#Kobe: Richter/moment?
15 messages in this thread
I keep hearing the Kobe quake described as "magnitude 7.2 on the Richter
scale." I thought everyone uses moment magnitude for the larger quakes. Can
someone shed a some light on this, and maybe help me understand the difference
in the two scales of magnitude? Thanks.
Hi, Tony–
Welcome to the forum/section!
The late Charles Richter, a seismologist at Cal Tech, devised his scale in
1931, basing it on the widest swing in the zig-zag line generated by a
seismograph's needle in a quake. It is not very accurate for the quakes above
8; it is based on readings on an obsolete type of seismograph used close to
quakes (within 100 miles or so, which is often not possible outside Calif.).
Thus, scientists long ago abandoned it, but the public had already grown
accustomed to it, so it lives on despite all attempts to "kill" it.
Since the early '80s, the most widely used measure has been moment magnitude.
It is based on the size of the fault on which an earthquake occurs and the
amount the earth slips. The bigger the fault and the larger the slip, the
higher the moment magnitude.
Seismologists at the USGS in Golden, CO prefer surface-wave magnitude, an
analysis of seismic waves crackling around the Earth's surface, and call this
Richter magnitude, which it somewhat resembles, though it is not precisely the
same.
The January 17, 1994 Northridge quake in Los Angeles had a moment magnitude of
6.7. The Loma Prieta quake that hit the San Francisco Bay area in '89 had a
moment magnitude of 7.0. The Landers, CA quake of June '92 was called
magnitude 7.3 by USGS Pasadena (moment magnitude), while USGS Golden called it
7.6 (surface-wave magnitude). The Alaska quake of '64 had a surface-wave
magnitude of 8.4, while its moment magnitude was 9.2.
–Doug
Thanks for that very clear explanation. I've since learned that the Japanese
use their own local scale for preliminary measurements. Under that scale, the
Kobe quake was 7.2, but under the moment magnitude scale it was 6.9, according
to Bill Ellsworth at the USGS in Menlo Park, CA. He says in a quake that big,
the difference between a 6.9 and a 7.2 is "whimsical."
Hi …
I pulled the USGS catalog on Kobe, and here's what it says …
Location: 34.67 N 135.04E
Depth: 33 km (a little more than 20 miles)
Surface Wave
Magnitude: 6.8
Quality of Read: A
I'm trying to get more information on it, but the computer lines have been
jammed into USGS and Caltech for days.
If I run across anything more, do you want it? I'm sort of waiting to find out
if the destruction was compounded by the fact that the waves ran into the
mountains and maybe backwashed against the oncoming waves. I think I heard
somewhere that it causes much more damage if you have a landform where that can
happen.
Thanks for posting your note, because I could not figure out why USGS and the
Japanese didn't match!
Many thanks …
Bye …
Yes, I'd like to hear any new theory by scientists. The idea of earthquake
waves "backwashing" against the mountains sounds pretty funky to me, but who
knows what we'll learn from this quake? About ten years ago, I hired a
geologist to look at a house and some property I was planning to buy on a Santa
Cruz, CA ridgetop. He assured me that ridgetops were the safest place to be in
an earthquake, since it was solid granite. The epicenter of the Loma Prieta
quake a few years later was five miles from my house, and structures on
hilltops experienced more shaking than buildings elsewhere. Afterwards, the
geologists decided that ridgetops "focus" earthquake waves. Gosh. Thanks a
bunch. Glad I was able to help the cause of science! (Of course, a magnitude 7
quake is going to shake up a lot of buildings, no matter what kind of ground
they're built on.)
Hi …
I meant to reply earlier, but got busy!
I wanted to clarify what I meant about "backwashing" quakes waves. The first
thing I should explain is that I am not a geologist. I am a business woman and
musician, not even remotely any kind of scientist.
However, one of the things I do in my work is that I am sent on research
missions for anything and everything my bosses want to know something about.
And when they wanted to buy some property on the West Coast, they decided
before they purchased anything they should probably find out as much as
possible about the whole area to see where they should live.
Fourteen books on geophysics and plate tectonics later, I was talking to the
director of emergency management for a major California city. I won't mention
his name for the simple reason that HIS superiors would kill HIM if they knew
he talked to me. We talked often. This was around the time of the Northridge
Quake, so we were talking a lot about that. But we were also talking about the
Loma Prieta earthquake and some of the things that they discovered from it.
One of the expected things was that the quake waves travelled in soft sand and
landfill much faster than in other densities of soil/rock. That's expected. You
build your house on sand, and God knows what will happen. In fact, I think
he/she issued a warning about that. Anyhow, quakes waves run down soft, loose
soil like fire runs down a line of gasoline. You don't want to be there when it
happens.
With Loma Prieta, one of the things that surprised them is that the earthquake
waves seemed to act like water. They literally took the path of least
resistance. For example, there is a HUGE solid granite boulder hidden mostly in
the ground south of the Bay Area. The township on the south side the form was
damaged. The township on the north side of it was, too, but there was no damage
in between. And they hypothesized that earthquakes waves of all four forms —
primary, secondary, Rayleigh and love waves — ran UNDERNEATH the boulder, and
surfaced on the other side, taking the path of least resistance. It kind of
sounds like the "aim" or "focus" of waves you were talking about.
In the Northridge quake of 1972 — which is, unhappily, a very vivid memory for
me since I lived a MILE from the epicenter — had something similar happen,
actually. The waves travelled down the San Fernando Valley, encountering
mountains on all sides until they came to the Cajuenga Pass, which connects the
San Fernando Valley with LA. The waves, sliding on the inside of the "bowl"
converged into the pass and ran into LA. There was serious damage in
Northridge, and serious damage at the portion of LA that lay at the bottom of
the Cajuenga Pass.
There is some hypothesizing going on out there with people who actually know
what they're doing (and are nice enough to talk to me every once in a while)
that says that earthquakes waves act like water in the bathtub. When
fast-moving waves of these frequencies encounter dense matter, some of the
wave-forms will be absorbed by the matter, and some will echo backward, like
sound waves hitting a wall.
That's one of my latest current hairbrain theories about why it's so bad in
Kobe. The waveforms ran across the seafloor from Awiji Island into the soft
sand and landfill of Kobe. Maybe what happened when the waves hit the mountains
is that some of the energy was volleyed back, only to meet other oncoming waves
of energy. If that is true (big IF!) then it would have complicated things very
serious, creating huge standing waves or vertical upshots of energy.
Another interesting little tip came from Nightline the other night when they
reported that there had been a 10-foot drop in the height of the land at the
port itself. Now that's real similar to what happened at Seward, Alaska, when
they had their 8.3 a few years back. They had terrible subsidence there. Aerial
photographs of it will just scare the daylights out of you.
If there were a 10-foot subsidence (literally, a sliding of sandy seafloor off
the shelf, resulting in a collapse of the land) at Kobe Harbor, that ALONE
would cause an immense, immense series of waveforms to add to the problems they
already had.
What I am searching for here is why a 6.8 earthquake that happened in Kobe is
so much worse than the quake in Northridge, which had a really violent ground
motion — more than the REAL scientists thought it should — and so much worse
than Loma Prieta, which I think was around the same moment magnitude. And right
now my mind seems to be thinking that there is something in the landform there
which compounded the intensity of waves.
So that is the source of my "pretty funky" comment! I can always be counted on
for a few pretty funky comments here and there. It seems to be part of my
nature, for good or ill.
May liquefaction never real its ugly head in your reality.
May it all be fun …
Morgaan
Morgaan,
>> That's one of my latest current hairbrain theories about why it's so bad in
Kobe. The waveforms ran across the seafloor from Awiji Island into the soft
sand and landfill of Kobe. Maybe what happened when the waves hit the mountains
is that some of the energy was volleyed back, only to meet other oncoming waves
of energy. If that is true (big IF!) then it would have complicated things very
serious, creating huge standing waves or vertical upshots of energy. <<
I don't think that's such a harebrain theory, but then I'm no seismologist
either. <G> I've also wondered about the tremendous devastation in Kobe from a
quake that was no more powerful than either Loma Prieta or Northridge '94.
I do recall reading somewhere that earthquake waves travel in substantially
different ways in loose soil than in rocky areas. Consider the New Madrid
quakes of 1811-12, which rang churchbells in Boston over two *thousand*
kilometers from the epicenter. Granted, those quakes were the most powerful
ever recorded in North America, but OTOH 2000km is one helluva long way for
waves to carry with that kind of strength.
I've wondered for a while if rough, heavily broken and faulted bedrock like the
West Coast tends to deflect or damp out seismic waves rapidly, while loose soil
like that of the eastern US carries them with less-than-average energy loss
over long distances. I also recall a 5.0 or so quake in 1985 or 86 — its
epicenter was under Lake Erie about fifteen miles north of Cleveland, but the
surface waves were noticeable (I know, I noticed 'em <g>) 150 miles south, in
Marietta, Ohio. That's a long way for waves to go from a 5.0 quake.
Jon W.
| What I am searching for here is why a 6.8 earthquake that happened in Kobe is
| so much worse than the quake in Northridge, which had a really violent ground
| motion — more than the REAL scientists thought it should — and so much worse
For what it is worth Radio Japan several years ago carried stories about the
controversy in Japan when a French politico on a visit that in France they
had rabbit hutches built better than most Japanese houses. I don't know, I'ld
sure like to visit and see for myself.
Hi, Martin–
I am none too sure a "French politico" would know much about quake safe
construction – I have to suspect he was referring to their qualifications for
everyday living. Part of the shock to the Kobe residents seems to have been
how hard they had tried to make the buildings quake safe yet failed. This makes
me more nervous about family and friends back in southern Calif., though I have
always figured destruction like Kobe could happen in L.A.
–Doug
Now that's interesting. I have always heard that wood frame construction was
the safest there is and certainly I saw that in Santa Cruz.
I hear conflicting stories about building codes in Kobe. Apparently much of the
housing was Grandfathered in with their newer construction standards, but I saw
many newer structures with nasty failures. In Santa Cruz the masonry stuff
downtown was toast, but the steel frame buildings did pretty much OK. I was in
the health department and thought the building had been hit by a large truck at
first.
Hi …
I don't know about the building codes in Kobe, but I did learn some interesting
stuff from some books and emergency management people when I was looking at
what to build/buy in California. Here were some things passed along to me:
(1) Wood-frame house are supposed to stand up best. They will
list wildly to port or starboard, but are less likely to come
down!
(2) Brick fireplaces, even in wooden stuctures, are bad news!
You'll almost always lose one of them, and some-
times they'll come down on the inside, rather
than the outside, of the house.
(3) Masonry is a disaster.
(4) Never build a house on land that has more than a 30% grade.
There are any number of cliff-dwellers who were
not told of this when the drew up the blueprints!
(5) Do not bury your fuel tank in the ground. Liquefaction can
cause it to "float" out of the ground like a
surfacing submarine, and the resulting breakage
of fuel pipes can start a fire that may run right into
your house. It's better to put it overground. This is
the one that scares me the most.
(6) Most people hurt in earthquakes are hurt breaking, flying
glass or by fire. This applies to buildings that don't
come down, but which sustain damage. In the 1972
Northridge quake, which was my most up-close-and-
personal experience, virtually everything made of
glass disintegrated. A Southern-born person, I was
completely shocked by the experience and didn't
know anything. So, I didn't go stand in the door
like any normally intelligent person would have
done. I tried to run east as fast as I could. On the
way from the bedroom down the hall, there was
"water" spraying out of the bathroom. On the
way through the living room, there was "water"
spraying out of the front of the television and
the doorway to the kitchen. But it wasn't water:
it was glass. There was glass everywhere.
When I got to the front door and opened it,
THEN there was water. Swimming pool water.
It was getting bounced off the bottom of the
pool and onto the 2nd-floor roof of the building.
That's when I gave up and starting having rapid
conversations with the deities of my choice.
I know that after Loma Prieta and Northridge a lot of the companies
that do earthquake evaluations went back to the drawing boards. I had some
friends building houses at the time in California and they had people come back
in for a second time. The second time they had a LOT more suggestions to make.
I guess this is an evolving issue and always will be as public health people
try to find a way to make us safe. I feel so sad for the Japanese. It seems
like the shock factor was so deep it was nearly paralyzing for a few days. …
I hope they heal fast.
Write more! What else did you find out in Santa Cruz????? <G>
| Write more! What else did you find out in Santa Cruz????? <G>
I am native born Californian and had always fantasized (being an RN) of the
heroic things I would do when a quake should give me the opportunity. Most of
what I learned at Loma Prieta was about people. I am a public health nurse.
To wit:
"People are sheep" … When I got home from the health department after we
were sent home. There were people sitting around on the streets drinking beer
and watching TV with a half hour of daylight and no power. I got out all my
camping gear, filled all my pots and set up in the backyard. The next
morning, they (the sheep) all figured out they were in a jam and showed up at
Safeway simultaneously. The line went 3 blocks.
The Red Cross are a pack of incompetent jerks. I've seen them in action. I
would crawl on my hands and knees clear to Salinas before I would stay in one
of their shelters.
Every charlatan in the world will come flocking to the disaster and have some
incredible scam usually based upon a claim for sympathy they don't deserve.
(Since often they were nowhere near the disaster in the first place.)
Every sucker in the world will come flocking in to get fleeced by the
charlatans.
Life goes on it just becomes INCREDIBLY tedious to do it. EVERYTHING takes
ten times longer to do.
Santa Cruz was saved because we got the power back in 24 hours. If the power
goes in the US after 24 hours things get real bad real fast.
Most of the suffering in an earthquake comes from plain old human pig headed
stupidity and greed. (Both before and after the fact)
In short, God forbid it should happen to my house in San Diego, but if it
does and I'm not kept here for duty. I intend to lock up the house and get
the hell out of town for a month. 90% of the mess in Santa Cruz would have
been infinitely less if only all those damn people had only gone the hell
away when it was obvius they weren't needed.
Whew! I hadn't realized how repugnant that two month's time was.
For what it is worth Radio Japan several years ago carried stories about the
controversy in Japan when a French politico on a visit that in France they had
rabbit hutches built better than most Japanese houses….
One wonders what the frenchman would think of modern american housing.
(Probably die laughing at his first sight of a trailer park). Actually,
Japanese housing is pretty well built, but not the way a European would build
it. The construction in areas that don't get heavy snowfalls is pretty light
so that they can open up as much area as possible in the hot humid Summers.
Also, they have a bad habit of putting heavy objects like tiles up high. I
suspect that the very high death toll in Kobe is largely due to the very high
population density and the difficulty of getting emergancy equipment in over
the narrow and infrequent streets. The death toll in California quakes would
be far higher if our residential housing weren't largely single family homes
set well apart separated by driveways and parking areas and accessed by broad
roads that are not blocked by debris.
Hi, Tony–
Different frequencies and types of quake waves have different characteristics
in terms of whether they focus at ridges, liquefy ground, shake buildings
horizontally or vertically, and so on. Each quake has its own combination of
these wave types and surrounding terrain, and so the experts keep getting
confounded until all the combinations have been seen closely. Even then you
cannot be sure which combination will come up – it will always be a matter of
where the best odds are, I suspect.
–Doug
| hilltops experienced more shaking than buildings elsewhere. Afterwards, the
| geologists decided that ridgetops "focus" earthquake waves. Gosh. Thanks a
| bunch. Glad I was able to help the cause of science! (Of course, a magnitude 7
Very interesting! I was in the county health department on Emline that
afternoon.
Do you think you had more damage than down town or in the SF Marina? What
sort of damage did you sustain to your house?