Colleagues.
The now attached message is from a recent ILRS mail, detailing some of the ongoing geodetic SLR and GPS analyses following the massive Earthquake in Peru.
Regards,
Graham Appleby
Dr Graham Appleby
NERC Space Geodesy Facility
Monks Wood, Huntingdon PE28 2LS
01487 772477
********************************************************************************
SLR Electronic Mail Wed Jul 18 18:55:19 CEST 2001 Message No. 0785
********************************************************************************
Author: Van Husson, ILRS Central Bureau
Subject: Status on the movement of Arequipa
Dear Colleagues,
A massive earthquake (8.4) struck beneath southern Peru at 20:33 GMT
on June 23, 2001. The earth quake was 175 km West of Arequipa, Peru.
There have been a number of large aftershocks with the largest (7.6)
being on July 7th at 9:38 GMT, which was 110 km south-southwest of
Arequipa. (Check out the graphics below:)
http://earthquake.usgs.gov/activity/latest/eq_01_06_23/index.html
http://earthquake.usgs.gov/activity/latest/eq_01_07_07/index.html
The June 23 shock originated just southeast of the source of a
magnitude 7.7 earthquake that occurred in 1996, and it appears to
have involved rupture of part of the plate-boundary segment that
producedan earthquake of magnitude approximately 9.0 in 1868. The
1868 earthquake was destructive in towns that were heavily damaged
in the June 23rd earthquake. The 1868 earthquake produced a tsunami
that killed hundredsof people along the South American coast. The
1868 tsunami also caused damage in Hawaii and alarm in Japan.
The initial onset on June 23rd consists of two events separated
by about 6 seconds. It was followed by at least one larger complex
event occurring about 40 seconds later. (Excerpt from USGS)
Arequipa's (SLR Station 7403) first pass after the June 23rd quake was
July 3rd (only a week after the event). To date (July 17, 2001), more than
25 passes of the geodetic Lageos satellites have been obtained by the
station.
RESULTS FROM NERC SLR (7403) ANALYSIS
*************************************
Graham Appleby, from the NERC associate analysis center, using a weeks
worth of 7403 LAGEOS data has solved for its coordinates.
Here are NERC and ITRF2000 7403 site coordinates:
X(m) Y(m) Z(m) Comments
------------- ------------- ------------ -------------------------
1942807.6450 -5804069.7160 -1796915.7580 NERC (epoch 1997)
1942808.1130 -5804069.7040 -1796915.4630 ITRF2000 (epoch 1997)
-0.4680 -0.0120 -0.2950 0.5533m change in position
This represents a 55cm change between the pre and
post quake 7403 site coordinates. Graham also computed these
changes in NEU and are -31cm in North, -44cm in East and -4.8 cm
in height.
************************************
RESULTS FROM DGFI SLR (7403) ANALYSIS
*************************************
DGFI, Germany, has also estimated a new position for 7403. Here are their
results using independent weekly LAGEOS-1 & LAGEOS-2 solutions
from July 3 to July 14:
X(m) Y(m) Z(m) Comments
------------- ------------- ------------ -------------------------
1942808.1157 -5804069.6997 -1796915.4595 DGFI,epoch 1997 a-priori
1942807.6240 -5804069.7180 -1796915.7700 DGFI,epoch 1997 solved
0.4917 0.0183 0.3105 0.5818 change in position
This is a total displacement of 58cm or almost 2 FEET.
*************************************
RESULTS FROM MIT GPS (AREQ) ANALYSIS
*************************************
Wolfgang Seemueller has provided the results from the
MIT global solution using GPS data one week before and one
week after the June 23 earthquake. Here are his results for AREQ
(GPS):
X(m) Y(m) Z(m) comments
------------ ------------- ------------- -------------------------
1942826.3487 -5804070.2723 -1796894.2092 DGFI (June 24 - June 30)
1942826.7935 -5804070.2452 -1796893.9329 DGFI (June 17 - June 23)
-0.4448 -0.0271 -0.2763 0.5243m change in position
This represents an approximate 52cm change between the pre and
post quake AREQ (GPS) site coordinates.
*************************************
RESULTS FROM SOPAC GPS (AREQ) ANALYSIS
**************************************
Matt VanDomselaar ([log in to unmask]) from Scripps Orbit and
Permanent Array Center has seen a horizontal coseismic offset of
almost 51 centimeters and a continuing postseismic signal of about
4 centimeters in the days following the quake.
**************************************
To conclude, the SLR results from NERC and DGFI are very much in
line with the GPS results from DGFI and Scripps. Bear in mind,
NERC and DGFI SLR results indicate more movement than the initial
GPS results, because the Arequipa SLR station did not obtain any
LAGEOS data until 1.5 weeks after the quake.
SLR analysts, who are currently using their old positions for
7403, are observing range and time biases on LAGEOS of +/-50cm
and 0 to +250 microseconds, respectively. These apparent biases
are caused by the movement of the station not by problems in the
system. NERC has re-analyzed the 7403 SLR data with its new
coordinates and the biases have returned to nominal levels.
NERC's short arc analysis of three simultaneous passes on
July 13 and July 16 between Monument Peak, Greenbelt, Arequipa,
Grasse (LLR), and McDonald is further evidence of the strength of
SLR. These analyses are located at:
http://nercslr.nmt.ac.uk/global_short_arc/Arequipa_lgb_2001-Jul-17_01.html
http://nercslr.nmt.ac.uk/global_short_arc/Arequipa_lga_2001-Jul-17_01.html
http://nercslr.nmt.ac.uk/global_short_arc/Arequipa_lga_2001-Jul-18_01.html
Both Graham and Wolfgang are waiting for results from other analysis
centers from not only ILRS, but IGS and IDS.
Van Husson, ILRS Central Bureau
From: "Husson, Van" <[log in to unmask]>
********************************************************************************
|
