On 5/30/14, 3:00 PM, Hal Murray wrote:
[Structure of Earth's core]
jim...@earthlink.net said:
Molten, but it's a composite material under a lot of pressure, so the
transition between "liquid" and "solid" isn't like between ice and water.
Think cold peanut butter.
Seismic evidence is how they knew it was liquid in the first place. As you
get better at doing the models, and getting better time measurements of the
seismic propagation with higher performance seismometers, you can get a
better model.
There was an article in Scientific American back in the early 1970s
discussing the structure of the Earth and all the tricks the seismologists
used to figure things out. I remember a diagram of the cross section of the
Earth with a blizzard of seismic paths, bending and bouncing at each boundary.
In that time frame, there was a lot of money for seismic research. It was a
key part of the test ban treaties.
What did seismologists use for timing back then?
A good wristwatch?
Most of the work was done with conventional ink on graph paper drum
recorders, and the sync was probably done with some sort of conventional
"derived from national time reference" source: WWV or similar in the US.
It's not like you need nanosecond precision when you're looking at
propagation times of minutes.
P wave propagates at about 5 km/sec
S wave propagates at about 3 km/sec
That's enough difference that you can tell how far the epicenter is from
you in a local earthquake. If it's "close" (within 10km), you feel the
earthquake as a single event, because the P wave arrives in a couple
seconds and the S wave is a second later, while the shaking is still
going on (unless it's a very small quake). If it's farther away (say
50km), then you feel two separate events (50 sec vs 80 sec).
I would say that most people who live in Southern California for a while
and are somewhat aware of it can tell the rough distance and magnitude
of an earthquake because of this, even if they don't know why. Duration
of shaking correlates well with magnitude. The short "thump" is probably
a 2 or 3 (was that an earthquake or a sonic boom or someone dropping a
truck load of something), a 4 will maybe be a rumble or a couple sways,
a 5 is "we're definitely having an earthquake" because it lasts
distinctly long enough for you to realize that something's going on
that's not a "short event", and a 6 is in the "seems to last forever"
category.
So if you get a "crack", "thump", "rattle" followed a few seconds later
by the lamps or cupboard door swinging a bit, you say "hey, a 3.5 10
miles away"
The P wave has a distinct "crack" and feels short to me, and the S
(shear, transverse to propagation direction) wave is more the rolling
and swaying (depending on the direction of motion).
It's kind of like counting seconds between lightning flash and thunder
(short duration event, delay, long duration event)
In any case, to propagate the 10,000-15,000 km across the earth takes
the better part of an hour.
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