[IGSMAIL-5082]: IGS and the Earthquake offshore Sumatra

[Gerd Gendt (forwarded by A. Moore)]

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IGS Electronic Mail      07 Jan 18:22:14 PST 2005      Message Number 5082
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Author: IGSACC
 
Dear IGS Community,
 
we all were shocked by the effect of the earthquake offshore the
coast of Sumatra on December 26 and the tsunami events associated
with it. The question naturally arises whether (a) the IGS products
were negatively affected by the event and whether (b) the event left
its traces in the IGS products.
 
Let me first answer the questions:
 
(a) Quality of the IGS products; IGS Network
 
    To the best of my knowledge as IGS analysis coordinator there
    was no negative impact of the earthquake on the quality of IGS
    products. It is in particular worth mentioning the that the "rapid
    orbit contributions" were of the same quality as usual. This
    also implies that the IGS rapid orbits for the time interval
    covering the terrible event are of comparable quality as for the
    days before and after the event. This is important to know for
    analysts using the IGS products for further analyses in the region
    affected. They can fully rely on the global terrestrial reference
    frame (ITRF2000/IGb00) as materialized by the IGS products.
    Of fundamental importance for the IGS is its global network. From
    all stations located around the northern Indian Ocean (BAKO,
    BAN2, COCO, DGAR, HYDE, IISC, MALI, NTUS, REUN, SEY1) only SEY1
    stopped its tracking because of the tsunami. Therefore continuous
    regional data are available for detailed analyses. High-rate data
    are recorded at BAN2, IISC, REDU, JOGJ.
 
(b) Visibility of the event in the IGS products.
 
  - In IGS mail no. 5069 sent out on December 29 (by Urs
  Hugentobler/CODE AC Team) possible effects on the earth rotation were
  mentioned. In essence the actual motion of the pole was compared
  with a predicted pole. Effects, mainly in the y-component of the
  pole of (at maximum) 2.7 mas were reported.
  - Today we can say that in essence the same motion of the pole
  was also observed by the other analysis centers (therefore also
  reflected in the IGS rapid product - see
  http://igscb.jpl.nasa.gov/images/Dec2004EQ.Gendt.gif ) and that it
  seems to be above the noise level.  No clear signal was found in the
  determined length of day (lod).
  - The IGS will continue to analyze the data to improve our
  understanding of the geophysical processes. We do not comment on
  the origin of the motion. Further analysis will show what part
  of the polar motion can be attributed to the observed excursion
  of the pole. The IGS only provides observational evidence, no
  interpretation.
 
  - First evidence that the stations NTUS and IISC(BAN2) (1000 km
  from the epicenter) are shifted ~1-2 cm in the eastern direction
  can even be obtained using the Precise Point Positioning with the
  IGS Rapid orbits and clocks (see attached plot), and are clearly
  seen from the Final analysis.
 
Station displacements will be best seen in the IGS Final products
(where all station coordinates are left free). This analysis will
become available about one week from now. Preliminary reports of the
IGS analysis centers indicate that station motions of the order of
1-2 cm are likely to have occurred. Some of the centers even excluded
stations from the list of fixed fiducial sites for their analyses. We
are not aware of "dramatic" displacements (5 cm or more) in the
stations defining our reference system.  More information concerning
station motion will be provided on the occasion of producing the IGS
Final products.
 
Let me point out that this is a preliminary summary. It is based on
 
 - reports by the IGS Analysis Centers,
 - the analysis performed in the context of the IGS combination,
 - personal communications
 
Please check our IGS web sites:
    http://www.gfz-potsdam.de/igsacc
    http://igscb.jpl.nasa.gov/network/netindex.html
 
 
Best regards

Gerd Gendt, IGS Analysis Coordinator