Newfound L.A. Fault Threatens Major Quake

Stefan Lovgren
for National Geographic News
April 3, 2003



Between the sun and the stars, Los Angeles sometimes seems like paradise. But life in the City of Angels comes at a price: earthquakes.
Now the threat of "the big one" may be greater than previously feared. Researchers have identified a buried fault that may have caused at least four large-magnitude earthquakes in the past 11,000 years and is still active.

Known as the Puente Hills Blind Thrust System, the fault is three to 17 kilometers (2 to 11 miles) deep and extends for almost 50 kilometers (31 miles) from northern Orange County, through Los Angeles, up to Beverly Hills.


Earthquakes pose a constant threat to California. A recently identified fault found beneath the Los Angeles basin suggests the threat to the city is greater than previously thought.


"In terms of location, it couldn't be much worse," said James Dolan, a professor at University of Southern California's department of Earth sciences, who led the study. "Downtown L.A. is sitting on top of this thing."

Paleoseismologists have previously pinpointed the locations, magnitudes, and dates of ancient earthquakes, but never in so-called blind thrust faults. These are faults that don't extend to the surface of the Earth. Scientists have in fact debated if such faults exist beneath Los Angeles. The new study shows they both exist and could pose a credible earthquake hazard.

Earthquakes New and Old

The researchers received help for their study from an unexpected source: the oil industry. Companies like Texaco, which have spent millions of dollars on geologic drilling research in California, provided scientists with invaluable research data.

Using that information and high-resolution seismic reflection data, Dolan and colleagues drilled 15 bore holes, up to 40 meters (130 feet) deep, to study sediment layers overlying the hidden fault. What they found was subtle folding of the sediments revealing a history of ancient earthquakes.

The study shows the occurrence of at least four earthquakes with a magnitude of 7.2 to 7.5 on the Richter scale during the past 11,000 years. Perhaps most importantly, the 6.0-magnitude Whittier Narrows earthquake occurred in 1987 along a segment of Puente Hills, demonstrating that the fault system remains active and dangerous.

Squeezing L.A.

Geodetic studies show that Los Angeles is contracting. The northern point of the L.A. basin is moving closer to the southern point. "L.A. is being squeezed from north to south at about 4 to 5 millimeters [0.15-0.2 inch] per year," said Dolan.

This shortening, part of which is happening on top of recognized fault systems, literally bends the rock in the ground. The process stores energy, and when this energy exceeds the strength of the system, the fault breaks, triggering an earthquake.

Scientists believe that up to half of the energy stored in this process could be released on the Puente Hills fault.

"The good news is that major earthquakes along this fault are very infrequent, it may not happen again for thousands of years," said Dolan. "The bad news is that it could be very strong when it does happen."

An earthquake with a magnitude of about 7.5 on the Richter scale probably occurred on the Puente Hills fault 7,000 to 8,000 years ago. That means it was 15 times stronger than the 1994 earthquake that hit Northridge, north of Los Angeles, and killed 51 people.

The Northridge earthquake, which measured 6.7 on the Richter scale, caused U.S $44 billion in economic damage and is the largest natural disaster in U.S. history.

The tremblor proved that an earthquake smaller in magnitude can cause greater damage than a more powerful earthquake. The shaking in Northridge was some of the worst ever felt.

Bowl of Jello

In bigger earthquakes, shaking lasts longer and is felt over a larger area. There is a difference in frequency content between small and large earthquakes. Small earthquakes have higher frequency energy and can be particularly harmful to homes. Big earthquakes have low frequency energy and may cause more damage to large structures like skyscrapers.

What makes Los Angeles particularly vulnerable to any earthquake is that part of the basin the city is built on is filled with weak sediments. "The fault will pump energy directly into the basin and cause it to shake like a bowl of Jello," said Dolan.

Mexico City has a similar problem. That city was severely damaged in a 1985 earthquake, even though the epicenter lay in far-away Acapulco.

Establishing what kind of earthquakes could happen is critical for seismic hazard zoning, emergency response, and risk mitigation strategies.

"You want to find out as much as you can about the threat," said Dolan. "That gives you a batter chance to prepare for it."

The main challenge is to build earthquake-proof structures. As seismologists are fond of saying, "Earthquakes don't kill people, buildings do."

A summary of the research appears in the current issue of Science, the journal of the American Association for the Advancement of Science.

I can guarantee you that they come quite suddenly and can be very violent. Example: in the 1971 Slymar earthquake I was thrown out of my bed and landed on my feet! Whilst waking up, it was one of the erriest and surreal experiences that I've ever had, especially because as I was trying to make sense of what was happening, the walls were cracking, pictures falling & dishes being thrown from the cupboards to the floor.

A friend of my brother's who had a clear view of the downtown L.A. skyline said that he could clearly see the skyscrapers swaying from side to side. He freaked out so bad that he & his girlfriend returned to Rochester, N.Y. within a couple of weeks.

During the 1992 Monrovia quake, I was suddenly awake to see & hear the falls cracking, I was out of the building in a flash. And several of my neighbours moved to other states because of their fright from that experience was so great. BTW: that building was only 1 block from Cal-Tech where the seismic monitoring facilities are.

Some amazing footage is of the Okland quake that caused the upper tier of the bridge to collapse. One car was literally hanging over the edge held only by it's rear wheels.

I suppose the feeling I got is of how fragile and powerless we humans are when faced with natural disasters.

Seismic zones are very interesting, about the best way to see dynamic geology in action, lol! However, I am glad I am from the Gulf Coast area... nice and stable... Just not a good feeling to feel the ground dancing under you...

Wow, if you survive the smog, gun violence, drugs and HIV you still have earthquakes to deal with.

XAOS

Which is why I am NOT on the west coast!

Excellent article! One question for my smarter brothers and sisters out there... any estimation as to when the big one will happen? (key word here is estimation).

I know cayce, nostradamus, etc. say by 2003, 2007.. what do you all think?

If you asked me that a couple of years ago, I would have said the Oakland Earthquake was the big one... The rocks associated with the San Andreas are highly fractured, and many geologists believe that they are so highly fractured to be incapable of storing sufficient energy to cause an earthquake significantly greater than about 7.5.

That was before I started looking into the newer theories regarding quake swarms and blind thrust faults. (Yes, I am a geologist, but I am NOT a seismologist. I am trained in petroleum geology, and seismology is sort of a hobby if anything for me). This new data looks like this previously unknown fault zone could store much more energy than the San Andreas, resulting in much worse damage.

The one seismic zone I can think of that would be equivalent would be the New Madrid Seismic Zone, located along the western boundaries of Kentucky and Tennessee. The last time this fault zone was active was in 1811. Although we have no solid proof of just how bad that quake was, it was estimated to be 9+ on the richter scale (higher than anything currently measured). It is known to have made the entire Mississippi river run backwards, and buried 2 towns under it.

My personal suspicion is that sometime in the next 5-10 years, something very large is going to cut loose on the west coast. I also suspect (granted I have no hard evidence for this, just a hunch) that such activity will be linked to volcanic activity in the Washington/Oregon area. I think that we are going to have a massive eruption with Mt. Ranier shortly, and it may trigger something very large....

DragonRider, I was just getting ready to speak of the new Madrid fault we have here. I live in KY and though not a geologist, I do work in the geotech field. I monitor that fault through a series of email notifications when an earthquake occurs there. It averages about 3-5 a week on a scale of 1.5-2.5. Nothin major but you're right as rain about the last one(s) in 1811,1812. We worry about Ky,s highway infrastructure during a quake, bridges and liquification of slopes. It is a buried fault, under a lot of glacial outwash brought down by the Ohio river. Hope I'm not close if it decides to really move but i would say, I'll be on my way out there if it does.

Actually, Nostradamus never said anything about California earthquakes, and Cayce said in the 1990's, when Atlantis would rise from the depths.

Astrocreep:

I will admit I dont have a lot of experience with active seismic zones, although the NMSZ is the one I would worry about over and above the San Andreas. For one, it has been largely inactive long enough to build up serious energy concentrations, meaning when it does rip loose, it is going to be spectacular.

As far as survivability, I would certainly take a look at your local flood and topo maps, make sure you are not in a drainageway or down gradient from a dam or other water body in the event that it happens. You are experienced in geotech ( by the way, I spent 2 years doing geotech, you learn some good practical geology that way), so you have an idea of what you are likely living on. If you are on solid limestone (which I think most of Eastern KY is) you should be ok. But, if you live on mainly sedimentary soils, you are SOL, as it will readily liquify in a serious quake.

In any event, if a serious event takes place GET OUTSIDE RIGHT NOW!

DragonRider, The flooding potential in West KY is very discouraging. Two lakes, the KY Lake and Lake Barkley ar the largest in the state and would probably do some damage to Paducah, the largest city out there in West KY. As for me, I'm in Central KY, I live on the border of the knobs region and the Eastern coal fields. Soil here is more clayey and safe enough from what my boss tells me, he's a geologist here. The real danger is the sandy silts out there in the west. Liquafaction is almost a certainty. They worry a lot about structures but not really about embankments. Hard to cross a bridge when you can't get to it. A couple years ago, they tried to get me a project to monitor land mass movement with GPS. I monitor localized landslides now with survey grade equipment. But, being a buried fault, it was thought of little use when no real surface movement would be detected until the big one hit. Plus, it was just too big for one guy to do anyway. Still, I can't help my own curiosity and hope I can do some work on that topic some day. They thought of letting me get some samples and doing some liquifaction frequency work but it hasn't panned out yet.

Astrocreep:

I can certainly understand your concern with the lakes in West KY. I would also be very concerned with Memphis TN to your south. I dont know about flooding potential there, but it is a very built up metro area, with a large population. If something around a 8 or 9 Richter hit, there would likely be massive loss of life there.

I have also read a couple of articles about splaying faults associated with the NMSZ. There is an older and largely inactive fault zone to the the north and west near Illinois/Chicago. A couple of theories are that these splaying faults might connect up to the fault zone near Chicago: if the big one rips loose, it could transmit enough seismic energy along these faults to seriously affect Chicago as well.

Your surface readings using GPS or laser sounds pretty interesting. They do the same thing in the pacific northwest on suspect mountains (there is a bit of interest in Mt. Ranier, which I sort of think is the next one to explode). I agree, such activities are of limited use, and mainly only useful in a Mt St. Helens type of scenario, where you are monitoring the swelling or shifting of a known slope. For the NMSZ they would likely do much better with buried monitoring points using stress/strain meters, temperature, dialectric, ect. Unfortunately, these are all very expensive prospects, which is why I suspect they are trying to run surface measurements instead.

As far as survivability, the tight hard clays are better than the sand/silt (the most dangerous areas by far), but I would prefer to be on good solid limestone if at all possible. If this were an option for you, I would seriously consider relocation to an area with a solid concrete foundation on solid limestone. Also, I would avoid any areas on top of or immediately under any slope greater than 15 degrees.

Maybe this great fault line has to do with the quake that's going to seperate LA from the mainland like some predictions state.

I dont know that California will ever fall off into the ocean (sorry, STILL cant get that beach property in Vegas!)

What is going to happen is that California will slide northward and eventually end up around Washing, while Baja Mexico will become the new west coast.

Hope you guys on the left coast like Mexican food!

I don't think it would go under either just move or branch off of the US mainland.

i lived there a few years back and to the west of their are some big lakes, don't remember their names though. uh i think there was a small shake while i was their too. i know there was an earthquake up here in new york in plattsburgh. didn't feel it though. east coast is better than westside. we don't fall into the sea.

Of course this action will take thousands of years, as the rate of movement is measured in millimeters/year.

There IS a good chance of a series of very intense quakes that would cause extensive damage to most of California however. I also see a very serious pacific Northwest eruption (Most likely Mt. Ranier) in the near future, that may well dwarf Mt. St. Helens, and likely with much worse loss of life.

just glad i live in south dakota. the only thing we have to worry about is tornadoes and you can see them coming for the most part. but i remember the news saying something about an earthquake here last year. it was small somewhere around a 3 or maybe a 4. just enough to make you go hmmm what was that?

Yes, I live in an area known to be seismically stable, but we still have (low level) earthquakes.

As an aside, not many people know that the oil industry for years has been artificially triggering earthquakes.

Even in stable areas, a certain amount of strain energy builds up in the bedrock. In most oil producing regions, many oil producers use injection techniques (drilling injection wells near oil bearing strata and injecting high pressure water to increase the water table, increasing the pressure and recoverability of the oil) in increase thier oil recovery. When enough water pressure is applied to substrata, it acts as a lubricant, allowing the strain to be released in a small quake.