Calif. quake
22 messages in this thread
Hi, folks–
This morning at 4:31 am Pacific time there was an earthquake, tentatively
identified as Richter 6.6, centered 20 mi SW of San Fernando, Calif.
The fault has been tentatively identified as a shallow thrust fault at a
shallow angle (less than 45 degrees from horizontal was the implication). This
suggests the Santa Monica mountains have resumed their march on Los Angeles;
this movement of the transverse (east-west) mountain range north of greater
L.A. generated the Sierra Madre and Whittier Narrows quakes of recent memory.
Like those quakes, this one was in a populous area (the San Fernando valley,
but it was significantly greater in magnitude. For comparison, note that the
Bay Area quake of 1989 was only somewhat larger in magnitude, and was 80 miles
from the area where hundreds died, while this one was directly under heavily
populated areas and "shallow". There are at this time (6:30 am Pacific) no
confirmed reports of fatalities from the quake, though it is too soon to assess
the damage.
Forum regular Dr. Rod Burroughs lives very near the quake site.
–Doug
ANIMAL PRECURSORS Yesterday Afternoon (4PM PST) I observed a neighbors horses
acting very strange. They were butting against their water troff and trying to
break through the pipe fence. They actually moved the water by a few feet. I am
located in the High desert near Hesperia, CA. This could have been animal
behavior prior to the San Fernando Valley Quake of '94. I would be interested
in hearing from any other people who are also observing strange behavior prior
to a major earthquake.
Thanks
Frank Condon
Frank, even if you get some other observations of strange animal behavior, it
doesn't prove anything….until you also keep track of strange animal behaviors
at times when there aren't earthquakes, i.e., all of the time. There HAVE been
observations of anomalous behavior that probably indicate some sense of
vibrations or whatever, notably before some Chinese quakes, but the typical
after-the-fact anecdotes aren't anywhere nearly statistically significant.
Also, this reminds me of "flashbulb memory" I think it's called, which commonly
shows up in courtroom testimony etc.
Here in Santa Clara county (San Jose etc) we are stricken with a county
geologist who publishes a monthly newsletter featuring such morsels as a "Pet
of the Month" column (including Twiggy, the tiny Yorkie, etc) but no
statistical analysis of off-quake behavior. It's frustrating. Richard
> Forum regular Dr. Rod Burroughs lives very near the quake site.
And I, for one, would like very much to hear from him! Hey, Rod – you OK?
Anything any of us can do to help??
BTW, I take it that this quake was not related to the CO quake? If memory
serves, the CO quake was two miles south of Santa Monica. Todays quake would
have been some distance (North?) East of Santa Monica. Is this correct?
-Guy
Hi, Guy–
To the best of my knowledge, you are right – the Santa Monica quake last week
seems unrelated to today's Northridge quake (that's what they are calling it
now – it may change again, like the Whittier Narrows quake kept changing its
name during the following days as they got ever-better fixes on the epicenter).
–Doug
Finally got a phone line to give the "latest" on the Northridge earthquake. I
live in Arcadia (next to Pasadena), and although the shaking was quite intense
here, there was very little serious damage here in the San Gabriel Valley (all
the damage-prone structures in this area were taken care of in the Whittier
quake).
Kerry Sieh (Cal Tech seismology scientist) was just on one the local TV
stations and believes the quake was a slip on the Oakridge fault, a fault which
runs from the San Fernando Valley westward to Ventura and offshore. This fault
has southern dip of 60 degrees (higher than the preliminary estimate). Since
4:31 this morning, there have been several aftershocks above 5.0, and numerous
aftershocks above 4.0. (The latest aftershock rumbled through about 5 minutes
ago, probably about 3.6-4.0).
All 7 local stations have been providing continuous earthquake coverage since
this morning. Because Arcadia is about 25 miles from the epicenter, you "see"
the aftershocks on TV, and three seconds later it arrives here (mostly Surface
waves, although in the larger aftershocks S and P waves can also be felt).
Most of the fires are out now (8:00 PM PST), but there are 15,000 homeless, and
many residents of the San Fernando have no water or electric service. Most
Angelenos concerned about the freeway situation; three have been closed because
of structural failure to overpasses. Depressing to many is the failure of an
overpass on the newly constructed Century freeway (it opened last fall). The
engineers will have their hands full with this one.
It'll be a long night (another aftershock just went through) for a lot of us,
hope we don't fall into the ocean overnight! (g)
Phil
Hi, Phil.
I lived in Glendale during the Sylmar quake in 1971. I am also a structural
engineer now residing in San Diego. The quake appears to have had a
significant vertical component. Since the Oakridge fault is a thrust fault
this is not surprising. The problem is that our building codes do not account
for vertical accelerations exceeding 15% of G. I think that will now need to
be revised.
This is important in buildings that have a "transfer" level of framing. An
apartment built over parking is an example. Because the spacing of parking
spaces does not fit neatly with the typical widths of apartments beams are
frequently used to transfer forces from walls above to columns below. With a
significant vertical component of acceleration these beams can be
overstressed and fail. Even if they don't fail they can be bent so much that
they will be pulled off their supports.
The San Andreas is a transverse fault and will generate far less in the way
of vertical acceleration. It's horizontal components, though, will be
whoppers. These are accounted for, however, in current codes.
It appears that a code revision is in the offing.
Mark
Hi Mark, I must be learning something because your explanation was crystal
clear! I will do some more reading and add that to my course this semester.
Did you even feel any of that EQ down in San Diego? ANY reports of shaking
etc inSan Diego County? We had diddly. Richard
I woke up at 4:29 AM and felt the whole thing. Frankly, I had no idea how
powerful it was. It could have been a five on the Elsinore fault for all I
knew. When I saw the news a few hours later I learned the truth.
Everyone concentrates on the plate boundary faults. That, afterall is where
the interesting science is. Considering the strike of the Oakridge fault I am
not surprised that buildings (all probably codeworthy) fell down. The parking
structure at Northridge Mall was designed by an eminent engineer.
This is how codes are written. Geologists try to prognosticate, but you
aren't usually going to investigate things that don't promise results. The
sound palms striking engineers forheads echo throughout the land.
It might not be as surprising as all that, though. In 1971 high vertical
accelerations were recorded at the Van Norman Dam. Maybe as high as 80% G.
But the engineers (I was still in high school) considered it anomalous. Maybe
not *so* anomalous, huh?
Later,
Mark
Did I say strike? I meant, of course, dip.
Mark
Hi, Phil
> Depressing to many is the failure of an overpass on the newly constructed
> Century freeway (it opened last fall).
Would that be the elevated freeway running west to LAX? It was under
construction last time I was in the vicinity in 1990. You're right – heads
(codes?) will roll over that deal.
Guy
Hi, Guy–
Yes, the Century freeway is the new freeway to LAX. It was only recently
completed and opened, yet it and the 5/14 junction are among the fallen. The
latter was rebuilt after the Sylmar quake of 1971 trashed it. Overpasses
falling during the Sylmar quake caused a lot of reengineering and much ballyhoo
about quake-proof intersections; but even those built after the lessons of
Sylmar proved unsafe. As Mark says, something more is still needed.
It was truly fortunate that the quake happened so early in the day. While
fewer would have been caught in the collapsing apartment buildings, a 9-story
medical center collapsed that might have killed hundreds had it not been
uninhabited due to the hour, and the freeway collapses would have taken a
larger toll as well.
–Doug
Hi, Doug
I just found my Alaska Airlines mini-map of LA, so now I can get a
better picture of the locales involved. I don't claim to be a structural
engineer, but based on the CNN pictures of the failed overpass on the Santa
Monica freeway I got the impression that the steel re-bar in the vertical
columns was not tied together very well, and there wasn't enough of it to
boot.
BTW, how are your folks doing? I'm a bit embarassed that I didn't think
to ask until just now. <g>
Guy
Hi, Guy–
There was a program to reinforce old bridges and overpasses, in motion since
the Sylmar quake of 1971, but it was underfinanced and moving very slowly,
having done only a fraction of the indicated work. Santa Monica freeway at
Fairfax may have been awaiting attention; two fallen overpasses in the valley
were due up soon on the list. But particularly frustrating were failures in
intersections built after Sylmar, which is why I mentioned the Century freeway
and the camera-worthy failure of the 5/14 intersection in particular. I
suspect that one of the "aftershocks" was really the sound Mark mentioned of
palms slapping engineer's foreheads.
Most of my relatives and friends were a respectful distance from the quake, and
aside from a scare had little trouble. Rod, as noted in another quake thread
branch, is another matter.
–Doug
Doug,
The Santa Monica overpass was not retrofitted. All other failures had been
retrofitted or redesigned after the 1971, San Fernando earthquake.
Hi, Gary–
Welcome to the forum/section!
I read that there were 8 damaged intersections. If you meant to exclude the
two failures on 118 which I mentioned that my OK newspaper says were due up for
retrofitting soon (can I trust an OK newspaper to get such fine points right
about CA?) and the Santa Monica freeway at Fairfax, that seems to say 5
retrofitted intersections were damaged by the quake.
I have a notion the retrofit was intended simply to prevent collapse. Some
cracking and shifting that should not immediately threaten life but might make
the intersection suspect enough to be closed for repairs was to remain possible
in a severe quake, or such is my understanding. I wonder how total those other
failures not detailed in my news sources are – at least I can tell they are not
as photogenic as the 5/14 junction collapse, which was a clear black mark for
the retrofit program.
–Doug
> Would that be the elevated freeway running west to LAX? It was under >
construction last time I was in the vicinity in 1990. You're right – heads >
(codes?) will roll over that deal.
That's the one. The failure was an overpass very near LAX – I have heard no
other detail as to the exact location, as the press is emphasizing the failures
of the 10, 5 and 14. Because it was near LAX, I'm sure the underlying soil is
not the best at transmitting seismic waves and as with most coastal areas,
amplifies the waves.
Phil
I heard the Century came through with no problems. There were no news reports
about much of anything south of Hollywood Blvd., with the exception of the
Santa Monica Freeway collaspe, and that IMO was treated as an afterthought to
northridge.
Hi, Harrison–
Welcome to the forum/section!
I heard from people freshly arrived from CA (funny thing; the morning after the
quake, our OK field office was overrun by people from southern CA, which is our
base) that part of the Century freeway had collapsed, and saw in my OK paper
that the 105 was closed due to "quake damage" – rumor could well have
exagerrated the degree of damage, but I assume something did happen there. Our
base building in El Segundo, farther from the epicenter than the Century, was
closed due to some cracks, I am told.
–Doug
Has anyone done a study comparing weather with devestating quakes, especially
in California. The four big California quakes during my lifetime have all
occured on cloudless days, above normal temperatures influenced by a high
pressure ridge off the coast. The same weather conditions existed for the 1906
quake in San Francisco. I don't have the weather conditions for the 1933 Long
Beach quake. With the exception of Loma Prieta, all the big California quakes
happened in the early morning between 4:30 and 6:30. Loma Prieta happen just
after 5 pm, thus all the big california quakes have occured within a couple of
hours of sunrise/sunset. Could weather/air pressure or the solar/lunar
position influence quakes? Maybe I could go into the earthquake non-prediction
business. I won't be able to tell you when we will have a major quake, but I
can say it won't be at 12 noon on a cold, cloudy or rainy day.
Hi, Harrison–
The only weather connection I have heard was the story that quakes are more
likely during rain, because the weight of the water on the ground helps make a
fault give way and/or water lubricates the fault. This came to me as folklore;
I know of no studies connecting quakes with any sort of weather, and would be
startled if such a connection emerged – the forces just do not seem right to
me.
I would more easily credit some involvement of tides. I do remember reading
of predictions based on the moon's position – the newspaper article implied
there had been a study that failed to relate tides to quakes, but this new
prediction (by Browning?) took a new factor into consideration, the degree to
which the moon was farther north or south than normal, presumably adding a
shear force to the tides. The ballyhooed prediction misfired. I have heard
here that Browning (if I am not confusing two stories here) was engaged in
commercialization of his quake predictions that might warrant some doubt as to
his motives.
–Doug
Hi, folks–
I called Rod last night and learned he is well and his house is intact, which
is quite lucky as he lives 2 miles from the epicenter. Of course all sorts of
things are broken and he is without power. He has people camping in his back
yard, perhaps because most of his utilities are intact.
His large mineral collection is all over the floor of his garage; it will be no
small task to get everything back into the right boxes. His hard hats with
lamps are proving quite useful – I doubt he ever expected to need them in his
home.
While his computer appears intact, and he has phone service, the computer was
misbehaving again before the quake, leading to the suspicion that the problem
which had him off-line for months is still present, and is likely a hardware
problem. It seems it will be some time before he will be on-line again.
–Doug