[time-nuts] Plate Tectonics was: GPS Antenna on Tower.

[Attila Kinali]

On Tue, 20 Jun 2017 13:29:24 -0700
Brooke Clarke <brooke at pacific.net> wrote:

> -------- Original Message --------
> > I have wondered how geologists are able to measure tectonic plate 
> > movements in the earths surface to a couple cm's when the sensors from 
> > what I see/read they are nothing more than sensors in concrete boxes?. I 
> > believe they use various technologies such as Very Long Baseline 
> > Interferometry<http://www.see.leeds.ac.uk/structure/dynamicearth/plates_move/active_tectonics/vlbi.htm> (VLBI)
> >  and Satellite Laser Ranging<http://www.see.leeds.ac.uk/structure/dynamicearth/plates_move/active_tectonics/slr1.htm> (SLR)

The first "proofs" for plate tectonics were not related to ranging
and movement at all. They were based on evidence burried in the ground:
http://hyperphysics.phy-astr.gsu.edu/hbase/Geophys/platevid.html

I don't know when the first proof using longitude measurments was made,
but it must have been fairly recent (i'd say last 100 years at most)
as pretty stable clocks were needed. Probably at least quartz
or maybe even atomic clocks.

Modern tectonic movement measurements are mostly based on GPS.
Even using a simple L1 C/A only receiver, one can achieve sub-cm
accuracy if phase data is recorded over several hours and post-processed.
Using an L1/L2 receiver, compensating for all those "tiny" effects
and averaging over a few days should result in sub-mm differences over
quite long baselines.

Side note:
Solid earth tides are in the order of 20-50cm vertical and 10-30cm horizontal.
(if i remember correctly)

> The first data on tectonic plate movement came from the Latitude 
> Observatories.  The Longitude problem was solved fairly 
> quickly by Harrison's clocks but the uncertainty of an observatories 
> latitude because of wobble of the pole took much 
> longer and was addressed by a hand full of Latitude Observatories all at 39 
> deg 8 min North, and I'm lucky to have one 
> in my town.
> http://www.prc68.com/I/UkiahObs.shtml
> Note this is more like an extremely accurate surveying instrument than a 
> telescope for star watching.  We will be having 
> star parties since it's summer and the sky here is dark enough to see the 
> Milky way.

These telescopes are called zenith telescopes and could only be moved
in one plane. This telescope is transit instrument and the modern version
of the mural circle. Transit instruments are used to measure when a star
passes through the north-south line/plane. This can then be used to
define local time. The modern variants of the zenith telescopes are used
by the IERS to measure siderial time and compare it to TAI/UTC.
The transit instruments in Greenwich defined the 0 meridian. Hence
every time the instrument got upgraded by a better one, the meridian
moved a bit. 

One thing I still haven't understood is, how these transit instruments
were "calibrated" and placed exactly on a north-south line.


> > All fascinating stuff....

Definitely!

			Attila Kinali


-- 
You know, the very powerful and the very stupid have one thing in common.
They don't alters their views to fit the facts, they alter the facts to
fit the views, which can be uncomfortable if you happen to be one of the
facts that needs altering.  -- The Doctor