On 06/12/2010 03:36 PM, jimlux wrote:
Magnus Danielson wrote:
On 06/12/2010 02:33 AM, Hal Murray wrote:
[email protected] said:
The Chilean earthquake changed the angular rotation rate (or,
probably more
accurately, changed the direction of the axis of rotation as well)
of the earth a small amount, as do most large earthquakes.
Has anybody measured that?
Is there a good URL on this? (predictions if not data) All I've found so
far is a small NASA press release predicting 1.26 microseconds per day:
http://www.nasa.gov/topics/earth/features/earth-20100301.html
(and a zillion news sources repeating it)
1 microsecond/day is 1 part in 1E11.
There is one place that keeps track of these things, the IERS. Their
Bullentin B provides monthly reports of earth rotation observatoins.
The bulletin for relevant period of time is:
ftp://hpiers.obspm.fr/iers/bul/bulb_new/bulletinb.266
ftp://hpiers.obspm.fr/iers/bul/bulb_new/bulletinb.267
Explanations is in:
ftp://hpiers.obspm.fr/iers/bul/bulb_new/bulletinb.pdf
Clause 3 is of most interest, look at the OMEGA column (appended the
bulletin b 267 data for a more complete time-series around the date of
interest).
3 - EARTH ANGULAR VELOCITY : DAILY FINAL VALUES OF LOD, OMEGA AT 0hUTC
LOD : Excess of the Length of day - 86400 s TAI
OMEGA : Earth angular velocity
Description: ftp://hpiers.obspm.fr/iers/bul/bulb_new/bulletinb.pdf
DATE MJD LOD sigma OMEGA sigma
(0 h UTC) ms ms mas/ms mas/ms
2010 2 2 55229 1.8681 0.0026 15.04106685346 0.00000000045
<snip>
55287 1.3068 0.0018 15.04106695117 0.00000000031
Yes, we see a dip there... but looking at the two-month data we se a
regular pattern creating a dip there... and the lack of jump is
interesting.
Essentially... I can't see it here.
And I think that's what I heard: it should have changed (permanently)
but that it would be impossible to see without years of data to remove
all the other effects.
My GPS friends (all in the same section as Richard Gross.. the same
section are the folks who do precision measurements of gravity (GRACE
and GRAIL missions), etc.) tell me that once you get down into a certain
range of uncertainty (10cm=ish for GPS), there's a whole lot of factors
that are all of about the same magnitude (tidal movement, atmospheric
delays not due to ionosphere, ionospheric effects, continental drift,
etc.) So making absolute measurements requires lots of data and
painstaking identification of each of the factors and removing it.
Actually, we should be quite happy that we are not seeing much worse
frequency jumps and angular movements than we do.
The main point of my exercise is to point into a bunch of aggregated
data-set that gives an overview at least. It uses several independent
sources (such as GPS, GLONASS, VLBI, SLR sources) and plots differences
such that effects particular to some systems is not allowed to dominate.
While it would be fun to know, the practical impact of such a change is
very, very small, to the level of being ignored. Considering of a major
event actually consisting of many hundreds of earth quakes spread over a
rather longish period, it becomes more interesting to see how the
aggregate behaves. But you also needs to understand what the effect was
prior to the event as it built up. What is the effect over a 1000 year
period (a short period in geological sense)? Consider the build-up of
pressure and magma under a volcano, which progresses over many years
before it finally accelerate. Gravimeters is routinely used to monitor
this build-up. There are thus more sources to consider. Someone is
probably studying that.
Cheers,
Magnus
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