1971 San Fernando, CA (Sylmar) Earthquake

Anyone else here live through the 1971 San Fernando (Sylmar), California earthquake?

Last night I stumbled upon a YouTube vintage video of the aftermath and it conjured some memories. My family lived in Torrance at the time (I was 8 years old). I remember waking up when it hit at 6:00 a.m. and subsequent aftershocks that would send a swinging lamp in our living room swinging a lot more than it was ever intended to. I also remember the pickup truck flattened under the freeway overpass and a subsequent school field trip to a damaged mission.

Torrance was a ways from the epicenter and I don't remember any fear but it may have been the beginning of my flirting with a career as a geologist (a road I ultimately did not travel....)

I did not realize until Googling the quake today that if the damaged San Fernando dam had collapsed approximately 100,000 people could have perished.


http://articles.latimes.com/1996-02-04/news/mn-32287_1_san-fernando-quake


"The 1,100-foot dam held 3.6 billion gallons of water on the morning of Feb. 9, 1971, but it was only half full; the water level was 36 feet below the lip.

"The top 30 feet of the edifice crumbled, leaving the water only six feet from the top and fresh chunks of earth falling off with each aftershock."

"...A year to the day before the quake, the dam held 6.5 billion gallons of water, and its level was eight to 10 feet higher than the level to which the top of the dam slumped in the quake. Engineers recognized that had such a vast quantity of water spilled over the top, the entire dam would have quickly been washed away."

"...Later, a UCLA study estimated that collapse of the dam would have brought flooding that could have killed between 71,600 and 123,400 people."

"...Lucy Jones of the U.S. Geological Survey commented, "San Fernando clearly nudged the Northridge plane toward having an earthquake. It didn't happen for 23 years, but it could have happened almost immediately as an aftershock.

"For some reason, Northridge wasn't quite ready to go, and we were fortunate it wasn't, because had Northridge occurred right away, then it's pretty clear we would have killed a lot more people."


Retrospective - 41 years later:

http://latimesblogs.latimes.com/lanow/2012/02/sylmar-earthquake-anniversary-dam-almost-collapse.html

"Quake damage also forced four freeways to be closed: Interstate 5, Highway 14, the 405 Freeway and the 210 Freeway."



Vintage video of the emergency response to the quake:

https://www.youtube.com/watch?v=fiabD0WBl7w

I was in Redlands then. I do remember it well. I slept on the top bunk, and was thrown across the room. I was much younger then!

Chilling image of a bunch of clocks salvaged from the severely damaged (later demolished) 3-month old Olive View Hospital -- frozen at the time of the earthquake:

http://framework.latimes.com/2011/02/09/sylmar-earthquake/#/6


USGS data on the quake (now estimated to have been the same magnitude as the '94 Northridge quake.

http://earthquake.usgs.gov/earthquakes/states/events/1971_02_09.php


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Interesting stories. always a good reminder.
Looks like LA escaped BIG trouble - that dam could have been a major problem. we still have dams over that way - I have never studied them in any detail. I wonder if they are less vulnerable now??

Northridge was 1994. That was the last major quake that was "under our feet". It has been 21 years and NO significant quake in LA since then. WE have felt a couple of tremors, but no direct damage. I CHECKED that statistics - So Cal should get two large quakes about every 25 years. That refers to quakes that are 6 or 7 magnitude. So we are getting well overdue for something. WHEN it does come, it will really rattle peoples' fears a lot.

Pete

There has been a lot of research on how a big earthquake like Sylmar '71 leads to Northridge '94 (study below says the former accelerated the latter by a factor of decades).

This study at the link below analyzes the degree to which the 1933 Long Beach quake triggered the 1952 Kern County quake which triggered the 1971 San Fernanco/Sylmar quake and, in turn, the 1994 Northridge quake. Each quake increased stress on the fault lines which slipped in these subsequent quakes.

I'm not sure "triggered" is the most apt term but I'm on my first cup of coffee....

https://profile.usgs.gov/myscience/uploa...cience_1994.pdf


"A model of stress transfer implies that earthquakes in 1933 and 1952 increased the Coulomb stress at the site of the 1971 San Fernando earthquake. The 1971 earthquake in turn raised stress and produced aftershocks at the site of the 1987 Whittier Narrows and 1994 Northridge ruptures. The Northridge main shock raised stress in areas where its aftershocks and surface faulting occurred. Together, M>=6 earthquakes near Los Angeles since 1933 have stressed parts of the Oak Ridge, Sierra Madre, Santa Monica Mountains, Elysian Park, and Newport- Inglewood faults by >1 bar.

"While too small to cause earthquakes, these stress changes can trigger events if the crust is already near failure, or advance future earthquake occurrence if it is not."



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If I were in southern California, I'd be seriously preparing for a Northridge-Sylmar magnitude (6.6-6.7) earthquake to occur at any moment. Especially if I lived in or near the San Fernando Valley.

Anywhere in California such prep would be prudent, of course.


1933 - Long Beach - 6.3
1952 - Kern County - 7.5
1971 - Sylmar - 6.7
1994 - Northridge - 6.7




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I totally agree (I live very close to the Oak Ridge fault). I have found this site veryn helpful in making these preps, although some updating and revising is probably worthwhile.

More fodder for the geology geeks (such as myself, obviously). Note that Northridge (6.7 mag) is considered a "moderate" sized earthquake and the kind of fault that generated it is more likely to trigger a later "moderate" earthquake than is a longer fault.

Very interesting graphics in this report:


http://www.whoi.edu/science/GG/people/jlin/papers/Stein_Lin_OFR_2006.pdf

"A fundamental feature of blind thrust faulting is the stress transfer along strike that leads to successive earthquake sequences or large off-fault aftershocks. M≤6.8 blind thrust faults tend to be square in aspect ratio; in other words, their length is similar to their width [Lettis et al., 1997]. Such short faults are highly efficient at increasing the stress beyond the fault ends along strike, whereas long faults increase the stress beyond the ends only slightly more [Lin and Stein, 2004] (Fig. 14). In contrast, the region of stress drop is much larger for the long fault. This means that M≤6.8 blind fault earthquakes are more prone to trigger successive earthquakes on adjacent fault segments than are M>7 events, which tend to be much longer along strike in comparison to their width (Fig. 14).

"This suggests that the moderate size events like the 1994 Northridge earthquake are capable of promoting further moderate size events in the surrounding crust."


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DAGNY - you are correct on all counts.

So if we follow your logic one step further - the '94 Northridge quake now switches the stress to another fault line. YES, I would say that this makes a LOT of sense. Our problem is ... we don't know the next fault line to go. The fault with the LONG BEACH quake is already overdue again, and will trigger major damage, especially if it ruptures further north (Newport-Inglewood fault line). BUT several other faults are also past-due.

To be honest, I will not be surprised if L.A. gets hit by TWO significant earthquakes in the next couple of years. So much stress has built up, that would hardly be a surprise.

Unfortunately the PEOPLE living here in L.A. are at an all-time LOW with regards to earthquake preparation. The fact that we have gone 21 years since the Northridge quake has lulled everyone into false security. Very few people have enough water to last more than a couple of days.

Pete

duplicate post deleted

Really? Is the glass half empty or half full? While I certainly don't want to claim that SoCal if "fully prepared," there are hopeful signs - the development of CERT, the development of regular EQ awareness events,some enhancement in building codes (although this is far from perfect), and the development and distribution of hazard maps. The mayor is pushing hard for infrastructure improvements which seems like a no-brainer.

EQ preps are interesting. You are getting ready for an event which may never occur in your lifetime,and yet you stockpile stuff and adopt a series of habits which will be useful if it starts shaking. And then you get on with life

Those of us on the Pacific Plate are moving past the NA plate about as fast as fingernails grow,and yes,stress is increasing. When and where this stress will release is anybody's guess. A great deal of the time it occurs on a fault not previously recognized (like Northridge)

One last encouraging note. When Northridge struck, I was on San Miguel Island(the westernmost of the Channel Islands). When Loma Prieta struck, I was again on San Miguel island. That's a 100%, perfect correlation. You will be happy to know I have no plans to visit San Miguel Island in the future,and, if I do go there, I will announce it in advance. That's the least I can do....

It does seem that the area governments are better prepared than ever (but have a lot further to go, especially in regard to dealing with the structural inadequacies of older buildings).

I don't know that the public is less prepared than ever, maybe they are I don't know, but I'd bet that most individuals will find themselves woefully unprepared for a major disaster.

That just seems to be human nature.



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