Subject: [IGSMAIL-4643]: Re: [IGSMAIL-4642]: IGS00 (v2)
From: Jeff Freymueller <[email-masked]>
To: "Ferland, Rémi" <[email-masked]>
Cc: "[email-masked]'" <[email-masked]>
Date: Thu, 9 Oct 2003 10:33:52 -0800

******************************************************************************
IGS Electronic Mail      09 Oct 11:33:54 PDT 2003      Message Number 4643
******************************************************************************

Author: Jeff Freymueller

Users of IGS2000(v2) should be cautious in constraining the position of=20=

the IGS site FAIR, as it is still undergoing time-dependent deformation=20=

following the November 2002 Denali Fault Earthquake.

In addition to the large (~6 cm) displacement at the time of the=20
November 3, 2002 Denali Fault earthquake, the site has experienced a=20
further ~2 cm of postseismic deformation. The postseismic deformation is=20=

a transient signal and even treating the site has having a new velocity=20=

after the earthquake is an approximation because the postseismic=20
deformation decays with time. For example, about half of the total=20
postseismic deformation to date occurred in the first two weeks after=20
the earthquake. The accuracy of the post-earthquake velocity thus=20
depends on the exact time interval used to estimate it. In particular,=20=

this means that if you extrapolate the immediate post-earthquake=20
velocity very far into the future you will run into problems as the=20
postseismic transient decays away.

Fortunately, the total postseismic deformation is fairly small, and=20
based on Remi's description I suspect that any errors  due to=20
postseismic deformation are presently at the 1-2 mm level, but I would=20=

not suggest extrapolating the post-earthquake velocity more than several=20=

months into the future in order to maintain this level of accuracy.

We are working on models to describe and explain the time-dependent=20
deformation following the earthquake, but due to a heavy fieldwork load=20=

it will be a few months before they are ready.

Jeff

On Thursday, October 9, 2003, at 09:57 AM, Ferland, R=E9mi wrote:

> 4)	Sites with different alignment & realization points
>
> Six sites used in the reference frame realization had
> experienced significant discontinuity since the original
> IGS realization and since ITRF2000 became official. In the
> IGS cumulative solution (updated weekly), at stations with
> identified discontinuities, the position/velocity were
> estimated independently. For the alignment/realization,
> the independent velocity estimates at a station were
> combined to provide a unique velocity per site. The sites
> with estimated discontinuity are:
>
> BILI COCO FAIR GUAM HOFN KOKB
>
> The cumulative solution for GPS week 1231 (IGS03P33.snx)
> was used for the proposed IGS2000(v2).
>

Dr. Jeffrey T. Freymueller         Office: 907-474-7286
Geophysical Institute              Fax:    907-474-7290
University of Alaska, Fairbanks    Home:   907-479-3550
PO Box 757320                      Cell:   907-322-7632
Fairbanks, AK 99775-7320           email: [email-masked] or=20
[email-masked]
URL: http://www.aeic.alaska.edu/Input/jeff/jeff.html

Download Alaska GPS data: http://www.aeic.alaska.edu/Input/akda/