[time-nuts] New tide gauge uses GPS signals to measure sea level change

Jim Lux jimlux at earthlink.net
Fri May 30 19:39:30 EDT 2014


On 5/30/14, 3:00 PM, Hal Murray wrote:
> [Structure of Earth's core]
>
> jimlux at 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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