the top of the other one. The exception to this is in Sothern Clalifornia, where the San Andreas between Frazier Park and Palm Springs takes a more east-west orientation and bears WNW-SSE. As a result of this bend, the mountains in the Los Angeles area associated with the San Andreas fault (the San Bernardino, San Gabriel, and Santa Monica Mountains) all follow this east-west oridentation (in contrast with the NNW-SSE orienation of almost all other mountains in California associated with the San Andreas).

But more importantly, that east-west trend in a fault where the two sides of the fault are trying to move north-south means that in that area the plates collide into each other instead of sliding past each other. That's why the mountains in that east-west trending stretch of the San Andreas (between Palm Springs and Frazier Park) are so much higher elevation than othe San Andreas related mountains.

Further, it's easy to see how, when two plates collide instead of slip, the movement gets locked up for a longer time. Then when the stresses become too great, the release is much larger and more violent. That east-west trend of the San Andreas fault is also the source of the concealed deep thrust faults that underly the LA Basin, such as the fault that caused the Northridge earthquake.

Bottom line is that the Los Angeles basin is the area of California with the most severe earthquake hazards.

As I understand it the damage done from the actual earthquake in 1906 was heavier here in Santa Rosa than in San Francisco. It was the fires in San Francisco that did the most damage. They were unable to put them all out and many of them were left to just burn out. They have been telling us to prepare for the "big one" for years. Most people know that it will probably come but unfortunately most people here also are not insured for earthquake damage because it is so expensive to add on to your home owners policy.




The Destruction of Santa Rosa
The San Francisco Calamity by Earthquake and Fire
Charles Morris, LL. D.
In Santa Rosa, sixty miles to the north of San Francisco, and one of the most beautiful towns of California, practically every building was destroyed or badly damaged. The brick and stone business blocks, together with the public buildings, were thrown down. The Court House, Hall of Records, the Occidental and Santa Rosa Hotels, the Athenaeum Theatre, the new Masonic Temple, Odd Fellows' Block, all the banks, everything went, and in all the city not one brick or stone building was left standing, except the California Northwestern Depot.

In the residential portion of the city the foundations receded from under the houses, badly wrecking about twenty of the largest and damaging every one more or less; and here, as in San Francisco, flames followed the earthquake, breaking out in a dozen different places at once and completing the work of devastation. From the ruins of the fallen houses fifty-eight bodies were taken out and interred during the first few days, and the total of dead and injured was close to a hundred. The money loss at this small city is estimated at $3,000,000.

The destruction of Santa Rosa gave rise to general sorrow among the residents of the interior of the State. It was one of the show towns of California, and not only one of the most prosperous cities in the fine county of Sonoma, but one of the most picturesque in the State. Surrounding it there were miles of orchards, vineyards and corn fields. The beautiful drives of the city were adorned with bowers of roses, which everywhere were seen growing about the homes of the people. In its vicinity are the famous gardens of Luther Burbank, the "California wizard," but these fortunately escaped injury.

Two theatrical people were in a hotel in Santa Rosa when the shock came. The room was on the fourth floor. The roof collapsed. One of them was thrown from the bed and both were caught by the descending timbers and pinned helplessly beneath the debris. They could speak to each other and could touch one another's hands, but the weight was so great that they could do nothing to liberate themselves. After three hours rescuers came, cut a hole in the roof and both were released uninjured.

The list of dead was not confined to San Francisco, but extended to many of the neighboring towns, especially to Santa Rosa, where sixty were reported dead and a large number missing, and to the insane asylum in its vicinity, from the ruins of which a hundred or more of dead bodies were taken.

swift said:

As I understand it the damage done from the actual earthquake in 1906 was heavier here in Santa Rosa than in San Francisco. It was the fires in San Francisco that did the most damage. They were unable to put them all out and many of them were left to just burn out. They have been telling us to prepare for the "big one" for years. Most people know that it will probably come but unfortunately most people here also are not insured for earthquake damage because it is so expensive to add on to your home owners policy.
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During an earthquake damage is much more closely related to subsurface soil conditions underlying a structure than to the distance to the rupture (as long as the rupture doesn't go directly under the building). When we lived in the Bay Area that was one of the factors we considered when we bought our house. Where we lived (in northwest CoCo County), half the house was directly on bedroock and the other half of the house had less than two feet to bedrock. We came through the Loma Prieta with no damage except for a few items that fell off of tables and shelves.

In the 1906 quake, damage in many aras in Santa Rosa (as well as Napa, Vallejo, Concord, Stockton and Sacramento) was for worse than in the hilly areas of San Francisco (which are close to bedrock). The areas that received heavy damage, such as Santa Rosa, were built on loosely consolidated soils with shallow depths to groundwater.

Areas of San Francisco and the East Bay that were built on similar materials also sustained very heavy damage in the 1906 quake (just as they did in the 1989 Loma Prieta Quake). This included most of North Beach, the Mission, the Marina, Noe, Cow Hollow - basically all waterfront areas and the flat valley lands between the hills.

There is quite a bit of myth about the fire after the San Francisco quake. The 1906 quake massively damaged San Francisco. There was an orchestrated coverup of the amount of damage caused by the quake because quake damage was not insured, but fire damage was insured. So San Francisco ran a very effective PR campaign to attribute most of the damage to the fire. In areas that were flattened by the quake, the City make little attempt to put out the fires (logically they were directing resources to protecting areas that were less damanged). The property owners then claimed the damage was caused by the fire, not the quake.

I was just wondering ... should I go to California to be shaken by an earthquake, pop along to Yellowstone to be blasted by a super volacano or sit on the east coast beach to be swamped by the tsunami that an underwater landslide in the Canary Islands will cause? Perhaps I can just wait for the comet to arrive.

Or shall I just stay at home with my head under my duvet?

Nah, don't like that last option at all.

ellend said:

I was just wondering ... should I go to California to be shaken by an earthquake, pop along to Yellowstone to be blasted by a super volacano or sit on the east coast beach to be swamped by the tsunami that an underwater landslide in the Canary Islands will cause? Perhaps I can just wait for the comet to arrive.

Or shall I just stay at home with my head under my duvet?

Nah, don't like that last option at all.

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You can always go the Caribbean and wait for a hurricane.

Or, go the western Pacific where you can experience all of them: earthquakes, volcanoes, typhoons, and tsunamis.

Well, this is all very interesting. I just hope I'm at home when that portion of the San Andreas goes, because then (if I'm lucky), my house will slide down the mountain it's perched on and I won't be around to worry about its loss.

Seriously, I worry far more about brush fires than I do earthquakes (they interfere with my daily life far more). The last time there was a major earthquake of the San Andreas around the Frazier Park area was in 1857, the Fort Tejon earthquake (estimated at around 8.0, initial rupture was apparently in Parkfield and traveled to the Cajon Pass area near San Bernardino, but Ft. Tejon was about the midpoint of the rupture and the only populated locality along the rupture).

Did you know that ground water availibility changes significantly - if you are on one side of the fault line there's pretty good water. A few hundred yards away there isn't half as much. That's another, more immediate hazard for this area - lack of water.

So take your pick - fires, earthquakes, no water. Trust me, you don't want to visit California, do you? Living there is out of the question (never mind that you can hike within 5 minutes of your home, there's snowshoeing and nordic skiing 10 minutes away or out your front door, no sirens at night to listen to and fabulous pine covered ridges to look out on), right?

mtngal said:

... Did you know that ground water availibility changes significantly - if you are on one side of the fault line there's pretty good water. A few hundred yards away there isn't half as much. That's another, more immediate hazard for this area - lack of water.

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That's one of the techniques geologists use to map a fault.

In the north San Bernardino area there are about 10 branch faults of the San Andreas that are very clearly delineated by changes in groundwater level.

Steve, that's all very interesting stuff. Thanks for posting so much info on it. I was reading about it in the paper this morning, but having been born and raised in So. Calif, and living through a few "big ones", I still know little about the physics of earthquakes, and I doubt if it would do me much good if I did.
We lived in Laughlin when the Northridge blast occurred. (My older daughter lived in Northridge.)
I remember being very shaken by an article in the Times Parade magazine in '89(?). It was a very descriptive depiction of what might happen in a worst scenario, and it focused on Marina del Rey, which is basically built on sand. It was very scary.
At the very least, it's a good reminder to examine and update our emergency supply kits, which we've carried since the 80's, when we lived in Florida. (Yes, we've updated them since then. )

Jaybee said:

I remember being very shaken by an article in the Times Parade magazine in '89(?). It was a very descriptive depiction of what might happen in a worst scenario, and it focused on Marina del Rey, which is basically built on sand. It was very scary.)

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Marina del Rey, as with many of the waterfront areas in southern California, will suffer extensive damage. The most vulnerable areas are those locales that are built on old filled areas, followed by areas that are builit on loose sedimentary soils (sands and silts) along the coast.

Other highly vulnerable areas include steep slopes on hillsides and canyons. Those areas are unstable to begin with, as can be seen by the frequency of landslides during heavy winter rains. Add some ground shaking, and the faces of of many of those hillls are going to come down.

T_R_Oglodyte said:

Marina del Rey, as with many of the waterfront areas in southern California, will suffer extensive damage. The most vulnerable areas are those locales that are built on old filled areas, followed by areas that are builit on loose sedimentary soils (sands and silts) along the coast.

Other highly vulnerable areas include steep slopes on hillsides and canyons. Those areas are unstable to begin with, as can be seen by the frequency of landslides during heavy winter rains. Add some ground shaking, and the faces of of many of those hillls are going to come down.

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.....as will the high rises when the vibration sends the water upwards, and there is no foundation. Isn't there a biblical quotation about not building your house on sand? Maybe the developers didn't read that part?
I also worry about south Florida, and all the high rises built on barrier reefs. Of course, they aren't worried about earthquakes, but a hurricane hitting the right area with a 12' tide, and WHAM!
So, it's been centuries since the south San Andreas had a big shaker. We watched a documentary about Mt. St. Helens the other day. We should never be complacent.

Jaybee said:

.....as will the high rises when the vibration sends the water upwards, and there is no foundation. ...

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Most high rise buildings are built on deep foundations, typically drilled piers or friction piles. In most cases they very safe places to be in case of an earthquake.

Many newer low rise buildings in seismically sensitive areas are also built on stable piers. Twenty-five years ago when I was involved with disaster planning, the planning assumptions for Emeryville and Foster City in the Bay Area in case of a monster quake was that the residential condos close to the water would survive intact, but all utility services and vehicle access would be cut off. That's because the builings were built with good foundations. The utilities, though, would simply float towards the surface when the soils liquified. The liquefaction also pretty much destroyed roads and parking lots.

The question the utility people needed to work on was how to respond to the people who were trapped in the buildings without utility services and with limited food and medical supplies.

ellend said:

I was just wondering ... should I go to California to be shaken by an earthquake, pop along to Yellowstone to be blasted by a super volacano or sit on the east coast beach to be swamped by the tsunami that an underwater landslide in the Canary Islands will cause? Perhaps I can just wait for the comet to arrive.

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Or you could go to Nashville and get hit by a quake on the New Madrid fault, or hit by a tornado (Just ask JLB on that one).